![advantages of digital terrain model advantages of digital terrain model](https://images.ctfassets.net/qfhr9fiom9gi/3htCL6yAQDjOBv1zgYf8e1/fa07f0fd133fa252da61a145cc5e1fd3/image17.png)
Try opening it in a text editor to see what I'm talking about.) DistributionĭEM tiles are available for free download through many state and regional clearinghouses. (Later in this chapter, you'll download a sample USGS DEM file. Other than the starting point, the positions of the other elevation points need not be encoded, since their spacing is defined. Records include the UTM coordinates of the starting point, the number of elevation points that follow in the profile, and the elevation values that make up the profile. Entities and attributesĮach record in a DEM is a profile of elevation points. Elevations in some DEMs are referenced to either NGVD 29 or NAVD 88.
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Some DEMs are based upon the North American Datum of 1983, others on NAD 27. They are cast on the Universal Transverse Mercator projection used in the local UTM zone. The target accuracy for large-scale DEMs is seven meters 15 meters is the maximum error allowed. The vertical accuracy of DEMs is expressed as the root mean square error (RMSE) of a sample of at least 28 elevation points. Others were produced photogrammetrically from aerial photographs. Some older DEMs were produced from elevation contours digitized from paper maps or during photogrammetric processing, then smoothed to filter out errors. The preferred method for producing the elevation values that populate DEM profiles is interpolation from DLG hypsography and hydrography layers (including the hydrography layer enables analysts to delineate valleys with less uncertainty than hypsography alone). The farther the quadrangle is from the central meridian of the UTM zone, the more it is tilted. See how the DEM tile in the illustration below appears to be tilted? This is because the corner points are defined in unprojected geographic coordinates that correspond to the corner points of a USGS quadrangle. In these large-scale DEMs, elevation profiles are aligned parallel to the central meridian of the local UTM zone, as shown in Figure 7.8.1, below. Coverage is complete at 30-meter resolution. The resolution of a DEM is a function of the east-west spacing of the profiles and the south-north spacing of elevation points within each profile.ĭEMs corresponding to 7.5-minute quadrangles are available at 10-meter resolution for much, but not all, of the U.S.
#ADVANTAGES OF DIGITAL TERRAIN MODEL SERIES#
Large-scale (7.5-minute and 15-minute), intermediate scale (30 minute), and small-scale (1 degree) series were produced for the entire U.S. Like other USGS data, DEMs were produced originally in tiles that correspond to topographic quadrangles. USGS DEMs are raster grids of elevation values that are arrayed in series of south-north profiles. Later in this chapter, we'll consider sources of global terrain data. Here we consider the characteristics of traditional DEMs produced by the USGS. The NED has been incorporated into a newer elevation data product at the USGS called the 3D Elevation Program (3DEP). Geological Survey produced a nation-wide DEM called the National Elevation Dataset (NED), which has traditionally served a primary source of elevation data. In general, a DEM is any raster representation of a terrain surface.