RiverCAD DEM Data

RiverCAD® can directly use Digital Elevation Models (DEM) for creating a 3D watershed model. Digital Elevation Models are data sets of regularly sampled topographic elevations typically derived from USGS topographic maps. DEM data is available from the USGS and many other sources, some of which are listed below. RiverCAD can directly read and triangulate DEM data very quickly, allowing you to generate a 3D digital terrain model representation — complete with contour lines — of your watershed. The 3D model may be densified honoring local slope and curvature for detailed maps, or rarified to lower the 3D TIN face-count. Digital elevation models are preferable to digitized contour maps, such as some DLG files, because DEM models use a uniform data spacing.

DEM Data versus Digitized Contour Lines

A DEM file contains topographic elevations sampled on a uniform grid spacing. Typically, the horizontal grid spacing comes out to about every 30 meters for 7.5 minute DEMs and 3 arc-seconds for 1 degree DEMs.

Digitized contours represent contour polylines at the elevation of the contour line. Digitized contours have many vertices along each contour, but no control points between the contours. This results in over-sampling along contours and under-sampling between contours. If the contour interval of the source map is small (therefore many contours), the surface model created from it is generally good. If the contour interval of the source map is large (therefore larger gaps between contours), the surface model created from it is generally poor, especially along drainages, ridge lines, and in undulating or hummocky topography.

As a general rule, a DEM model will produce a better model than digitized contours. With RiverCAD you may work with either DEM data or digitized contours but not both.

Types of DEM Data


RiverCAD currently supports the two most common flavors of digital elevation model data.

7.5 Minute DEM (1:24000)

7.5-minute DEM (30 x 30 m data spacing, cast on Universal Transverse Mercator (UTM) projection or 1 x 1 arc-second data spacing). Provides coverage in 7.5 x 7.5 minute blocks. This type of DEM provides the same coverage as a standard USGS 7.5 minute map series quadrangle. Coverage: Contiguous United States, Hawaii, and Puerto Rico. These are available for a fee from the USGS.

This data is typically in units of meters for x, y, and elevation. They contain elevation measurements for a uniform grid with a cell size of approximately 30 meters. A typical 7.5 minute DEM has about 170,000 points, which RiverCAD can read, triangulate, and contour in less than 15 seconds.

1 Degree DEM (3 arc-second data)

1-degree DEM (3 x 3 arc-second data spacing). Provides coverage in 1 x 1 degree blocks. Using two DEM files of this type (three in some regions of Alaska) would provide the same coverage as a standard USGS 1 x 2 degree map series quadrangle. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by USGS in the DEM data record format. Coverage: United States. These are available for download at no cost from the USGS.

This data is typically in units of arc-seconds (1/3600 of a degree) of longitude and latitude for x, y, and meters for elevation. They contain elevation measurements for a uniform grid with a cell size of 3 arc-seconds. A typical 1 degree DEM has about 1,440,000 points, which RiverCAD can read, triangulate, and contour in less than two minutes.

Other DEM Formats

There are special formats for Alaskan DEM data which compensate for their far northern latitude. At this time, this format is not supported by RiverCAD.

New 30 arc-second format data sets are becoming available. Although this format may be supported in a future release of RiverCAD, it is not currently.


Where to Find DEM Data

Availability of DEM models is improving daily. There are many sources of DEM data files, some for free and some for a nominal cost. A good place to start is at the source of U.S. DEM data: the U.S. Geological Survey (USGS). A few of the many sites offering DEM data are listed below: