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.

Unzip the file (it will have a name like 3097420g.zip) and you should see a new set of files. The file that ends in .dem is the actual digital elevation model in USGS format (e.g., 3097420g.dem)

Important information! The TNRIS DEMs are usually in the UTM coordinate system, NAD 27, 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 ArcMap to determine this if possible.

Other DEM sources
DEM data is available from the Federal government in SDTS format, and from many state agencies. The following resources contain some useful pointers:

• Bruce Gittings' Digital Elevation Data Catalogue

• USGS Geographic Data Download

• MapMart

• The GISuser State Clearinghouse Directory

• GISuser Guide to downloading Free 7.5 minute DEMs

• GISuser Guide to downloading Free 1:250K USGS DEMs

• USGS Seamless Data Distribution Center
  
  
  Importing a USGS format DEM in ArcGIS
  Note: The following instructions will only work if you DEM is in USGS format. Most DEMs from the USGS are not in USGS format (go figure...) - it is in SDTS format (but the USGS is also now producing DEMs in other formats including ArcGrid). If  your DEM is in SDTS format (Spatial Data Transfer Standard), you will need to use the Import SDTS Raster to Grid tool in ArcToolbox.

After you download the DEM, you need to unzip it. Once it is unzipped, you can import it into ArcGIS.

• Start ArcToolbox, and choose Conversion Tools - Import to Raster - DEM to Grid. Read the Help information before you begin, then follow the instructions.

• The USGS DEM is the file that ends in .dem (e.g., 3097420g.dem)

• Leave all the defaults as they are for now - we will talk about these in class.

• For output grid, you can navigate to your personal folder and type in a new name - this will be the final ESRI formatted DEM that you will work with, so give it a good name, like the name of the quad map it represents (e.g., austin_west). But read this carefully - you cannot specify a folder that has spaces in the folder name; in fact, no folder in the entire path on the computer can have spaces in it (don't ask me why). So the folder path to this new grid needs to be something like C:\temp\edwards_group\dem\austin_west

• Once the importing process is done, take a look at the results in file manager or explorer - notice that there are now two folders, one called the name that you gave the grid (e.g., austin_west) and one called info. This is an ArcInfo formatted GIS data set.

• Next add the new grid to a data frame in ArcMap.

Using a DEM in ArcGIS

• To use a DEM in ArcGIS, you need the Spatial Analyst extension. In ArcMap, go to Tools - Extensions and make sure that Spatial Analyst is checkmarked.

• Next, choose View - Toolbars and turn on the Spatial Analyst toolbar.

• To create a hillshade map once your DEM is in view, click on the Spatial Analyst toolbar and choose Surface Analysis - Hillshade. Accept the default settings first, then try again and play around with the settings. Be default, this will create a temporary file. If you want to create a permanent file, click on the Output Raster section, navigate to your personal folder , and give the hillshade file a new name.

• To create a slope map, you can use the Surface Analysis - Slope command. Beware, 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 need to do some additional preparation! See Working with x, y, and z units in Digital Elevation Models for details and instructions.

• If you need to re-project the grid to a different coordinate system, ArcToolbox has projection tools for grids under the Data Management area. The define projection wizard for grids is different than for shape files, and a little trickier. The necessary parameters are: the projection is UTM, zone 14, meters, no x,y shift, and the datum is NAD 1927 US Nadcon

• For more information about Spatial Analyst, go to ArcGIS Desktop Help or the digital manual, Using ArcGIS Spatial Analyst.

For more tutorials please see this section titled ArcGIS Desktop 8.x Tips and Tutorials

Author

Thanks to Barbara Parmenter and The University of Texas at Austin School of Architecture for this contribution. See http://web.austin.utexas.edu/architecture/