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Export DEM in .dem File Format

rreedrreed Global Mapper UserPosts: 30Trusted User
edited September 2013 in Technical Support

I've displayed a number of DEMs that had various input formats, e.g. Lidar, Ascii Grid, etc. When I use the export function to create a new DEM in the USGS .dem file format, the elevation values in the created file are truncated (rounded?) to an interger value, i.e. 4532.3234 becomes 4532. To confuse the issue more, when you open the created file in GM, the "elevation tracker" continues to dispaly elevations with decimal precision, but if I open the file in ArcGIS, the "pixels" only have interger precision.

I do not believe this to be an ArcGIS issue because I've re-exported the .dem file to an ascii grid file format and reviwed the ascii file format elevation values in Excel - the elevation values in the .asc file are truncated to an interger. However, if I "create" the new DEM as an ascii grid or a TIFF originally, and not use the .dem file format, the elevation value precision is retained (I've confirmed this via reivew of the .asc elevation values in Excel, as well as verified the precision in ArcGIS).

I have looked for a user input field that would drive the output to an interger value, but I don't see one. Is this a user input issue, or is interger outpu the standard precision for a GM .dem file format. As a note, the USGS NED DEMs do have greater precision than an interger.

Thanks for your help.



  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2013

    The USGS DEM format stores elevations as integers. You are seeing decimals in GM when you mouse over because by default bilinear interpolation is enabled for elevation values, so you are seeing a blend of the 4 adjacent elevation values depending on where your cursor is. If you change resampling to nearest neighbor on the Options dialog for the DEM you can see the raw values.

    You can change the units to something more precise like decimeters or decifeet (centimeters usually overflows what DEM can handle), or use a different format, including Arc ASCII Grid or one of the grid formats that uses full floating point, like 32-bit GeoTIFF.

    Let me know if I can be of further assistance.


    Global Mapper Guru
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
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