Digital Elevation Model (DEM) is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The standard DEM consists of a regular array of elevations cast on a designated coordinate projection system. The DEM data are stored as a series of profiles in which the spacing of the elevations along and between each profile is in regular whole number intervals. The normal orientation of data is by columns and rows. Each column contains a series of elevations ordered from south to north with the order of the columns from west to east. The DEM is formatted as one ASCII header record (A-record), followed by a series of profile records (B-records) each of which include a short B-record header followed by a series of ASCII integer elevations per each profile. The last physical record of the DEM is an accuracy record (C-record). A 30-minute DEM (2- by 2-arc second data spacing) consists of four 15-by 15-minute DEM blocks. Two 30-minute DEMs provide the same coverage as a standard USGS 30- by 60-minute quadrangle. Saleable units are 30- by 30-minute blocks, that is, four 15- by 15-minute DEMs representing one half of a 1:100,000-scale map. Coverage includes the contiguous United States and Hawaii.
DEMs can be used as source data for digital orthophotos, and for earth science analysis as layers in geographic information systems. DEMs can also serve as tools for volumetric analysis, for site location of towers, or for drainage basin delineation. These data were collected as part of the National Mapping Program.
Supplemental_Information: The 30-minute DEM's rows and columns are based on the geographic coordinate system. The use of this system results in a rectangular DEM which shares a common edge and duplicate points with other adjacent 30-minute DEMs. The 30-minute DEM is distributed as four 15-minute DEM units, and corresponds to the same coverage as the east half or west half of a 1:100000-scale topographic series USGS quadrangle map.
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Data Set Citation
U.S. Geological Survey
2-Arc-Second (30-Minute DEM) Quadrangle
Dataset Release Place:
Reston, VA, USA
U.S. Geological Survey
Data Presentation Form:
Attribute_Accuracy_Report: The accuracy of a DEM depends upon the source level of detail and the resolution of the data samples. The primary limiting factor for the source level of detail is the scale of the source materials. The proper selection of grid spacing determines the level of content that may be extracted from a given source during digitization. DEM data accuracy is derived by comparing linear interpolation elevations in the DEM with corresponding map location elevations and computing the statistical standard deviation or root-mean-square error (RMSE). The RMSE is used to describe the DEM accuracy. For 15-minute DEMs derived from vector or DLG hypsographic and hydrographic source data, an RMSE of one-half contour interval or better is required. The 1-degree DEM data have and absolute accuracy of 130 meters horizontally and 30 meters vertically.
Logical_Consistency_Report: The fidelity of the relationships encoded in the data structure of the DEM are automatically verified using a USGS software program upon completion of the data production cycle. The test verifies full compliance to the DEM specification.
Completeness_Report: The DEM is visually inspected for completeness on a DEM view and edit system for the purpose of performing a final quality control, and, if necessary, edit of the DEM. The physical format of each digital elevation model is validated for content completeness and logical consistency, both during production quality control and prior to archiving. Due to the variable orientation of the quadrilateral in relation to the Universal Transverse Mercator (UTM) projection grid, profiles that pass within the bounds of the DEM quadrilateral may be void of elevation grid points and are not represented in the DEM. This condition occurs infrequently and is always the first or last profile of the dataset. Level codes of data quality are assigned depending on the specific production process used. Level 1 DEMs are created by auto correlation or manual profiling from aerial photographs. Level 2 DEMs are created from digital line graph contours. Level 2 DEMs may contain void areas due to interruptions to contours in the source contours. Void area elevation grid posts are assigned the value of -32,767. In addition, suspect elevation areas may exist in the DEM but are not specifically identified. Suspect areas can be located on the source graphic as a disturbed surface and are symbolized by contours overprinted with photorevised or other surface patterns.
Positional_Accuracy: Horizontal_Positional_Accuracy: Horizontal_Positional_Accuracy_Report: The horizontal accuracy of the DEM is expressed as an estimated root mean square error (RMSE). The estimate of the RMSE is based upon horizontal accuracy tests of the DEM source materials that are selected as equal to or less than the intended horizontal RMSE error of the DEM. The testing of source material horizontal accuracy is accomplished by comparing the planimetric (X and Y) coordinates of well-defined ground points with the coordinates of the same points as determined from a source of higher accuracy. For 15-minute DEMs derived from vector or DLG hypsographic and hydrographic source data, an RMSE of one-half of a contour interval or better is required.
Vertical_Positional_Accuracy: Vertical_Positional_Accuracy_Report: The vertical RMSE statistic is used to describe the vertical accuracy of a DEM and encompasses both random and systematic errors introduced during production of the data. The RMSE is encoded in element number 5 of record C of the DEM. Accuracy is computed by a comparison of linear interpolated elevations in the DEM with corresponding known elevations. Test points are well distributed, representative of the terrain, and have true elevations with accuracies well within the DEM accuracy criteria. Acceptable test points include, in order of preference: field control, aerotriangulated test points, spot elevations, or points on contours from existing source maps with appropriate contour interval. A minimum of 28 test points per DEM is required to compute the RMSE, which is composed of a single test using 20 interior points and 8 edge points. Edge points are those which are located along, at, or near the quadrangle neatlines and are deemed by the editor to be useful to evaluating the accuracy of the edge of the DEM. Collection of test point data and comparison of the DEM with the quadrangle hypsography are conducted by the quality control units within the USGS. There are three types of DEM vertical errors: blunder, systematic, and random. Blunder errors are those errors of major proportions and can be easily identified and removed during interactive editing. Systematic errors are those errors that follow some fixed pattern and were introduced by data collection procedures and systems. Random errors result from unknown or accidental causes.
There is no guarantee of warranty concerning the accuracy of these data. Users should be aware that temporal changes may have occurred since the data was collected and that some parts of these data may no longer represent actual surface conditions. Users should not use these data for critical applications without a full awareness of their limitations. Acknowledgement of the originating agencies would be appreciated in products derived from these data. Any user who modifies the data set is obligated to describe the types of modifications they perform. User specifically agrees not to misrepresent the data set, nor to imply that changes made were approved or endorsed by the U.S. Geological Survey. Please refer to http://www.usgs.gov/privacy.html for the USGS disclaimer.
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