The USGS Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. The DEM data for 7.5-minute units correspond to the USGS 1:24,000- and 1:25,000-scale topographic quadrangle map series for all of the United States and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator (UTM) projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series. The UTM-based 7.5-minute DEM data are available for most of the contiguous United States, Hawaii, and Puerto Rico.
The digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program. Five DEM tiles were used to complate a subset DEM of the Rincon area in Puerto Rico.
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Public domain data from the U.S. Government are freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey as the originator of the dataset.
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A representative sampling of test points is used to verify the accuracy of a DEM. A minimum number of 28 test points per DEM is required (20 interior points and 8 edge points). Specific standards regarding statistical accuracy and related testing criteria are described in Standards for Digital Elevation Models Part 2, Specifications, section 2.1.4 (http://rockyweb.cr.usgs.gov/nmpstds/demstds.html).
The computer program, DEM VERIFY, is the standard software used to verify the logical and physical format as part of the data base entry procedure for the original DEM files provided by U.S. Geological Survey (USGS). Validation by this software applies to all DEM software systems of the USGS and other government agencies or contractors whose data are intended for submission to the NDCDB. In all cases where the integrity of the logical or physical DEM structure is in question, DEM VERIFY is the standard software for format verification. Additional testing is performed using a DEM Editing System (DES) to aid in the identification of blunders such as irregularly gridded data, mistagging of tops and depressions, and spikes. These blunders are generally identified by displaying the DEM with the aid of DES options, which include color banding of elevation gradients, stereoscopic viewing using anaglyphic filters, and shaded-relief enhancement. An elevation matrix is analyzed in suspect areas and corrected as required. Additional checks for Mean Sea Level were performed on the five input DEM tiles by comparing the vertical GRS80 Ellipsoid heights with a GEOID99 dataset from NOAA.
These data have been clipped to the coastline boundary and exclude all ocean pixel values contained within the original data.
Horizontal accuracy of DEM data is dependent upon the horizontal spacing of the elevation matrix. Within a standard DEM, most terrain features are generalized by being reduced to grid nodes spaced at regular intersections in the horizontal plane. This generalization reduces the ability to recover positions of specific features less than the internal spacing during testing and results in a de facto filtering or smoothing of the surface during gridding.
The vertical accuracy of 7.5-minute DEMs is equal to or better than 15 meters. A minimum of 28 test points per DEM is required (20 interior points and 8 edge points). The accuracy of the 7.5-minute DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:24,000-scale.
The method of determining DEM accuracy involves computation of the root-mean-square error (RMSE) for linearly interpolated elevations in the DEM and corresponding true elevations from the published maps. Test points are well distributed and representative of the terrain. Collection of test point data and comparison of the DEM to the quadrangle hypsography are conducted by USGS quality-control groups.
The 7.5-minute DEM data are produced in 7.5- by 7.5-minute blocks either from digitized cartographic map contour overlays or from scanned National Aerial Photography Program (NAPP) photographs. Interpolations are produced from digital line graph (DLG) hypsographic and hydrographic data.
Five tiles (134.txt, 135.txt, 168.txt, 169.txt, 202.txt) provided by the USGS were used to complete a subset DEM of the Rincon area. These data were received projected horizontally to UTM Zone 19N, NAD83, GRS80, meters and vertically to GRS80 Ellipsoid heights (meters). Features were converted to a raster grid using the ArcGIS Spatial Analyst extension (5-meter grid cell).
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To best approximate Mean Sea Level (MSL), a second dataset of GEOID99 geoid heights was created using the National Geodetic Surveys Tool Kit (http://www.ngs.noaa.gov/TOOLS/). Each of the five point files were re-projected to geographic coordinates (Lat/Long) to facilitate use with the Tool Kit and input into the application. The resulting output (x,y,z) was then linked back to the original UTM coordinates in MicroSoft Access . Features were again converted to a raster grid using Spatial Analyst.
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The raster calculator was then used to provide the final DEM subset by subtracting the GEOID99 geoid heights from the GRS80 Ellipsoid heights, providing an updated z-value at each x,y location. These data were then clipped to the coastline boundary, excluding all ocean pixels. Note that all ocean pixels (no data) were recoded to zero (0) to resolve errors when using the DEM in the geometric correction (i.e., the program does not know how to resolve image portions that overlay onto no data areas).
384 Woods Hole Road
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Although these data have been used by the U.S. Geological Survey (USGS), U.S. Department of the Interior (DOI), no warranty expressed or implied is made by the U.S. Geological Survey as to the accuracy of the data. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
ESRI ArcRaster grid
DEM format raster grid with associated files and metadata
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