DEM files are the most important source of surfaces in the United States. The USGS (www.usgs.gov) publishes vast amounts of data as DEM files for free download from its web site. In addition to the DEM files published by USGS, DEM-style files are used with ESRI's ArcINFO package, with GLOBE worldwide digital elevation data and with GTOPO30 worldwide digital elevation data. All forms of DEM are slightly different and require a different importer.
All DEMS listed below will be imported as georegistered surfaces. Cool!
USGS publishes digital elevation data in two resolution levels:
1:250K DEM - 1:250,000-scale Digital Elevation Modules (terrain elevation) suitable for use as surfaces at the county level. These files have a .dem extension. Import them using the DEM Files (*.dem, *.*) setting.
1:24K DEM - 1:24,000-scale Digital Elevation Modules published in SDTS format suitable for use as surfaces at the local level. These correspond to USGS "Quad" maps and are extraordinarily detailed. These files follow SDTS file naming conventions. Use the SDTS drawing importer to import these files (a surface will automatically be created even though a drawing importer was used). Note that unzipping STDS files for a particular USGS Quad will often result in three compressed files. The largest of these files is usually the full accuracy DEM. The others are interpolated to smaller sizes and less detail.
The Global Land One-km Base Elevation (GLOBE) project provides the latest, most modern, public domain terrain elevation database for the entire world. Currently at http://www.ngdc.noaa.gov/seg/topo/globeget.shtml, the data set may be downloaded in 16 "tiles," most of which are very large files (over 100MB each) when unzipped. When unzipped, the GLOBE files have no extensions but will have names like g10g. Import using the DEM GLOBE Files (*.*) setting in File - Import - Surface. GLOBE tiles are so large that most users will crop them to the area of interest using the Crop transform toolbar operator.
GTOPO30 is a global digital elevation data set with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). GTOPO30 was derived from several raster and vector sources of topographic information. GTOPO30 was completed in late 1996. It was developed over a three-year period through a collaborative effort led by staff at USGS's EROS Data Center. Downloads are currently at http://edcdaac.usgs.gov/gtopo30/gtopo30.html and (after unzipping) may be imported using the DEM GTOPO30 Files (*.dem) setting in File - Import - Surface. Note that although GTOPO30 files use the .dem extension (as well as other file extensions in the GTOPO30 file set) they must be imported using a different importer than the USGS 1:250K-scale DEMs.
GTOPO30 files import into Manifold with invisible pixels at sea level. This makes them handy for use as components in maps showing large areas, since seas may be colored.
DEM files from ArcINFO are often saved using either a .grd extension or a .bil extension. Import DEM files from ArcINFO using either GRD Files (*.grd) or BIL Files (*.bil, *.hdr) in File - Import - Surface.
The Alps from GLOBE tile g10, seen as a layer in a map projected into Orthographic projection.
Norwegian fiords from GTOPO30 W020N90 tile seen in a map in Orthographic projection. The surface at left has no background while the one at right has a blue background.