Digital Elevation Model (DEM) Terrain Displacement

Now that you have all of your DEMs downloaded you can extract the files. The *.gz file format is simply a compressed file that WinZip or WinRar should be able to extract with no problem. When extracting the files I tend to use the "Extract To" method and accept the default sub folder name. This is why you want to append the quad name to the beginning of the files when you are downloading them. That way you are left with a subfolder structure that is easily recognizable when browsing your project. When you extract the file you should have about 18 *.ddf files and a README for each quad.

NOTE: Occasionally I've ran into an issue where I download the quad *.gz file and when I extract it there is only one file inside. I have not yet figured out what causes this, but usually re-downloading and extracting the file again will fix it. You may need to try downloading from a different machine. That's usually what I do.

Now you are ready to start converting all of these files to something that Photoshop can read. For this we will be using a program called PullSDTS. In order to use this program you must have the Python plug-in installed on your machine. Launch PullSDTS and click the [Select File] button. Locate the first quad folder and you will note that only one *.ddf file is listed. Select and load that file. Now click the [Copy] button to copy the header text to the clipboard. Launch a text editor like notepad and paste the text into the new document. Repeat this process of copying and pasting the headers into this new document. This header information makes up what will be our rangeStatistics used in the conversion process. Information such as elevation, quad size, and world coordinates are in this file. I've attached a copy of my completed text file here for you to download. What you need to do is scan through all 15 headers and determine what the lowest (min) elevation is and the highest (max) elevation for our entire range. You also will need to figure out what the Northwest and Southeast coordinates are for later. We will use the east and west coordinates to determine how wide our range is, too. I've done this for you in the text file and added this to the top of our rangeStatistics file. After you get all of this required information, it's now time to convert the data. Go back into PullSDTS and click the [Select File] button again. Re-load our first quad, then click the [Load DEM] button. You will now see a greyscale image of that quad in the left side window. Click the [Save RAW] button.

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