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How To: Convert GTOPO30 DEM data into a grid

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

  1. Uncompress the TAR file using WinZip 6.2 or higher.
  2. Change the file extension from *.dem to *.bil. You can do this from Windows Explorer or a Command Prompt window.
  3. In ArcView, add the *.bil to a view as an image theme.
  4. Make the BIL image the active theme in the view.
  5. Select Convert to GRID from the Theme menu.
  6. Specify a name and directory location for the new grid on the Convert dialog box.
  7. Click OK.
  8. Select Yes when asked to add the grid to the view.
  9. Make the new grid the active theme in the view.
  10. Select Map Calculator from the Analysis menu.
  11. 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.
  12. Click Evaluate and close the Map Calculation dialog.
  13. Make the new Map Calculation Theme the active theme.
  14. Select Map Calculator from the Analysis menu.
  15. 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.
  16. Click Evaluate and close the Map Calculation dialog.

Related Information