Summary
You can use ArcView Spatial Analyst to convert a USGS GTOPO30 DEM to grid format while retaining the digital elevation data values.Click the link in the Related Information section to download a GTOPO30 DEM. This file comes as a .TAR file, which contains the *.dem and *.hdr files. Both files are required for this operation.
Procedure
- Uncompress the TAR file using WinZip 6.2 or higher.
- Change the file extension from *.dem to *.bil. You can do this from Windows Explorer or a Command Prompt window.
- In ArcView, add the *.bil to a view as an image theme.
- Make the BIL image the active theme in the view.
- Select Convert to GRID from the Theme menu.
- Specify a name and directory location for the new grid on the Convert dialog box.
- Click OK.
- Select Yes when asked to add the grid to the view.
- Make the new grid the active theme in the view.
- Select Map Calculator from the Analysis menu.
- Enter the following equation into the expression box on the Map Calculation dialog.
Code:
([in_grid] >= 32768).con([in_grid] - 65536, [in_grid])
Note: in_grid is the grid created in steps 4-6. - Click Evaluate and close the Map Calculation dialog.
- Make the new Map Calculation Theme the active theme.
- Select Map Calculator from the Analysis menu.
- Enter the following equation in the expression box on the Map Calculation dialog.
Code:
([Map Calculation 1] = -9999).setnull([Map Calculation 1])
Note: Map Calculation 1 is the grid created in step 12. - Click Evaluate and close the Map Calculation dialog.