What is an ASCII File?
ASCII files are files converted to a standardized numeric code. ASCII code is universally recognized across platforms and computers and is thus a common format for transferring files. ASCII files are designed to be useful to different end users employing a variety of different computer systems and applications.
Some WAGDA data - in particular the 10M DEMs covering Western Washington - are compressed and stored as ASCII files for you to download and use in ArcInfo or ArcView.
The first step in obtaining a useful DEM stored in ASCII is to locate the particular quad map you're interested in. Use the index provided for the 10M DEMs to identify your quad via the map or the searchable list.
What do I do once I've located my file?
Click on the link that bears the name of your file. Your web browser prompts you with a note that you're downloading a file. Select "OK and direct the file to a local directory to which you have access. The browser will quickly download the compressed file to your directory.
Navigate your machine to the directory where you've placed the file. Like most of the compressed files you find on WAGDA, you'll see that the new file is compressed in a Winzip format. If you don't already have the Winzip utility to extract the compressed file you can obtain it here.
Double click on your file. This should launch Winzip. You are presented with an open archive with the name of the ASCII file you downloaded in the archive. Click on it once so it's selected and choose "Extract" from the buttons on the Winzip menu.
Winzip prompts you for the directory where you want to extract the file to. Navigate to the appropriate location and click "Extract". An ASCII file by the name of your downloaded file will appear in your directory. It carries a <.dem> extension.
How do I import the file into ArcView?
Launch ArcView. Be sure Spatial Analyst is running. Open a new View and then go under "File" and choose "Import Data Source". ArcView will prompt you with a window requesting you select the file type. In this case, choose "USGS DEM".
ArcView then offers you a browser window to navigate your local directory. Navigate and select the ASCII file you've just uncompressed. ArcView then prompts you to choose a location and a name for the new grid it will create from the ASCII file. Name the DEM with a title that identifies the region it represents and place it in an accessible directory.
ArcView will read in the new grid. This may take a few moments. It will then ask if you want to add the new grid to your view. Click "Yes" and turn on the new grid in the Table of Contents.
Launching Spatial Analyst: Spatial Analyst is an ArcView extension that permits you to manage grid files. To be sure it's running, go to the Project Window (the one with the icons representing "View", "Table", "Charts" etc. on the left). Under "File" Choose "Extensions". Scroll the list of available extensions and place a check next to "Spatial Analyst".
How do I import the file into ArcMap?
Launch ArcToolbox. Go to Conversion Tools>Import to Raster>DEM to Grid. A pop-up box will appear. Navigate to the DEM file you want to convert in the “Input DEM file” box, and then in the “Output grid” box navigate to where you would like the grid to be stored and give it a new name. Click “OK”. Add the Grid to a view in ArcMap as you would any other theme by clicking the “Add Data” button and navigating to the desired file and clicking “OK”.