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Soil Survey Manual - Chapter Four (Part 3 of 4)Mapping TechniquesTable of Contents
Field and Office ActivitiesPreliminary ResearchThe soil survey party leader should arrive in the area before soil mapping begins and generally before the other party members do. This allows the party leader time to become familiar with the area, review preliminary data, investigate the major soils and their pattern of occurrence, review the stated purposes of the survey, check the adequacy of the base map material, and prepare a preliminary mapping legend. During the general premapping appraisal of the survey area, the party leader also assembles the information needed to schedule survey operations. A well-established principle of research is to assemble the existing information about a subject first. Time and effort are saved and costly errors are avoided if what is already known is used. The time required to find and appraise existing information is usually small relative to the time required to compensate for failure to use the information. Even for areas about which little is thought to be known, a diligent search usually uncovers useful information. In addition, information about adjacent areas can often be applied to the survey area. If an older soil survey has been made, it is generally the most important reference available. Soil surveys made in the United States before 1920 emphasized the character of the parent material. The maps commonly provide some of the best information available for dividing the survey area into sections within which parent material is reasonably uniform. Many soil surveys made between 1920 and 1930 provide most of the information needed to broadly characterize the area and its soils. Those made between 1930 and 1940 provide a very important part of the information needed for identifying map units. The earlier surveys are also useful for identifying map units, but they must be used in conjunction with a systematic preliminary field study. It is helpful to examine mapping and examples of established soil series in nearby areas that have been recently surveyed. Unpublished soil surveys of scattered farms are another source of information about the area. The value of this information depends on the quality of the legend and consistency of mapping over long periods. Regardless of the quality of the legend, the scattered farm mapping should not be made a part of the modern soil survey without careful field checking. A soil survey is a study of the geography of soil. Maps detail geographic information. Aerial photographs, topographic maps, and other maps are useful references whether or not they are used as the mapping base. Each kind of map shows features that the others do not. Topographic maps are the best references for appraising relief for most areas. Maps and texts on geology for many areas have been published by the U.S. Geological Survey and by comparable State agencies. The publications are on various subjects, such as bedrock geology, surficial deposits, and water or mineral resources. The maps were made at various scales and degrees of detail. Almost all contain important information about the parent material of soils and related factors. Although not as extensive as for geology, maps showing vegetation have been published for many areas. The U.S. Forest Service and State agencies are likely sources. In addition, climatic maps that are commonly at small scale and general in nature are available. The cartographic staff of the Soil Conservation Service, local libraries, and university libraries are good sources of information about what has been published and where it can be obtained. Local sources—libraries of local schools, universities, municipals, historical societies, State agencies—are sources of published material on soils, agriculture, geology, geomorphology, hydrology, climate, engineering, biology, history, and related subjects. If a university is located within reasonable distance of a survey area, graduate theses may provide significant material. Local weather stations can provide data on temperature, precipitation, and other weather events. Reports of the Bureau of the Census and of USDA's Economic Research Service and National Agricultural Statistics Service are authoritative references on land use and crop production. A computerized bibliographic search service can also provide references for publications about the survey area. Faculty members of universities often have information that is not available in published form or know of published information that the party leader has not found. The local representatives of the Cooperative Extension System, area and district conservationists of SCS, and vocational agriculture teachers may also be sources of knowledge that is not generally available. Representatives of planning boards, sanitation departments, highway departments, and the like are knowledgeable about matters that are important for interpreting soils and designing map units. Strong working relationships with the office of the State geologist and with geologists working in the survey area are very important. They can provide much information that is helpful in understanding soil-rock relationships. Some information not directly related to soils is also helpful in planning, organizing, and conducting a soil survey. Questions that should be answered include:
Not all of these questions are universally important, nor is the list complete. The answer to these questions, however, can be important in satisfying the needs for the soil survey. Promising sources of reference material have been mentioned. The amount and significance of existing information varies widely, but in most parts of the United States it is substantial. Preliminary research can provide much, if not most, of the information about the soils of the area and their geography that is needed to start field studies and prepare a preliminary mapping legend. Preliminary research provides the basic data for interpreting the soils. Preparing the Mapping LegendPreparing the mapping legend is the principal duty of the party leader after preliminary field studies have been completed. The purposes of the survey having been stated in the memorandum of understanding, the party leader consults with other specialists and determines what soil areas are significant. Soils and map units that can be consistently identified and mapped are then described, and names and symbols are proposed for them. The mapping legend is composed of two parts: (1) the descriptive legend, which contains descriptions and classification of the soils, the identification legend, the legend of conventional and special symbols, and the general soil map and (2) mapping aids such as a genetic key, table of soil characteristics, and notes about individual soils or map units. The mapping legend contains the primary references and the principal guides for each survey party member. It is designed to serve the purposes of the soil survey and is unique to each area. Preliminary studies are made in a survey area to identify sets of soil properties that are repeated in characteristic landscapes and are mappable. Not all of the soil map units needed for the complete survey can be anticipated at the start. An initial mapping legend is prepared after preliminary investigations and test mapping. The initial mapping legend should include only the descriptive legend and mapping aids for those soils, map units, and other features that have been definitely identified as needed. The number of map units in the initial legend depends on the scope of the initial studies, complexity of the area, and intensity of the survey. Map units must be defined and described carefully. These descriptions are the guidelines for mapping soils and the standards against which possible additional map units are evaluated as the survey progresses. The mapping legend should be made available to each member of the party before mapping begins. It is revised as needed during the soil survey. As the survey progresses, other material is added to the mapping legend. This makes a soil handbook for the survey area. The soil handbook contains all of the information and other related facts about the genesis, morphology, classification, and interpretation of the soils of the survey area. By the time mapping is completed, the soil handbook should contain all of the material needed for the published soil survey. Field OperationsSoil mapping is a technical art. It requires sound training in soil science and familiarity with the principles of the earth sciences. A skilled soil scientist is a perceptive observer and understands the significance of landscape. Subtle differences in slope gradient or configuration, in landform, and in vegetation can be important indicators of soil boundaries. The soil scientist must learn to associate sets of landscape features with sets of internal soil properties to be able to visualize the pattern of the soils. A skilled mapper is able to abstract the essentials of the soil pattern and sketch this pattern on a map. Above all, a good soil scientist strives for accuracy and is truthful about the reliability of the maps. The demanding standards for soil mapping must be maintained throughout such a survey regardless of vegetative cover. Even though the map scale is adequate and the legend is well designed, the legibility and usefulness of the maps depends on the skill and judgement used in applying the legend. Some soil boundaries are more important than others and require greater accuracy. Time and effort must be spent to delineate small areas of soil that contrast with neighboring soils. In mapping consociations, for example, boundaries between highly contrasting soils, such as a wet soil and a dry soil or a clayey soil and a sandy soil, must be located as correctly as possible. The greatest time and effort is spent delineating dissimilar soils that are more limiting for use than nearby soils. Small areas of some soils are deliberately mapped with their more extensive neighbors if the two kinds perform similarly for the purpose of the survey. Useless detail is avoided. Special symbols are used to indicate significant areas too small to be delineated. The skill and judgment of the mapper are part of the art of separating the landscape into meaningful units of soil and then recording the units on a map. Using Aerial Photographs.—Aerial photographs provide important clues about kinds of soil from the shape and color of the surface and the vegetation. The relationships between patterns of soil and patterns of images on photographs can be learned for an area. These relationships can be used to predict the location of soil boundaries and kinds of soil within them. Light and dark tone on panchromatic photographs and color differences on color photographs, for example, are records of light reflected when the photographs were taken. These records must be interpreted by relating the visual pattern on the photographs to soil characteristics found by inspection on the ground. Using the aerial photographs of an area, a soil scientist learns many relationships between the photographic images and soil and landscape features, but many uncertainties inevitably remain. Awareness of the factors that affect an image is required to interpret the aerial photographs as fully as possible. The techniques used to predict specific kinds of surface features, landforms, attributes of soils, and soil boundaries from photographs are continually being refined. Published material provides information about the techniques and the kinds of clues used by photo interpreters. Some publications provide helpful illustrations of specific features. Nevertheless, reliable predictions of many features in a particular area require experience in relating the images on the photographs to what is actually on the ground. Such features, as roads, railroads, buildings, lakes, rivers, field boundaries, and many kinds of vegetation can be recognized on aerial photographs. Relief can be perceived by stereoscopic study. Shadows and differences in tone between slopes that faced the sun and those that did not at the time of photography also help show relief. Relief features help locate many soil boundaries on the map. Relief also identifies many kinds of landforms which are commonly related to kinds of soil. Many landforms—terraces, flood plains, sand dunes, kames, eskers—can be identified and delineated reliably from their shapes, relative heights, and slopes. Their relationship to streams and other landforms provide additional clues. The soil scientist must understand geomorphology to take full advantage of photographic imagery. Some landforms are less easily identified, but most images contain clues that narrow the choices of the kinds of landforms represented. Experience in interpreting tone patterns, configuration of relief, and patterns of drainageways commonly permits correlation of these patterns with kinds of geologic deposits and geomorphic features in an area. As the survey progresses, experience generally increases the reliability of predictions. Differences in tone or color may reflect soil differences. Differences caused by man-imposed land use usually can be recognized by the angular shapes and abrupt boundaries of the areas. Other tonal differences may reflect differences in vegetation that relate to soil or differences in the surface of bare soil. Certain patterns of tone or color may reflect local soil patterns within areas that can be mapped in one day. Different soil associations have distinctive patterns that can be recognized on photographs. These patterns serve as bases for drawing tentative soil boundaries and for predicting kinds of soils. These predictions of soils and boundaries must be verified in the field. Accurate soil maps cannot be produced solely by interpretation of aerial photographs. Time and place influence the clues on the photographs. Shades of gray commonly reflect the state of the soil moisture when the photograph was taken; but the soil moisture changes with time. Clues to soil boundaries that are evident on photographs taken at one time are not necessarily evident at another time. The activities of man have changed patterns of vegetation and confounded their relationships to soil patterns. The clues must be correlated with soil attributes for each set of photographs, and predictions of soil properties from such clues must be verified in the field. The accuracy of maps improves as fieldwork and experience increase. Stereoscopic examination.—Before an area is surveyed, making a careful stereoscopic study is helpful (fig. 4-1 (no longer available)). The area is scanned with a stereoscope for a general impression of farming, relief, geology, landforms, kinds of soils to be expected, soil moisture states, and so forth. Important features that can be accurately identified are sketched lightly on the photograph. Some features can be determined with more certainty than others. Images that help identify obscure features can be marked. The following steps are commonly used in preliminary studies.
Routes of traverse can be placed during these preliminary studies. Obstacles can be identified and plans made to avoid them. Enough field checking is planned to ensure maximum accuracy with a minimum of walking per unit of area mapped. As experience is gained in an area, many soil boundaries and kinds of soil can be tentatively predicted on the photographs. These predictions must be verified in the field, but preliminary interpretation can increase the quality of mapping. During such preliminary studies, a map should not be cluttered with conjectures. Only features that can be predicted with confidence are marked. After fieldwork, mapped sheets are examined again while the landscapes are fresh in the mind and can be related to the stereoscopic images. If considerable time elapses, details may be forgotten. Questions that the examination may raise become more difficult to resolve, and a special trip to the field may be needed. Because dense vegetation or other conditions may obscure the image on a photograph, some drainageways, slope breaks, and soil boundaries that are observed in the field may be impossible to place accurately on a photograph. These features can be sketched tentatively in the field, and their locations later checked by stereoscopic study for necessary revision. Thorough stereoscopic study of areas that have been mapped commonly reveals places where soil boundaries or stream symbols need to be refined to conform to relief. The traces of roads in heavily forested areas may be obscure on single photographs but evident under the stereoscope. If some boundaries inadvertently were not closed during field mapping, they can often be closed with confidence on the basis of stereoscopic study. In the field, roads, houses, streams, field boundaries, individual trees or bushes, and the like are used to identify locations on the ground with points on the base map. The photograph can be oriented so that the relative position of its images corresponds to the relative position of ground features from the vantage point of the surveyor. The photographic images of surface features that mark soil boundaries can be followed in the sketches of the boundaries. Boundaries that are not evident on the photograph can be sketched in relation to identifiable ground features. In some areas a stereoscope used in the field with stereoscopic pairs of photographs is helpful. A pocket stereoscope can be used on the hood of a vehicle or on a dropleaf shelf (fig. 4-2 (no longer available)). It can be carried while walking. The stereoscope and pairs of photographs can be used to relate the landscape features to the stereoscopic images. Kinds of soils and the location of boundaries can be predicted from the stereoscopic image. Boring or digging is needed to identify soils positively and to verify predictions, but stereoscopic study commonly reduces the number of borings that are needed to locate the boundaries of an area. Plotting soil boundaries.—A soil scientist plans the day's work as a series of trips across the area to be mapped. Proceeding along these routes, the soil scientist predicts soil areas, the kinds of soil in the areas, and the boundaries that separate different kinds of soil. These predictions are checked as the areas are crossed. Finally, boundaries and kinds of soils are plotted on the map. Thus, fieldwork consists of a sequence of predictions and verifications. To the extent feasible, mapping is scheduled to proceed systematically across contiguous areas. When mapping is resumed each day, the mapping of the previous day provides points of reference. The boundaries that were projected tentatively the day before are predictions to be verified. The soil patterns and the clues for interpreting the landscape are already understood. Mapping systematically across contiguous areas contributes greatly to both efficiency and quality of the work. Ground traverses are planned to cross as many soil areas as possible. Soil areas generally conform to the orientation of relief, which is commonly related to drainage courses. Consequently, most soil areas and most soil boundaries can be crossed by traveling at an angle to the secondary or tertiary drainage courses. The traverses are spaced so that the boundaries that are identified and projected on one traverse can be identified and continued on the next. Traverse spacing depends on the complexity of the soil pattern, visibility, and amount of detail required by the survey objectives. In fairly detailed surveys, for example, traverses are planned to pass within 200 to 400 meters of every point in the area, thereby permitting detection of small areas of contrasting soils. Where aerial photographs are used as the mapping base, a predetermined line of traverse need not be followed consistently if there are sufficient reference points for accurate location. A traverse can deviate from a planned route to cross landscape features that may be marks of soil boundaries. Wandering from place to place at random, however, should be avoided. Aerial photographs assist in avoiding obstacles on the route. If boundaries are observed to run in a different direction than had been anticipated, the plan can be adjusted. From any point of observation, the soil scientist looks along the projected route and predicts the kinds of soils on the landscape ahead. A break in slope gradient, a change from convex to concave slope configuration, a change in the color of the surface of a plowed field, the margin of a swamp or forest, the edge of a stony area, a change in kind or vigor of crops—these observable features can be related to soil boundaries. If possible, these features are identified on the aerial photograph. Some may already have been marked during the stereoscopic examination. If soil boundaries follow identifiable features, they are lightly traced on the photograph in pencil. Boundaries that are not evident on the photograph are sketched on the map in relation to identifiable features. Most features must be located and sketched by estimating location in relation to the point of observation and other known points. Tentative soil boundaries are sketched for perhaps 100 to 200 meters ahead and on either side of the point of observation. Natural and cultural features that are immediately ahead, such as a stream or drainageway, are also sketched on the aerial photograph. Some soil boundaries are sharply defined (fig. 4-3). Others are plotted as lines midway in zones of gradual transition from one soil to another (fig. 4-4). A judgment is made about whether a broad transition zone is a discrete mappable soil unit or should be split and its parts included with the soils on either side. Every part of the mapped area must be enclosed in a boundary and assigned a symbol. After predictions are made about the soil areas and boundaries are sketched on the map, the soil scientist walks across the predicted boundaries. The course is adjusted as necessary to investigate the transitional zone and any unusual features. Slope gradient is estimated or measured with an Abney level or a clinometer. As a predicted soil boundary is approached, especially in a broad transitional zone, the soil is examined to locate the significant changes in soil properties. As a projected delineation is crossed, the distribution of microdepressions, microknolls, tiny areas of different vegetation, convexities and concavities, and other features too small to delineate are observed. The soil is examined at a place where the microfeatures suggest that the predicted dominant soil should be best expressed; and this portion of the delineation is identified positively. The prediction may be confirmed, or a different kind of soil may be found. Where microfeatures suggest important inclusions, additional observations are made to ensure that the evaluation of the whole delineation is good. Sites for examination are not chosen at random if reasons exist for dividing the projected delineation into parts that are the predicted soils and parts that are not.
The number of places at which observations are made depends on the certainty of the predictions and the objectives of the survey. If predictions about the kind of landscape under examination have been valid many times before, soils need be examined in only a few places. If the landscape features have not been consistently related to kinds of soils, many places must be examined. The depth of the examinations depends on the depth of differentiating criteria for the map unit and on the confidence in the predictions about the kind and uniformity of soil material at a given depth. The examination itself is rapid and is mostly a search for a few properties that identify the soil. A small sample of a pedon is observed; seldom is an entire pedon studied. After a delineation has been identified and crossed, the soil scientist turns and looks back on the landscape from a new vantage point. A final judgement is made on the boundaries and symbols. If mapping is done on an aerial photograph, the photographic images are checked against the landscape features before the final boundaries are sketched. Soil boundaries are projected on either side of the traverse as far as they can be seen and identified with reasonable certainty. The ends of their projections are checked from the next traverse. Many boundaries can be seen throughout their lengths. Other boundaries can be predicted on the aerial photographs with a high degree of certainty. In forests, for example, visibility may be a few tens of meters or less; but, where a slope break that marks a soil boundary can be seen under the stereoscope, the boundary can be plotted much more accurately by a study of the photographs than by an observation on the ground. A soil boundary that is found at one point to correspond to a change in color on the photograph is commonly continued along the change on the photograph even though the boundary itself is not visible on the ground during mapping. In detailed soil mapping many boundaries between traverses are drawn on the basis of variations in the photographic image. In mapping, a pattern of soils and landscapes is conceived, rather than a group of individual map units. Certain soils are typically found together. The number of soils in any locality is usually small. In most places landscape features mark the kinds of soils. But landscape features do not identify soils everywhere, and by no means can all internal soil properties that are used to define map units be correlated with external features. Where soil boundaries cannot be predicted with confidence, they may be identified by direct examination of the soil. In some areas, important attributes of the subsoil or substratum are not related to surface features. Depth to bedrock, layers of contrasting texture, salt in the substratum, and similar attributes may have no visible relationship to the vegetation or other natural features but may be important when the soil is used. When desert is irrigated, when wet soils are drained, or when highways are built, soil differences that are not reflected in landscape variations may become important. Conditions of this kind occur in most survey areas. If common mapping techniques are used, the predictions frequently turn out to be inaccurate for some areas. Unless the mapper can reappraise the landscape and reliably predict the extent of the soil, the boundaries must be determined by actual examination. In large areas where landscape has low predictive value, geologic history and geomorphology may provide guides to stratigraphy, depth, and distribution of the kinds of rocks that are related to specific soils. The general hydrology of an area may indicate where salt-charged water has moved and where the salt has concentrated. Streams and their traces help in locating areas that have layers that differ in texture. As much preliminary information as possible is assembled to help determine the pattern and scale of soil variability. This information helps in planning the route and spacing of traverses and the spacing of samples within the traverses. In survey areas that are to be irrigated, samples of critical layers may be taken for special field-testing or examination to determine boundaries. These samples may be taken at points on a predetermined grid or at predetermined points along lines of a traverse. Where internal properties of soils are used for locating boundaries, a predetermined line of traverse is generally followed. Side trips are made wherever landscape features or experience with the soil pattern indicates that there is probably a significant soil change between traverses. Generally, the soil is examined at some standard interval along the traverse to locate important differences. If properties deep in the soil are important, the plan may require observations at fixed depth-intervals to a certain depth, such as 1 meter, and with layer depth intervals to greater depth. In most areas, some feature of the landscape or some aspect of the pattern of soils already mapped on an adjacent traverse provides a basis for predicting the location of soil boundaries. As evidence of change is observed, preliminary observations are made. Where the evidence indicates an important soil boundary, the soil is examined in more detail or to a greater depth to verify the prediction. Where power equipment can be moved freely across the countryside, it can be used to examine the soil to considerable depth at close spacing. Map units that are based on soil properties deep below the surface can be delineated with increased accuracy and the rate of progress can be greater if the geographic distribution of these properties is consistent with the scale of mapping. Neither standard intervals between traverses nor intervals for investigating the soil within traverses can be specified with certainty. The plan is adjusted to the direction and scale of the soil boundaries and the variability of the important properties. This kind of evidence is commonly obtained as the survey progresses, and the mapping plan can be altered to fit the accumulated evidence. A great deal of skill and judgement is required in areas of low predictability. Rarely are the soils at two sample sites exactly alike. Study of a single site is not enough to identify a significant area. Map units are defined to include the variability within areas large enough to be meaningful for the objectives of the survey. Using preconceived ideas of significant limits of definitive properties to define map units without regard to their geographic distribution generally results in unmappable units. Meaningless boundaries may result. Delineations should show the pattern and scale of orderly variation of soils. The kinds of variability over short distances should be noted in the descriptions of the map units. In all soil surveys distinctive landscapes are outlined on the map first. In surveys where most map units are fairly large and contain more than one kind of soil, landscape patterns are identified mainly by interpretation of aerial photographs, by aerial observation, and by study of topographic maps, geologic maps, land-use maps, and other available information. The size of the outlined areas depends on the objectives of the survey and the landscape pattern. Preliminary areas are of course no smaller than the smallest delineation that will appear on the soil map. They are often much larger. In 3rd-, 4th-, and 5th-order surveys, however, most map units are made up of more than one kind of soil or miscellaneous area and usually coincide with the landscapes outlined in preliminary work. Traverses of the preliminary delineations may be desirable, depending on the level of generalization required for the survey and the complexity of the soil patterns. The plan of traverses usually is based on interpretations of photographs, but this plan should be tested in the field. In areas of low accessibility, roads or trails may be traveled; but the mapper must understand that roads and trails commonly follow the easiest routes and avoid the steepest slopes, the wettest areas, and the other places that are difficult to cross. Such places are integral parts of soil associations and should be observed by the mapper on the ground. Transects are commonly used to determine the composition of map units. In transecting, a planned line of travel is followed as closely as possible and the soils are observed at predetermined regular intervals. In transecting, routes of travel are systematically planned to give a valid sample of the area. Taxa phases and other features are identified and recorded. Distances or number of points along the route identified by each taxon provides estimates of the composition of the map units. In surveys without easily predictable patterns, soils are sampled most efficiently if the transect lines are selected at random. Lines oriented to cross the drainage pattern often provide the most information about the pattern of soils. Sample blocks, instead of transects, are used in some surveys to determine the composition of map units. Blocks do not replace transects, however, they permit one to observe spatial patterns not always evident from transects. Sampling by transects is usually more efficient than block sampling for estimating map unit composition. Methods for sampling by blocks vary among soil surveys. One method imposes a grid of appropriate divisions on the entire area. Grid segments are numbered, and sample blocks are selected by drawing numbers at random. Each sample block is remapped in greater detail, and the area of each kind of soil is measured. These data provide estimates of the kinds and proportions of soils in each map unit. The number of blocks and their sizes are determined by statistical principles with consideration of mapping scale, the limits of confidence required for the survey, the general pattern of soils, and the relative size of soil areas. Mapping of organic soils follows the same general principles as mapping of mineral soils. Organic soils, however, have some special relationships to landscape and vegetation. These relationships affect mapping of organic soils at all levels. In preparing the mapping legend, systematic investigation of organic soils is required as for other kinds of soils. A thorough knowledge of the genesis of organic soils is required, as well as high-quality imagery and appropriate tools. The kind of organic soil in many areas is closely related to the kind and pattern of native vegetation. Since many areas of organic soils are comparatively undisturbed, reliable relationships between soils and plant communities can be established. Thus, high-quality imagery from aerial photography and other forms of remote sensing can be very useful in preparing legends and in mapping these soils. Where organic soils have formed directly on a mineral substratum, the environment may be rather uniform over extensive areas. Although the kind of organic material can vary with depth because of changes in climate over the period in which the soils have formed and because of differences in rate of decomposition that result from the accumulation of the organic material, such variations commonly are uniform over large areas. The properties of a large area of organic soils, therefore, can be accurately estimated from the properties of a small sample. Organic soils are not uniform in some areas that have microrelief of hummocks and swales. The hummocks commonly contain fibric material, and the swales contain hemic and sapric material. In such landscapes, many more sites must be examined to determine the nature of the soils. These relationships and processes generally apply where organic soils are formed by lake filling. Each basin in which organic soils have formed has a unique local environment, and the organic soils in adjacent basins may differ considerably. This is particularly true in irregular glacial moraines. For example, limnic materials may be covered by only a thin mantle of organic material in some basins and by several meters of organic material in others. Areal relationship must be kept in mind when estimating the extent of the different soil components within basins, particularly small basins. For example, one kind of organic soil occupying a rather narrow fringe of a bog may cover a greater area than the organic soil in the center. In northern glaciated areas in particular, organic soils may form around the edges of swamps that have open water in the center while adjacent swamps lack surface water. In some areas, a layer of water can underlie the organic soils at a relatively shallow depth. Such areas may not support much weight and should be investigated with caution. Organic soils of some coastal wetlands lack distinctive landscape features and, additionally, are poorly accessible. In these areas, the soil scientist relies on other features to predict kinds of soils. Patterns and kinds of soils in many coastal areas can be related to the position of such natural features as shores, deltas, streams, and adjoining higher lands. The soil scientist must have a thorough knowledge of the geomorphic history of the area in order to make reasonable predictions related to such features and to determine the places where transects and other field checks will best verify the predictions. Completing Field SheetsMost soil survey field sheets are individual photographs or compiled photobase maps. As each field sheet is completed it is joined with adjacent sheets and checked for errors. Joining field sheets.—Each pair of adjacent field sheets shares a common match line. During mapping, soil boundaries are commonly extended beyond the match line to be transferred to the adjacent sheet; but when the field sheet is completed, soil boundaries and other features may be discontinued at the match line. The mapping on each field sheet should be carefully matched with that on adjacent sheets to check boundaries and delineations. Roads and streams also should be continuous from one sheet to another. Special care is needed at the corners where four field sheets join. If soil boundaries are sketched on overlays, field sheets are matched before soil lines are transferred to the soils overlay. Matching should be completed while the photographic background is available. The mapping on one field sheet can be matched with that on an adjacent sheet in several ways. For aerial photographs, the mapped field sheet and an adjoining unmapped field sheet can be placed under the stereoscope and the images meshed. The soil boundaries and other features on both sides of the match line can then be transferred from the completed field sheet to the unmapped sheet. Another method, that is particularly useful if adjoining sheets vary in scale, is to transfer boundaries by reference to the photographic images. The relationship of the soil boundaries to images of isolated trees, clumps of bushes, field corners, and the like are observed along the match-line. Images of the same features are located along the match line of the adjoining photograph, and the boundaries are transferred or checked in relation to the images. When the second field sheet is mapped, boundaries of delineations that cross the match-line may be altered. Consequently, the boundaries at the match-line must be rechecked after both field sheets have been completed. If different individuals map adjacent field sheets independently and the completed sheets are joined, a match indicates the uniformity of fieldwork. If there is no systematic method of joining sheets, errors are easily made that may require additional fieldwork before the final map can be compiled. Figure 4-5 illustrates some errors on unmatched field sheets.
Inking field sheets.—After mapping has been completed on each field sheet, it may be inked to provide a permanent record and to provide a map from which copies can be made (fig. 4-6). All soil boundaries and symbols and important drainage features should be inked. Cultural features needed on the soil maps are determined before mapping starts and are specified in the legend. Inks or leads that are reproducible photographically and are readable by automatic scanning equipment are preferred. The ink or lead used should be compatible with the base material, and the lines should be opaque. Several kinds of inks and leads are suitable. Commonly, pens that store carbon-base ink in a reservoir are used. Several pens that make uniform lines of different thickness are needed for inking different features and for lettering. Line widths recommended for different features are indicated in the list of conventional symbols on fig. 4-7. Different groups of features generally are inked in separate operations. Drainage is inked first and inspected to see that individual streams are properly joined, matched, and classified. Then, culture is inked. The classification of roads and other features is checked at the same time.
Soil boundaries and symbols are inked next. Finally, the place names are lettered. In some surveys, however, certain features may not be inked. For example, if the photographic image of all roads is pronounced, they do not need to be inked. If photobase map sheets are used as field sheets, the inking can be done on transparent overlays. As many as three overlays can be used: one for culture and drainage, one for soil boundaries, and one for symbols. Together these form a composite overlay and can be used in printing the final map. The individual overlays can be used in printing special purpose maps. Adhesive-backed, clear stripping film with printed symbols can be applied to the overlay to save handwork. In inking soil boundaries, a good procedure is to close each boundary within one section of the field sheet. When the boundary of a small area is closed, its symbol is placed as near the center of the area as practical. More than one symbol is placed in areas that extend for long distances and in those that have intricate shapes. Mapping along the match lines may be left in pencil until the field sheets have been joined. Soil symbols on all sheets should be positioned to be read horizontally, or as nearly so as possible, when the map is oriented in one direction. Usually, north is toward the top of the map. If an area is too small to contain a symbol, the symbol may be placed outside it and a leader used to indicate the area to which the symbol applies. The leader should be so placed that it cannot be confused with a soil boundary. Place names should be inked last so that they may be placed where they will not obscure soil symbols and other details. Place names should be arranged so that they clearly identify their features. Names of features expressed as lines on the map are oriented parallel to the lines. Names of other features are usually oriented horizontally, with north at the top. Important features that serve as landmarks should be named on each sheet. Names of streams should be so positioned that no confusion arises about which branch is meant. Incorrect and correct placement of names are illustrated in figure 4-8.
Neatness and legibility are important in lettering. Maps with many soil symbols, boundaries, cultural features, and the like become confusing unless the lettering is done with special attention to high standards. Every soil scientist should learn the art of freehand lettering. A simple style of lettering should be used. Freehand styles that use single strokes are best for inking field sheets. The pen is held as in writing and the strokes are made with an even steady motion. Slant or vertical lines are made with a downward stroke: horizontal lines are made with a stroke from left to right. The slant of the letters is kept uniform.
Checking field sheets.—Each field sheet should be checked for open boundaries, areas without symbols, and other errors. Fieldworkers usually check their own sheets, and another person may check each sheet for completeness and legibility. The party leader should be responsible for checking the mapping of each party member. The mapping of beginners generally needs much checking. If different soil scientists map adjacent field sheets, the party leader can compare their mapping in the areas where the sheets join. During field reviews, supervisory soil scientists responsible for technical standards often check boundaries and symbols on samples of the field sheets of each soil scientist. After mapping has been completed, the party leader should check all of the field sheets to see if any corrections and remapping are required. Omissions and inconsistencies increase the cost of map construction and delay publication. In order of frequency, the most common mistakes are:
Failure to close soil boundaries is a common error. Figure 4-9 has three open boundaries—between map units 11 and 31, between units 62 and 25, and between units 38 and 62. The person who inked this sheet probably overlooked the lack of closure because of the drainage lines and railroad symbols that cross the area. Whatever the cause, the user cannot tell where one unit ends and the other begins. Someone may have to make a special trip to the field to close these boundaries. Each area of a mapping unit must be separated from all adjoining areas by a soil boundary or the boundary of a body of water. Neither the single lines representing streams nor the conventional symbols for roads, railroads, and the like can serve as map-unit boundaries. Each area also must contain the symbol for only one kind of map unit. Errors in symbols take various forms. A delineation on a field sheet might be closed without a symbol in it, or a symbol not listed in the legend might be used. Symbols might be illegibly drawn on the field sheet. Practice and care in lettering, good judgement in placing symbols, and care in erasing and reinking mistakes ensure legibility.
Various methods are available for checking field sheets. A good method is to color each delineation by hand on photographic copies of the filed sheets. A color check reviews each delineation and inspects boundaries throughout their length. In another method, each field sheet is divided into sections of perhaps 50 to 100 square centimeters. The delineations within each section are checked one at a time, special care being taken at the edges of the section. If the map checkers are not familiar with the legend, they must be especially diligent in checking the symbols against the legend. Incorrect placement of drainage or cultural features on the map can seriously reduce the accuracy of map unit boundaries. The location of streams, roads, and the like must be correct. Most errors in placement cannot be checked with precision except by stereoscopic study or field investigation. If accurate reference maps are available, locations of features on the field sheets are checked against them. If possible, locations are checked while mapping is in progress. Place names are verified with an authoritative source. Keeping records of field sheets.—Each field sheet may be identified by a number that locates it on an index map of the area. The index map outlines and identifies all of the field sheets of the survey. Where single-lens aerial photographs are used, several hundred individual sheets are required and a systematic means of identifying their locations in the survey area is needed. Photographic indices are available for most areas where photographs are used. An index map can be made from a small-scale map by plotting the match-lines of the field sheets. Each field sheet should contain the name of the survey area, the State, the date of the survey, and the names of the soil scientists who mapped that sheet. Names of others who inked or checked the sheet and its scale may be given also. Commonly, all this information can be put only on the back of the sheet. If it can be put on the front, it may be placed so that it will appear on photographic copies. A stamp can be used to provide spaces for the information. The adjacent field sheets are identified on the margins of each sheet. When adjacent field sheets have been joined, the margins of the field sheets may be marked and initialed by the persons responsible. The joined match lines can also be marked on a transparent overlay over the map index. Measuring the areas of map units.—Soil maps show both the location and the extent of map units. Measurements of the area of each map unit are needed. Planners, for example, need to know the extent of areas that have certain potentials or problems. Processors of farm products frequently need to locate areas that are suited to growing a certain crop. The data on an area are used to help decide whether certain map units of small extent are important enough to be retained on the published map. Measuring the areas also checks the map for open boundaries, delineations without symbols, and unidentified symbols. Measurements on field sheets are subject to errors caused by distortion of the photographs. The area of map units can be measured for the entire survey area; or sample areas can be selected and the extent of map units in them measured and expanded to represent the entire area. The accuracy of the estimate, based on sample areas, depends on the size of the sample and where the sample areas are located. If the sample is less than 5 percent of the total area, estimates are subject to relatively large errors. Generally, the sample should be at least 10 percent of the total area. Even with this large a sample, map units of small extent are likely to be either missed entirely or overestimated. The estimates of extensive map units by sampling methods are likely to be reasonably reliable. Estimates based on sample areas can be satisfactory for most uses of data for the major soils in the survey area. If a sampling procedure is used, dividing the area surveyed into soil associations and sampling each association separately is helpful. The most accurate estimates can be derived from sample strips running all the way across each association and oriented at right angles to the prominent unit boundaries. The strips can be spaced to provide the needed sample size. Square areas and rectangular areas oriented at random yield less accurate estimates, especially if the sample is small. Several methods can be used for measuring the area of map units. The dot-grid method uses a transparent sheet or card on which dots are evenly spaced vertically and horizontally. Each dot on the grid represents a small square, which has a unit area. The transparent sheet is placed over the map and the dots in each delineation are counted. Dots that fall on the boundary of a delineation are alternately counted. The dots in each delineation are summed for the map unit. The land area represented by each dot can be calculated on the basis of the map scale and the spacing of the dots. Less extensively used are grids that have a network of small squares instead of dots. The squares that fall within a delineation are easily counted. Squares that fall on a boundary of a delineation are averaged during counting. The averaging depends on the judgment of the person who is counting. If it is carefully done, the use of squares can result in somewhat greater accuracy than the use of dots. The former method is more time consuming. The dot-grid method is simple, inexpensive, efficient, and convenient. For these reasons, it is the method most commonly used in field offices. A short-coming of the dot-grid method is that it is not well adapted where there are long, narrow delineations on the map. It is sufficiently accurate for most purposes, however, because the land areas for the map units tend to be averaged if large areas, that is, entire field sheets, are measured. The electronic area calculator is an alternative method to the dot-grid method in that it electronically counts squares on a grid. By using an appropriate grid to fit the desired map scale and a wired pencil assembly to trace the map unit boundary, area can be determined easily from a numerical display. This method compares closely in accuracy to the dot-grid method but is much quicker and more convenient. It is initially much more expensive than the dot-grid method. The planimeter is an instrument used to measure area by measuring the length of the boundary of the delineation. This is done by following the outline of the delineation with a tracer. The value indicated on the planimeter is converted to land area by using an appropriate conversion factor related to map scale. The use of the planimeter is an accurate method to measure maps, but it is very slow and tedious. Accuracy depends on the skill and patience of the operator and on the care taken to convert measured value to land areas. Computer-based digitizing systems have the capability for measuring the area of map units. These systems are not only very accurate but serve as an excellent final check for errors on field sheets—open boundaries, areas without symbols, and the like. They can replace color checks and other methods of checking field sheets. Cultural FeaturesThe various mapping agencies of the United States Government have agreed on standard symbols for most cultural and natural ground features that are commonly identified on maps. Most of the symbols used on soil maps follow these standards. Some soil maps show special features that are not included in the standard list. The symbols for these must be compatible with symbols used by other mapping agencies. Different symbols are not used for the same feature, nor is the same symbol used for different features. Conventional and special map symbols must be functional and readily identifiable on the map. Conventional signs and symbols used in soil mapping are shown on figure 4-7. Some of these are described in the paragraphs that follow. Boundaries of cultural features are shown on soil maps by standard conventional symbols. These include the boundaries of nations, states, counties or parishes, minor civil divisions, reservations (including Federal or State parks and forests), land grants, parks, and cemeteries. U.S. Geological Survey (USGS) maps are the primary source of cultural boundaries. Where USGS maps are not available or must be supplemented, local sources are used. County or State assessors, planning and zoning officials, and reservation superintendents are authoritative sources. Boundary monuments are located in the field and boundaries are plotted during soil mapping only where boundary location cannot be plotted accurately from references. Boundaries are verified as a precaution against errors. Where cultural boundaries of different classes coincide, the symbol of the major subdivision is used, for example where a State boundary coincides with a county boundary the State boundary has priority. Where a boundary obviously follows a stream or road for a short distance, the boundary symbol may be omitted. In some places, the road or stream may be labeled for clarity: "Road is county boundary" or "State boundary is center line of stream." Township and range numbers are shown along the margins of field sheets for all lands that have been sectionized. Section lines are not shown. In some surveys all sections corners are shown; in others, only those that have been located are shown. In a published soil survey, section numbers are printed in the approximate center of each section. Published topographic quadrangle maps show the land grid, though some old ones may need correction. Soil scientists working in an area should be familiar with the local land survey system and its intricacies. Cemeteries are outlined to scale on field sheets using dashed lines. The name is usually placed within the outline of a large cemetery and outside a smaller one, although the smallest cemeteries are usually indicated by a cross and not named. A feature such as a road or stream may serve as a boundary for a cemetery. The identification of airports and landing fields is optional on field sheets. Boundaries of large municipal, commercial, and military airports and landing fields are shown by the symbol for a reservation. The runway pattern is not delineated if it is apparent on the aerial photograph. Small airfields can be shown by a dashed line symbol similar to that used for a cemetery, or the symbol for a "located object" can be used and labeled. Each airfield that is identified is labeled by its proper name or "airfield," if the name is not known. Roads are identified on soil survey field sheets by symbol or name. In towns and cities only major roads are identified. Standard emblems are used to designate interstate, Federal, State, and other roads. Route numbers are placed in the emblems. If roads are shown, a simple and explicit classification is used. The mapping of trails depends on their importance for proper map orientation and the help they will provide in locating specific areas on the map. In sparsely settled areas having few readily observable landmarks, important trails are shown and named. In more densely populated areas where roads are common, trails generally are not shown. Railroads are shown on field sheets by conventional symbols. They may be labeled "railroad" or by the name of the line. Electric trolley lines both in urban areas and beyond city limits are shown by the standard railroad symbol and designated by operating name and type. In large railroad yards with parallel spur tracks and switches and sidings alongside single tracks, only the main track is shown. Pipelines are shown on soil maps if they might be important as landmarks. A pipeline crossing a remote section of a survey area may be important. A similar pipeline in a populated area may be difficult to locate accurately and may have little value as a landmark. If shown, a pipeline must be accurately located. Trunk power-transmission power lines are normally not shown on field sheets unless they have value as landmarks. They must be individually evaluated. Lateral distribution systems are not shown. The symbol for power-transmission lines, if used, begins and terminates at towns, power stations, and survey area boundaries. Levees are indicated by short ticks. If a road or railroad is on the levee, the ticks extend from both sides of the road or railroad symbol. Large permanent dams are shown to scale on field sheets. Thin lines are used to delineate the base of the dam. Smaller dams are indicated by single, heavy lines. A road following the top of a dam is shown in its correct place, and the road line on the upstream side is thickened to represent the dam. A dam symbol is inked to its scaled length. Important dams are named. Permanent buildings.—rural dwellings, public buildings, and farm homes—are shown on most published soil maps but are optional. In some areas, buildings are constructed so rapidly that the map is out of date before it can be published. In such areas, omitting symbols for all buildings other than churches and schools is best. In most soil surveys, churches and schools are shown on the published map and may be named. Symbols for individual houses are commonly not shown in urban areas. Prominent landmark buildings—large schools and large churches—may be shown, but they are not drawn to scale and are identified by the conventional symbols. The cross or pennant of a church or school symbol is oriented at right angles to a nearby roadway. A building used as both a school and a church is marked by the school symbol. If churches or schools are omitted from large urban area but mapped in rural areas, the notation "omitted in urban areas" is made on the legend of conventional symbols. Open pits, mines, and quarries smaller than the minimum area for delineation are shown only by conventional symbols. Larger areas are delineated, classified, and correlated as kinds of soil or miscellaneous areas. Producing oil and gas wells may be shown. Where the number of wells is so large that the symbols are closely spaced on the map, the approximate outline of the field may be shown by dashed lines and the delineated area identified as "oil field" or "gas field" without the conventional symbol. Streams and rivers are shown on the field sheets, and perennial and intermittent streams are clearly differentiated. The pattern of drainage and the classification of the drainage must be complete. If the main drainage courses are identified by stereoscopic study of aerial photographs, the lines must be confirmed and the drainage classified in the field. Most distinct drainage courses more than 1 cm long on the field sheets are shown. Drainage courses are mapped to scale if wide enough to be shown legibly or by single lines if narrow. A perennial stream is one in which water flows constantly except during periods of unusual drought. That a stream is perennial must be verified, especially in semiarid and arid regions where the water in streams and waterholes is vitally important. Mapping large rivers that change course and width from time to time is difficult. The shorelines shown on a soil map generally mark the areas covered with water for so long that little or no vegetation grows during low water and unvegetated riverwash persists from year to year. Areas that are covered by flood water for only short periods are excluded. Areas that are uncovered only during very low water stages are included. The level of river stages varies widely, depending on characteristics of the river in relation to the climate of its watershed and other factors. Where the flow of rivers, though active for brief periods, dwindles or ceases altogether for many months, the normal stage is very low. Thus rivers, such as the Platte and much of the Rio Grande, are normally braided, and the boundaries of the river are usually placed at the outer limits of the area of braided channels. Unstabilized sediment that is washed and rewashed and supports little or no vegetation but persists from year to year may be identified as riverwash. Areas within a flood plain that can support vegetation are shown as soil. Some streams, especially in areas underlain by limestone, enter abruptly into caverns and may flow for long distances through subterranean channels. The points where the streams enter and emerge are located accurately, but only the surface drainage is shown. An intermittent stream is dry each year for extended periods, usually for more than three months. In arid and semiarid regions especially, intermittent streams are distinguished from perennial streams because they are not reliable sources of water. Poorly defined water courses are not shown. Aggraded flats or valley floors without well-defined stream channels or scars are shown as soil. Canals and ditches, whether for navigation, irrigation, or drainage, are plotted to scale if they are wide enough. Otherwise they are shown by the single-line symbol. Arrows indicate the direction of flow. Generally, both the main ditches and important laterals of irrigation systems are shown. Large canals and ditches are named on the field sheets if they have names. On the map, canals and ditches must be distinguishable from roads. Lakes, ponds, and reservations are delineated to scale on field sheets. The boundary marks the normal water level, which may not be the shoreline observed and recorded at the time of the survey. Normal water level may be marked by a line of permanent land vegetation, but many lakes are bounded by wave-washed beaches above the normal water level. Many reservoirs are bounded by areas that are submerged when the water level is high. The shore line that is evident on aerial photographs may be used to delineate the normal stage of a lake, pond, or reservoir. If a high water level other than wave-washed beaches can be identified, it is shown on the map by the intermittent water symbol and is identified. The area between high water level and normal water level can be defined as a soil map unit if the area warrants it. The intermittent water symbol is not used in these areas. The intermittent water cover is described in the map unit description. Reservoirs surrounded by an impounding structure are outlined. Some reservoirs have flood-pool lines that are determined from available sources. They are shown on the map by a dashed line and given an appropriate label, such as "approximate flood-pool line." The shoreline of an island is determined at the same water stage as the adjoining mainland shoreline. Islands exposed at a lower stage are not shown. Tidal shorelines present special problems. The mean high tide level (determined excluding the semimonthly highest tides) can be used where the land rises to elevations well above high tide within a short distance from the shore. Where broad marshes mark the transition from sea to land, the shoreline is the outer boundary of the area that supports plants. The soil boundaries extend to that line. The shoreline of a body of water is not broken for wharves, piers, and similar structures that may be built over the water. Seawalls and retaining walls that are part of a shoreline are shown as the shoreline. Intermittent lakes are shown on the field sheets as kinds of soil or miscellaneous areas. The dashed line symbol shows the area covered by water part of the year. Marshes and swamps are mapped as soil unless they are too small to be delineated. If too small, they are shown by the conventional marsh or swamp symbol. Springs are shown on the soil map if they are important in the area. Springs of all kinds are shown in arid and semiarid regions. In humid regions, only large and dependable springs are shown. Some springs have names, which may be printed on the soil map. In arid regions, intermittent springs or springs that have salty or otherwise impotable water are so identified by notes on the map. Walled-in springs are shown by circles, like those for wells. A spring that is a source of a stream is shown by a circle where the stream symbol starts. Artesian wells and wells for irrigation are shown on soil maps where they are important sources of water, as in arid and semiarid regions. Artesian wells are designated by a conventional symbol, whether or not they flow at the surface. In regions of few wells, all are shown; but in thickly settled areas that have many nonflowing artesian wells, they can be explained in the report without being shown on the map. A wet spot is an area of wet soil that is too small to delineate. It is usually somewhat poorly drained or wetter and at least one drainage class wetter than the soil around it. Wet spot symbols are not placed within areas that are mapped as a wet soil. Special symbols are used to identify small areas of various kinds of soil, miscellaneous areas, and special soil features. These are commonly used for areas that are too small to delineate but large enough to significantly influence use and management. If a specific kind of area is shown by special symbols, all such areas of that land are shown; the symbols are not to be used haphazardly. The symbols must be defined in terms of the kinds and size of areas each symbol represents. In some places, the pattern of mappable areas is so complex that symbols and leaders clutter the map. Special symbols used with moderation reduce the congestion of lines and symbols, although many special symbols in a small area reduce legibility. It may be preferable to map as complexes many areas of intricately associated kinds of soil. Special symbols show relief features that are too small to show as map units; for example, bedrock escarpments, short steep slopes, and gullies. Natural depressions or sinks such as those common to limestone areas, may be shown by the depression or sink symbol. Small areas of rock outcrop in an area of otherwise deep soil are obstacles to tillage and should be shown. In addition, small areas of saline soil and very stony soil, in areas otherwise suitable for crops should be shown. Special symbols are used for small areas of some kinds of soil that contrast sharply with surrounding soils in their management needs or productivity, even though they are suited to the same uses. Small areas of gravelly soil in gravel-free areas, sand spots in areas of finer textured soil, and small areas of severely or moderately eroded soil in areas of noneroded soil are examples.
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