Data for this Project
Large-scale (1:24,000) SDTS data comes in two distinct types—DLG vector data and DEM grid models. All 1:24,000-scale data is developed using the USGS 7.5' quadrangle series maps as a base.

The model for using the SDTS DEM described here primarily uses data found the 7.5' Moab, Utah, quadrangle that was downloaded from the EROS site.

Recent mapping of jeep trails using global positioning system (GPS) equipment has greatly improved descriptions of the area's many jeep and mountain bike trails. The sample data set for this article includes two area jeep trails, Moab Rim and Hell's Revenge, along with the main route of the famous Moab Slickrock Bike Trail, as well as 1:100,000-scale DLG vector shapefiles and Geographic Names Information System (GNIS) place name points for the Moab quad.

The modeling methods described here will allow ArcView GIS users to use DEM data to create similar models almost anywhere in the continental United States.

Downloading DEM Data
The 1:24,000-scale DEM data at the EROS site is divided just like 7.5' paper maps into 7.5 x 7.5-minute sets indexed by the USGS quadrangle name. Use the FTP by Alphabetical List site if the name of the USGS map is known. If the name of the map is not known, use the FTP via Graphics to search for the desired data. The data is registered using metric UTM values according to NAD27. Grid cell elevations are presented in meters.

Now is a good time to set up the folders that will be used while building the Moab model. Using Windows Explorer or some other file manager, create a directory called UT at or near the root of the drive that will be used for modeling. Under the UT directory, create a subdirectory called MOAB. Under MOAB, create two subdirectories named DEMFILES and SHPFILES.

Once the Moab DEM is located at the EROS site using either FTP by State or FTP by Graphics, take a careful look at its file name. This DEM requires a number of files that are all compressed inside a Tape Archive (TAR) file named 30.2.1.1006298.TAR.GZ. This file may be renamed or have the name truncated by Windows-based Web browsers when it is downloaded and stored in \UT\MOAB\DEMFILES. If the file is downloaded using Windows long file name rules, the resulting file name might be 30_2_1_1006298_TAR.GZ. Windows replaces all the decimals except the last one with an underscore character.

Unzipping DEM Data
Loading SDTS DEM data into ArcView GIS requires unzipping the TAR file and converting it to a UTM NAD27 DEM. An unzipping utility, such as WinZip, that will accommodate both GZ and TAR files is required.

To decompress the TAR file, rename it so that the file extension (i.e., the string to the right of the "dot") includes both TAR and GZ. Rename the TAR file MOAB1158.TAR.GZ. The use of 1158 will become apparent shortly. After renaming, open the TAR\GZ file with the unzipping utility. When prompted to decompress one file named MOAB1158.TAR into a temporary directory, answer Yes. The decompression utility will open 20 files. Most of these files begin with 1158. Next, extract these into the DEMFILES directory. Use Explorer or some other file manager to verify that all 20 files were extracted. Note that most files include the extension .DDF. The next step requires a DEM conversion utility.

Acquiring the DEM Conversion Utility
Sol Katz of the BLM in Denver, Colorado, developed a very functional and easy-to-use utility that converts the complex SDTS DEM file set into an ArcView GIS-ready format. This conversion utility, contained in a file named SDTSEDEM.ZIP, is available for free from the U.S. BLM FTP site.

The utility is in PKZIP compressed format that yields an executable file named SDTSEDEM.EXE and a C program file named SDTSEDEM.C when unzipped. Store both files in a directory with other utilities so that both files may be easily located and copied into any project directory.

Katz has developed many other DEM and DLG converters focusing primarily on the complex SDTS format that are also available at the BLM FTP site. Look at the other utilities available and download the readme files that explain how these utilities work. The DOS executable, SDTSEDEM.EXE, released in mid-1998, is the best DEM conversion utility he has developed.

Processing DEM Files into an ArcView GIS-Ready Format
Place a copy of SDTSEDEM.EXE in the DEMFILES directory. The number 1158 will be very important in processing the Moab file. This utility requires a DOS window to run. Simply double click on the SDTSEDEM.EXE file in Explorer to bring up a DOS window and start the program.

1. As the conversion utility initiates, it requests the first four characters of the base SDTS file name. Type in 1158.
2. The program requests the name of the output DEM. Type MOAB1158. Do NOT type the DEM extension (.dem) because the program creates it.
3. Finally, the program asks for the two characters in positions 7 and 8 of the CELL file name. These are usually L0, so type L0 (the letter L and the number zero). Hit enter and watch the program process all 20 files into one file.

The conversion utility can be run from Windows Explorer.

Importing DEM Files with ArcView Spatial Analyst
Start ArcView GIS and load either the ArcView Spatial Analyst or ArcView 3D Analyst extension, or both. Either extension will import the DEM into ArcView GIS and create a grid. ArcView Spatial Analyst will create a two-dimensional hillshade of the gridded DEM. 3D Analyst will post the DEM in a three-dimensional view.

Maximize the ArcView GIS application on-screen and open a new view. Using View Properties, title the new view 1: Moab Quad UTM NAD27 Zone12 or something else appropriate. Choose Set Working Directory from the File menu and enter the path to the Moab data in the dialog box. Save the project.

1. Make the view active and select File, then Import Data Source from the File menu.
2. Select the import file type USGS DEM from the drop-down box.
3. When prompted for a DEM source, navigate to \UT\MOAB\DEMFILES and select MOAB1158.DEM.
4. Name the output grid MOAB1158 and store it in \UT\MOAB.
5. Answer Yes when prompted whether to add grid data to the view. ArcView GIS imports and grids the DEM.
6. Double click on the grid legend to open the Legend Editor.
7. Select Elevation #1 or Elevation #2 as the color ramp, click the Apply button, and turn on the grid.

Creating a Hillshade
Accept the defaults of Azimuth 315 and Altitude 45 or experiment with different values. After the hillshade builds, turn it on and observe a grayscale model of the heart of Canyon Country. Turn the hillshade theme off, but don't delete it from the view. Double click on the legend for the DEM theme to open the Legend Editor. Click on the Advanced button in the lower-left side of the Legend Editor. In the drop-down box for Brightness Theme select Hillshade of Moab1158 and accept the default Cell Brightness Values. Apply the enhanced legend, close the Legend Editor, and admire a 30-meter DEM of a very rugged piece of real estate. Save the project.

When creating and sizing a 3D Scene, leave a small portion of the associated 2D View visible underneath the 3D legend window. To quickly move back to the 2D View, click in the exposed legend area. Copy the desired themes up to the Theme Clipboard, then click in the gray title area of the 3D View to jump back to the 3D View. Paste the selected themes into the 3D legend window and edit their 3D properties as appropriate.

Looking at Moab in 3D Analyst
Next, load the 3D Analyst extension, if it is not already loaded, and open a new 3D Scene.

1. Using 3D Scene Properties, specify a vertical exaggeration of 2.0 and a light blue as the background color.
2. Return to the two-dimensional view, turn the grid off, and select Edit, then Copy to place it on the Theme Clipboard.
3. Return to the 3D Scene view and paste in the grid using Edit, then Paste.
4. Verify that the grid is active.
5. Open Theme 3D Properties and select Moab1158 as the Assign Base Heights From.
6. Check the box to Show Shading for Features located in the lower left of the dialog box.
7. Click OK to return to the 3D view and turn on the Moab grid.
8. Zoom in and pan around.

Summary

These procedures can be used to download, convert, and import available USGS 7.5' SDTS DEM data into ArcView GIS. The data can be modeled with ArcView Spatial Analyst and/or ArcView 3D Analyst to create very striking digital terrain. Now users can use two of the sources of free data from the EROS Web site: DLG-O and SDTS DEM data.