About DEMs
A DEM is a digital elevation model. It can be used in ArcView's Spatial Analyst, Model Builder, and 3-D Analyst extensions. To read up more about DEMs, go to the USGS National Spatial Data Clearinghouse DEM web site.
DEM Sources for Texas
TNRIS serves the 1996 Digital
Elevation Models at 30 meter pixel resolution - see the
TNRIS Digital
Data-DEMs site for more information. DEMs at TNRIS are identified
by their USGS Quad Map name - see the USGS
Quad and Quarterquad Locator Map to know which DEM you will need. DEM
data from TNRIS is in USGS DEM format.
Important information! The TNRIS metadata link is currently not working, but I belive that these DEMs are in the UTM coordinate system, Zone 14 for most of central Texas, and map units (x,y horizontal units) are meters. Some of the elevation data in these DEMs is in meters and some is in feet. It is very important that you know which! Once you bring up the data, examine it in ArcView to determine this if possible.
Other DEM sources
DEM data is available from the Federal government in SDTS format, and from
many state agencies.
USGS Geographic Data Download (SDTS format - see SDTS info below)
Importing a USGS format DEM in ArcView
Note: The following instructions will only work if you DEM is in USGS
format. Most DEMs from the USGS are not in USGS format (of course...) - it
is in SDTS format! If your DEM is in SDTS format (Spatial Data Transfer
Standard), you will need to use a different process. See the instructions
for SDTS DEM Import below.
After you download the DEM, you need to unzip it using Winzip. Once it is unzipped, you can import it into ArcView.
Start Arcview, and load the Spatial Analyst extension (choose File-Extensions, then scroll and check mark Spatial Analyst).
Choose File - Import Data Source
In the scroll list, choose USGS DEM.
Press OK.
Now the DEM should be available for adding to the view. Choose View - Add Theme
A DEM is a Grid data source, so under Data Source Type, choose Grid.
Navigate to where you unzipped the DEM and add it to your view.
To create a hillshade map once your DEM is in view, choose Surface-Compute Hillshade.
To create a slope map, you can use the Surface - Derive Slope command, but you must be aware of how this works. Also, your horizontal map units must be the same unit as your elevation units (e.g., both feet or both meters) - often they are in different units, in which case you can't use the Derive Slope function in the menu! See Creating a Slope Grid in Spatial Analyst for details and instructions.
If you need to re-project the grid to a different coordinate system, use the GridProjector extension available from ESRI's Scripts web site. This should be loaded as an extension on most of our computers.
For more information, see the Spatial Analyst manual or the Extending ArcView GIS book - both are on the GIS shelves in the lab. Also look at Terrain Modeling with ArcView GIS (from ArcUser magazine).
Importing DEMs in SDTS format (Spatial Data Transfer Standard)
SDTS is the Spatial Data Transfer Standard. All GIS data sets created by US federal government agencies are supposed to provide data in this format in an effort to standardize GIS data formats. Most DEMs from the USGS are in the recently approved official format called SDTS Part 5: Raster Profile and Extensions (FGDC-STD-002.5-1999). To extract these formats from SDTS into something that ArcView 3.2 can read, follow the instructions at the GeoCommunity web site: Using USGS DEMs in ArcView GIS (http://data.geocomm.com/dem/sdts2dem_demo.html).
Note that when you download a DEM in SDTS format the file you downloaded usually is named something like this: 1673704.DEM.SDTS.TAR.GZ. The .TAR and .GZ are two compression formats. Unzip the file using Winzip, then unzip the resulting file using Winzip again. You will finally end up with the SDTS DEM files themselves. Then follow the directions given in: Using USGS DEMs in ArcView GIS (http://data.geocomm.com/dem/sdts2dem_demo.html).
ArcView 3.2 comes with a stand-alone utility for translating DEM data in an older version of the SDTS format into a grid that ArcView can read, but it does not work on the newer SDTS format. This utility is not found within ArcView, but should be part of the ESRI folder structure created when ArcView 3.2 was installed. It should be available by from the Windows Desktop by choosing Start-Programs-ESRI-SDTS Raster to Grid. For instructions on how to use this utility, see Use Raster to Grid Utility to Model SDTS Format DEM Data (from the January 2000 ArcUser magazine).
See also Everything You Want To Know About SDTS from the GeoCommunity web site for tutorials, software utilities, etc.
