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Methods of Digital Parcel Mapping Cont'd (pg 2)
3.2.6 Summary of Control Surveys
Establishing an accurate geodetic control for parcel mapping is imperative for viable parcel mapping projects. Even if the GIS-based parcel map is established initially for general planning, it may expand eventually into applications that require more accurate data. If constructed properly, the geodetic control will support all parcel mapping activities in the long run, thus enabling the local government to mandate tie-ins to the network of all new and old surveying and mapping activities. The result of such a requirement is an incremental construction of an accurate parcel-based GIS with minimal expense.
The following sections describe methods for generating base maps and compiling parcel information. In all of these methods the basic geodetic reference system is assumed to be present.
3.3 Photogrammetry: Introduction
Photogrammetry is the art and science of deducing the physical dimensions of objects from measurements on photographs. Its ability to capture geographically referenced information over large areas makes it an ideal tool for developing a GIS base map. Historically, the main uses for photogrammetric information gathering has been to support civil engineering projects. Today, with advancements in computer technology, information can be gathered, assembled, sorted, and reported in expanded ways. These developments have thrust photogrammetry from a limited support service role to the forefront of digital interactive information gathering. The photogrammetric plotter, now computerized and coupled to data records, permits rapid acquisition of geographic information in digital form. The uses of this information may be wide and varied--from managing cities and municipalities, to managing utilities and industrial complexes, to gathering information for civil engineering design.
The gathering of land information is historically the fundamental role of photogrammetry and its civil engineering upbringing has demanded that positional (or geodetic) accuracy be its hallmark. Much of the nation's infrastructure, its interstate highways, dams, water and sanitary systems, its airports, housing, and so on, have been designed using a photogrammetrically compiled land base. It is only natural that the same process that was used to design the nation's infrastructure be employed to manage it.
3.3.1 Photogrammetry: A Brief History
From the 1930s through the early 1970s, the photogrammetric stereo plotter remained relatively unchanged. It consisted of a series of two projectors upon which overlapping aerial photographs were placed. The overlapping images were projected onto a movable stage and either polarized lenses or oscillating machinery were used to permit the operator to view the stereo photography in three dimensions.
Optical stereo plotters were developed in Europe prior to World War II. However, widespread use in the United States did not occur, until the late 1960s. By employing fine optics and high order optical magnification, the precision of the stereo plotter was nearly doubled. This permitted the photogrammetrist to obtain much higher altitude photography while still maintaining required map accuracies. The mechanics of the stereo plotter were replaced with electronic encoders driving a series of stepping motors, which produced the map on a flat bed plotter. In the late 1970s, computer microprocessors were employed with the stereo plotter to cartographically enhance the map.
It was not until the early 1980s that interactive computers were used to store the mathematics that represented buildings, roads, and contour lines. Developments in minicomputer and photogrammetric collection software permitted encoders to transmit digital impulses which were stored by the computer as different ground features. This gave birth to Computer Aided Drafting (CAD).
For a five-year period in the mid-1980s, there were few who could use the actual digital information. Therefore, a computer was coupled to an output plotter and hardcopy mylar maps were still the final product of the photogrammetric process.
During the early to mid-1980s, a new photogrammetric stereo plotter, the analytic stereo plotter, became available. This new stereo plotter was so named because the orientation of photographs was generated by a computer based on the principles of spatial analytic geometry. The pure computerized mathematics of the analytic stereo plotter resulted in increased precision. The digitalization of the photogrammetric process was now an integral part of the stereo plotter's design.
In the 1990s, the advancement of the fourth generation of photogrammetry in the form of the softcopy process is emerging. The softcopy system consists of a powerful UNIX or Pentium/NT workstation that has the capability of simultaneously displaying the digitally scanned images of two adjacent aerial photographs. This creates a stereographic single image which the operator perceives by using the appropriate polarizing or alternatively shuttering eyeglasses. All of the geometric and photogrammetric algorithms are computed "on the fly" by the computer. Therefore the operator captures data that is orthogonally correct, devoid of the distortions caused by camera tips and tilts, relief displacement, or lens aberrations. As the operator traces the outline of the objects to be digitized, the resulting vectors are overlaid to the stereo image in three dimensions. This is one of the major advantages of the softcopy process. Overlaying three dimensional vectors on three dimensional images is a quality control tool that allows the photogrammetric technician to check the accuracy, both positionally and altimetrically, of the data being captured. It also allows the technician to identify immediately those objects that have been omitted or overlooked in the digitizing process. Most softcopy systems also can automatically generate Digital Elevation Models through image autocorrelation in areas of open terrain.
In the mid-1980s, the engineering community began to employ computers and engineering software for design and computer-aided drafting purposes. There were few standards at the time, however, while dozens of engineering software packages became available. The photogrammetrist was burdened with the formidable task of translating the CAD map into the seemingly endless software packages that the engineering community was employing and all too often much of the data was lost in translation.
By the late 1980s, Intergraph's IGDS and AutoCAD's DXF emerged as relative standards in the engineering industry. At the same time, the concept of using CAD as a management rather than as a design, tool began to filter through organizations such as the Urban and Regional Information Systems Association (URISA).
Early GIS’s were no more than computer-aided drafting systems used to manipulate CAD map features. Electronic data tables could be "hung" onto map entities, but the concepts of relational databases and topology were far from practical applications in a municipal management role. Many an early GIS found itself floundering in the never-never land of computer technology, fiscal economics, and practical application.
By 1988, the concepts of a topologically-structured database, polygon processing, buffer generation, network analysis, and digital terrain analysis came to life in functioning GIS software. GIS software packages now demanded much more than the CAD map had to offer. To function in the true GIS environment of the 1990s, photogrammetric collection methods had to evolve into a GIS land base. For example, network analysis for vehicle routing cannot occur without street centerlines. Buffer generation and polygon overlay are impossible unless the mapped land base features have been electronically closed on themselves. Slope/aspect angle calculations cannot be completed without the three-dimensional capabilities of a digital terrain model.
The practical realities and dilemma of any land base are:
- How much is enough?
- Who has responsibility for completeness?
- What is the intended use of the land base?
- What are the political and fiscal processes within the community?
a. How to account for the endless technological advances and changes in computer hardware, the Internet, and GIS software?
The information contained in this Chapter and in this publication should help to begin to formulate the answers to these questions.
3.3.2 Photogrammetry: Aerial Photography
After an accurate geodetic control is established, aerial photography is the critical next link in the land base chain of events. The acquisition of aerial photography must be planned carefully. The parcel map will be built from and referenced to features that are visible in the aerial photography. For a successful project, several factors must be taken into consideration for aerial photography planning, including overall project timing, size of the project area, and scale.
Timing -- The project must be scheduled to allow for a time lag between contract initiation and the land base completion. Flights should be planned for those times of the year when:
u trees are absent of foliage;
u the ground is free of snow;
u the sun is at the highest possible angle;
u it is not cloudy, raining, or snowing;
u streams and rivers are within their banks; and
u air traffic control permits the mapping aircraft to enter the project area.
Plan ahead, allowing at least a six-week window for the flights. It may be necessary to re-fly the project area if there is unexpected interference with picture clarity. Once the flights are completed, adequate time should be allotted for processing of the aerials before the land base is completed.
Project Size -- The size of the project area will affect both the time for completion as well as cost. Typically, multiple flight paths with photographs taken at controlled intervals are used to produce overlapping aerial photos for any given area. Therefore, mapping a large land base would be relatively more time consuming and significantly more expensive than a small land base.
Scale -- The goals and needs of each project directly influence the scale for which aerials should be flown. Scale is the ratio comparison between measurements on a map, or photo, and actual ground distance. Scale can be expressed as a ratio (1:10,000, where 1 inch on the map equals 10,000 inches on the ground) or as an equation (1 inch = 12,000 feet). During the flight itself, scale varies as a function of the plane's altitude, such that a lower level flight will produce a larger scale aerial photograph. The smaller the number represented by such a ratio, the smaller the scale of the map and the larger the area shown. Thus, a scale of 1" = 40,000' is smaller than a scale of 1" = 12,000'. In general, larger scales such as 1" = 12,000' might be warranted for engineering level parcel mapping, while smaller scales may be adequate for planning activities. The scale of the aerial photo affects the size of features that can be seen and recognized. For example, smaller features such as manhole covers might be discernible on a large scale (1" = 500') photo, but would not be visible at a smaller scale such as 1" = 24,000'. As a practical matter, flight altitudes relative to land base detail and map scale are well established, as indicated by the following table:
| Base Map Scale and | Aerial Photography | Flight Altitude Above |
| | | |
| Contour Interval (CI) | (Negative) Scale | Ground Level (AGL) |
| 1"=20', 1" CI | 1"=200' | 1200' |
| 1"=50', 1' CI | 1"=350' | 2100' |
| 1"=100', 2' CI | 1"=700' | 4200' |
| 1"=200', 5' CI | 1"=1600' | 9600' |
The mixing map scale and contour interval will result in adjustments to aerial photography scale and flight altitude. For example, a 1"=50' land base with a 2' CI would most likely be flown at 3,000' AGL. This would result in 1"=500' scale photography (also called the "negative scale" of the photography). The negative scale and map scale relationships displayed in the table are relative to stereo photogrammetric plotter capability.
A discussion of aerial photography would be incomplete without mentioning the aerial camera. These cameras are highly sophisticated imaging instruments and can cost $300,000 or more. Their handling, care, maintenance, and calibration is an industry unto itself.
3.3.3 Photogrammetry: Control Surveys
Control surveys are described in Section 3.2. They are survey methods that establish the exact location of points on the ground. The purpose of control surveys for aerial photography and photogrammetry are to determine the exact position of the aerial camera at the instant of exposure and to establish known reference points for parcel mapping.
As an airplane flies over the terrain, it snaps many exposures at specified intervals. Without control surveys there would be no way of determining exactly where the aircraft was at the instant of exposure. Control surveys are used to determine the precise geometric relationship between the physical position of the aircraft camera (its altitude and attitude) and the internal spatial geometry of the camera itself.
To accomplish this task, targets are physically placed on the ground at specified locations within the project area. These targets are then surveyed to establish their location. For proper visibility, targets must be sized according to the photo scale necessary. The following table lists some photo scales and their corresponding target size:
| Photo Scale | Target Size |
| 1"=200’ | 2’ x 2’, 4" thick |
| 1"=350’ | 4’ x 4’, 6" thick |
| 1"=700’ | 6’ x 6’, 12" thick |
| 1"=1600’ | 12’ x 12’, 18" thick |
The number of targeted control points and their overall positions within the project area is relative to the flight altitude. Lower aerial photography requires more control points than higher aerial photography.
In some cases the targeted points used for the aerial photography may also be established points used for parcel boundaries or other survey applications. In these cases the targeted control points should be permanently monumented. There may also be cost considerations, since establishing a permanently monumented point costs more than a non-monumented point. For more information on methods of establishing control see Section 3.8
Another approach to establish control for aerial photography is to use the technique of Airborne GPS. Airborne GPS involves the use of GPS receivers mounted on board aircraft. The GPS receiver records the position of the aerial camera at the time of the photo exposure, enabling the photography to be controlled from the air instead of on the ground. As a result, fewer ground control stations are required.
Although this is a relatively new technology, Airborne GPS has several positive benefits. It is an unobtrusive and non-invasive method of determining positional information without targeting control points. Targeting takes time and the presence of the targets in a project area may require obtaining landowner permissions.
Before ending this discussion on survey control, it may be helpful to look at analytical aerotriangulation, a most cost effective method for reducing survey control requirements. This technique, often called analytics (or AT) for short, is a computerized, mathematical photogrammetric procedure for densifying survey control for base map compilation. Measurements based on the precise geometry of the camera and basic survey control can be employed to calculate X, Y, and Z coordinates for use as photo control. The root mean square (RMS) error of analytically generated photo control points should be no greater than 1:10,000 of the flight altitude. For example, if the flight altitude is 2100' AGL, the maximum allowable error in the X, Y, and Z coordinates of any analytically generated control point should be no greater than 0.21'. The use of AT reduces the total number of required targeted points.
3.3.4 Photogrammetry: Stereo Digitizing the Land Base
The third critical link in the chain of compiling a GIS land base is the stereo photogrammetric digitizing process. The major component of this phase is the stereo photogrammetric plotting instrument. At this point in the aerial photography process, the actual capturing of the land base occurs.
To begin the digitizing process the following information is needed:
u the camera calibration report for the aerial camera used to obtain the photography;
u the survey control;
u the aerial photography in the form of 9" x 9" film diapositives forming a stereo pair; and
u the analytic aero-triangulation results.
These are physically loaded into the stereo photogrammetric plotting instrument. The instrument is then mathematically oriented to create a stereoscopic three-dimensional image of the earth's surface in its precise geometric proportion. The operator views the three-dimensional image and compiles or digitizes the features that can be viewed physically through the optics of the stereo plotting instrument.
The actual point of digitizing occurs through a white or black dot of light that the stereo plotter operator moves through the three-dimensional stereo model. This dot is sometimes called a floating point since it appears to float in space in the three-dimensional model. Buildings, for example, are collected by occupying their edge of roof line with the dot. Roads are collected by following the edge of pavement. Utility poles are collected by physically occupying the center of the pole with the dot, and so on.
Elevations and Contours
The collection of contours is another matter. Contours can be collected by either of two methods: stream digitizing or electronically through a digital elevation model (DEM). In the stream digitizing mode, the stereo plotter operator follows his or her interpretation of the physical elevation of the ground with the digitizing dot, thus creating an isoline (contour) of coordinates that exist at the same elevation. This is not a task that can be done quickly or without a great deal of care.
The second method for creating contours is by creating a DEM and employing a computer program known as a contour interpolation program. The DEM is created when the stereo plotter operator digitizes a network of spot elevations and compiles all break lines. Break lines are the lines at which contours would change directions abruptly, such as curbs, ridge lines, valley bottoms, retaining walls, and so forth. A digital terrain model (DTM) contains break lines and points of notable elevation. The difference between a DTM and a DEM is that a DTM does not have a regular grid of elevation points. Both can be used to generate contour values.
A contour interpolation program is used to evaluate the DEM or DTM to create the contours via computer. The DEM and DTM procedures are faster and require less computer space because far fewer elevation points need to be stored. Contours are generated rather than directly observed.
Cartographic Editing
Upon completion of the land base compilation, the data must be cartographically edited. This process occurs on a digital edit station which consists of a computer, a digitizing surface, and a digitizing puck. At this stage, any overprinting of data is corrected, contours are smoothed, and street and place names are inserted. In general, the land base is given final cartographic quality and completeness. At this point in the process, the data are translated from the photogrammetric collection software to either the final GIS file format or a generic file format.
Field Editing
A field edit is completed by personnel physically walking throughout the project area. Survey crews use hardcopy of the land base and measurement tools, such as steel tape, a measuring wheel or even GPS, to compare the hardcopy of the land base to the ground, visually locating any missing detail. The missing items are positioned by measuring from features that have been properly located on the land base. The missing items are then scaled into their actual position and physically plotted onto the hardcopy land base. Back at the office, the printed map is electronically registered to the land base at a digital edit station and the detail obtained from the field edit is transferred into the digital land base via a digitizing puck.
The field edit process is both time-consuming and costly. The need for and extent of field edits should be evaluated carefully. Some spot checks are important to establish the accuracy and completeness of the land base.
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