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"CERTIFIED" NHD DATA: Transport reaches are defined on nearly all features of type: stream/river, canal/ditch, artificial path, pipeline, and connector. Water body reaches are defined on the subset of lake/pond features identified as water bodies. Geographic names, where present, were carried over from both 1:24k DLG hydrography data and from the best available source data. Near the international boundaries with Canada, only the parts of features within the United States are delineated. Detailed capture conditions are provided for every feature type in the Standards for NHD (USGS, 1999), available online through <http://mapping.usgs.gov/standards/.>
This dataset does not claim to include all potential or conducted river restoration sites in Lamoille County, VT.
Tested (value varies by sub basin-see below) meters horizontal accuracy at 95% confidence level.
These values do not account for the error of the source digital orthophotos (approx. +/- 6 meters) on which the data is based. Horizontal accuracy was assessed by comparing 20 well defined "check" points of the source RF 5,000 scale digital orthophotos against derived surface water data. This approach is detailed in the FGDC Geospatial Positioning Accuracy Standards (<http://www.fgdc.gov/standards/status/sub1_3.html>). Digitized data was overlaid upon the RF 5k scale digital orthophotos. Check points were assessed on-screen to determine the spatial difference between the delineated vector data features and the location of the features on the orthophotos. Check points are any combination of the following features: 1) Confluences of one dimensional streams ("1d" or single line streams); 2) Confluences of two dimensional streams ("2d" or double line streams represented as polygons); 3) Polygonal lake and pond features; and 4) Distinct island features. Personnel other than the originator conducted accuracy analysis. Data is believed to be of uniform positional accuracy. Potential errors were deemed to be random, thus check points were dispersed across the extent of the data.
Additional enhancement to data accuracy involved the use of RF 40,000 scale 1992-93 NAPP color infrared aerial photos (Lamoille and White River Sub basins only). The RF 5,000 scale surface water data was plotted at RF 20,000 scale and overlaid onto enlargements (RF 20,000 scale) of the photos to determine errors of omission of water polygons. Errors noted on Mylar were digitized on-screen using digital orthophotos.
River restoration sites were GPS'd using Trimble Pathfinder Basic Plus. Points were verified using benchmarks and found to be within 2 meters of accuracy.
The accuracies reported were generated under the following conditions:
1) Personnel other than the original editor conducted accuracy analysis,
2) The best "independent source of higher accuracy" available is the RF 5000 scale Vermont Mapping Program digital orthophotos on which the original editing is based, and
3) Potential errors were deemed to be random, thus test points were dispersed across the entire extent of the data.
OVERVIEW - Twenty random "check" points were generated along the finalized surface water drainage network and compared against their actual "test" points on the ground that were visible using the RF 5,000 scale digital orthophotos.
Horizontal accuracy was assessed by conducting the following steps in ArcView® for each sub basin:
1) The finalized surface water data was assessed to ensure that a single route represented the entire 8 digit cataloging unit (sub basin). If one didn't exist then it was created.
2) Using the ArcView® extension "randpts.avx", a user extension created by Jeff Jenness, a Wildlife Biologist and GIS Analyst with the US Forest Service at the Rocky Mountain Research Station in Flagstaff, AZ, 20 random points were generated as "check" points. This extension is available from the ESRI Arc Scripts website.
3) The "check" points and surface water data were overlaid upon the RF 5000 scale digital orthophotos to assess "check” points with their true locations, i.e., "test" points. Test point locations were stored in a separate point theme. A "key" field was maintained between these data sets. In some locations it was not possible to determine a "test" point with a high level of confidence so the "check" point was moved to the closest location along the same linear feature where a "test" location could be adequately determined on the orthophoto. Check points included any combination of the following features: A) Confluences of one dimensional streams "1d" or single line streams); B) Confluences of two dimensional streams ("2d" or double line streams represented as polygons); C) Polygonal lake and pond features; and D) Distinct island features.
4) Using the ArcView® sample script "addxycoo.ave", X,Y coordinates were added to the features tables of both the "check and "test" point attribute tables. These were then exported to comma delimited text files.
5) Using an Excel spreadsheet, created by the Minnesota Planning Land Management Information Center in a publication titled the "Positional Accuracy Handbook: Using the National Standard for Spatial Data Accuracy to measure and report geographic data quality " (http://www.mnplan.state.mn.us/press/accurate.html), the delimited files were imported. This spreadsheet contains all formulas necessary to generate the NSSDA statistic representing average horizontal positional accuracy.
Source data for each sub basin is a composite of surface water data from different sources created using varying methodologies by numerous entities. Therefore, reported accuracy values are reported for this composite data. Features in this data are believed to be of uniform positional accuracy. Accuracy value does not take into account the horizontal positional accuracy of the source digital orthophotos, on which the data is based.
Attributes are present that were not developed as part of the NHD dataset and standard, and that are not part of any standards. Note the Attribute Definition Source to identify source agencies.
Attributes are present that were not developed as part of the NHD dataset and standard, and that are not part of any standards. Note the Attribute Definition Source to identify source agencies.