Instructions for Downloading and Processing USGS SDTS DEM Files

Dean Wallraff, Chair, Angeles Chapter GIS Committee deanraff@arsnova.org

The USGS provides high-resolution (10-meter) Digital Elevation Model files in a format known as SDTS (Spatial Data Transfer Standard). Each of these corresponds to a USGS 7.5-minute quad and contains an raster of elevation values for each point within the quad, spaced in a 10-meter array.

Since ArcMap can't deal with these files directly, we want to convert them into ESRI grid files, with the elevations in meters and a UTM 1927 Zone 11N projection.

Before You Start

You need to get set up first, with the following:

• ArcView Installed on your desktop, with the ArcView 8 tools available. In ArcCatalog, select View-> Toolbars -> ArcView 8x tools and dock the little “Conversion Tools” window somewhere up top. This is where you’ll find the “SDTS Raster to Grid” tool referenced below.
• A decompression program capable of decompressing .tar.gz files (this is a unix data compression standard). Winzip works fine. You can download a free trial from www.winzip.com. There are other programs available as well. If you already have one installed, use that. The native .zip file handling on Windows XP will not serve this purpose, however.
• Set up two directories (folders) on your machine:
  • Source directory, where you'll download the SDTS files and create temporary folders to hold their unzipped contents
  • Target directory, where you'll create the final ESRI Grid files

Step 1 - Download Some Files

• Get the names of some quads in LA and Orange Counties to process from me.
• Surf your Web browser to www.mapmart.com, click on "Digital Elevation Model" on the right.
• Under "USGS SDTS Digital Elevation Models (DEMS)" click on "Begin Search" in the "High Detail" category.
• Select "DEM" on the left navigation strip, then click on the Los Angeles area several times until the "Begin Ordering" button appears at the top. If your browser tells you that pop-ups are blocked, turn the pop-up blocker off temporarily. Click "Begin Ordering" at the top.
• At this point, the quads should be labelled in faint black slanted lettering. Click on the quads you want (that I assigned to you) to put them into your order basket.You may select up to 10 of them in one order, and there will be no charge.
• When you are done selecting, click the "Check Out" button to be taken to the order page. For each quad, select the 10-meter resolution and meters as the units (these are horizontal resolution and vertical units), leave the rest of the top part the form along (SDTS format, UTM projection). Fill in your contact information below, leaving "FTP Download" set as the delivery method, then click "Submit Order."
• The Website will respond with a screen showing a download link for each quad, and a link to download a file named filenames.txt that will translate the arcane names of the files you will download into quad names. Download this file first into your source directory. Then download each of the SDTS quad files by clicking on the Download link at the right of the table, then :"Save" then selecting your source directory. Don't rename the file when you download it.

Step 2 - Unzip and Convert the files. For each SDTS file you downloaded in the step above, do the following:

• Unzip all of its contents into a new folder on the source directory. The easiest thing is to give this directory the name of the quad you're working on (use filenames.txt to translate if you don't remember). The steps to do this with Winzip are:
  • Run Winzip, click on "Open," select your downloaded SDTS file whose name ends with .tar.gz
  • Say "Yes" to "Should Winzip decompress it to a temporary folder and open it?"
  • Click on "Extract" and then select a new folder within your source folder as the destination. I recommend you give it the same name as the quad you're working on. Click "Extract."
• Convert to ESRI Grid by doing the following:
  • From the "Conversion Tools" (i.e. ArcView 8 tools) menu you enabled above, select the "SDTS Raster to Grid" tool. In the dialog box that opens, click on the button to the right of the "Input Prefix" box and navigate to the folder you created when you unzipped the archive in the previous step. When you double-click on that folder name in the open dialog, one "prefix" should appear in the window on the right in the open dialog box. Select that number, then click "OK." That selects your SDTS data for input.
  • In the Raster to Grid dialog box, click on the button to the right of the "Output Grid." field. In the dialog box that opens, navigate to the target directory where you want to create the grid file, enter the name of the grid you want to create (which should be the same as the quad name), then click "Save."
  • Finally, Click "OK" to do the conversion.
  • I find that it often doesn't work right the first time, but that I can always get the conversion done by repeating it. So check the grid by previewing it in ArcCatalog. If it doesn't display right, then delete the files for this quad in your target directory with Windows Explorer, then delete the related files that show up in ArcCatalog using ArcCatalog's delete. These are separate and I don't know where ArcCatalog stores them.
  • When you're done, you should have a directory named with your quad's name (it will contain several files), and two other files: a <quadname>.rrd and a quadname.aux>. I will need all of these.

Step 3 - Send Them To Me. Each quad takes about 6 Mb, so they're a little large to email en masse. There are two good methods:

• FTP: Zip them into a single file (taking care to preserve the directory structure), and then upload them to our AngelesGIS.org ftp server. Please contact me to get the ftp login information.
• Send me a CD: If you do this, please don't zip them, just copy your target directory to the CD with its directory structure intact. Mail to me at: Dean Wallraff, 10211 Sunland Blvd., Shadow Hills, CA 91040