How DEM To Raster works

Digital terrain data covering most regions in the United States is available in the USGS DEM format from the U.S. Geological Survey. For more information, see the USGS Rocky Mountain Mapping Center web site or write to the National Cartographic Information Center, 507 National Center, Reston, VA 22092.

It is important to note the differences between raster datasets created from USGS 7.5-minute and 1-degree DEMs. The following table compares raster datasets created from these two sources.

Factor	Raster dataset from a 7.5-minute DEM	Raster dataset from a 1-degree DEM
Geographic extent
in latitude and longitude	7.5' x 7.5'	1° x 1°
in kilometers (approx.)	12 km x 14 km	90 km x 110 km
Projection	UTM	Geographic
Zone	1 to 60	N/A
Ground Units	meters	arc-seconds
Surface ZUnits	meters	meters
Spheroid	Clarke 1866	WGS72
Xshift	0.00000	0.00000
Yshift	0.00000	0.00000
Distance between elevation points in x and y	30 m x 30 m	3 arc-seconds 75 m x 75 m (approx. in the U.S.)
Number of elevation points in x and y	(varies) 380 x 450	(constant) 1201 x 1201
Variation in ground distance between elevation point spacing across the raster dataset	no variation, spacing is constant	varies with the latitude

Raster datasets created from two different DEMs

Raster datasets constructed from USGS 1-degree DEMs are not immediately suitable for volume, slope, or accurate visibility analysis because the x,y locations are measured in latitude and longitude, but the z-values are measured in meters. Consequently, the actual distance on the ground represented by one Ground Unit is not constant, and the Ground Units and the Surface ZUnits are not in the same units of measure.

Unlike USGS 7.5-minute DEMs that contain surface elevations at a constant spacing of 30 meters, 1-degree DEMs contain ground elevations at an interval of 3 arc-seconds. The actual distance in ground units represented by 3 arc-seconds varies with latitude even within the raster dataset. For example, in the vicinity of Southern California, at the top of the raster dataset, 3 arc-seconds measure 76.86 meters in x and 92.36 meters in y. At the bottom of the raster dataset, 3 arc-seconds measure 77.11 meters in x. In the region of the North Pole, the distance in x approaches zero.

The 7.5-minute USGS DEMs are a by-product of the orthophotomapping program. Some orthophotos were produced using semiautomatic profiling devices that yield elevations along profiles. “When these heights are resampled to a 30 meter square grid, the interpolation technique produces estimated elevations which have a much stronger spatial autocorrelation along the direction parallel to the profiles than in the orthogonal direction. Many images produced from these models have clearly visible ‘stripes’ parallel with the scanning direction of the profiler.” For a reference on this topic, see: Mark, D., "Automated Detection of Drainage Networks from Digital Elevation Models", in Proceedings of the Sixth International Symposium on Computer Assisted Cartography (Auto-Carto VI), Vol. 2, 1983, pp. 288–298.

You can check the data for this effect by creating a hillshade of the raster dataset with the Hillshade tool and displaying it. If any striping is visible, before using the elevation raster any further you may wish to first smooth the values out with the Filter tool and specifying the LOW filter type

Projecting DEMs

To make a surface model suitable for volume, slope, and accurate visibility analysis, the ground units in the 1-degree USGS DEM must be projected to a nonangular unit of measure, such as UTM or Lambert. In addition, the surface ZUnits should be expressed in the same units of measure as the Ground Units.

Use the Define Projection tool or Project Raster tool (with a BILINEAR or CUBIC resampling technique) to convert a raster dataset to the desired projection or to change datums.

The WGS72 datum does not share a common transformation method with NAD27 or NAD83, so an equivalent datum must be used.

Learn more about datums

Note:

The Project tool does not perform ZUnits conversion. Use the Times tool to convert the ZUnits by multiplying by a factor (for example, raster * 0.3048).