hillshade "terraces" in exported GM raster

emkleb
emkleb
in Raster Data
I am using the lidar extension of GM (v16) to create rasters and hillshade for geologic mapping. The hillshades and rasters look fine rendered in GM.  I'm trying to make it so the rasters will work with Arc's toolboxes since that's what the mappers use. The hillshade  seems a good test for that.

When I try to create a hillshade with the raster I exported from GM in ArcMap, I get a terraced effect, where flat areas have artificial "terraces." I figure it must be something with the re-sampling during the export, but I'm not sure. I've tried nearest neighbor, bilinear, and no interpolation so far and they all have the same issue.

Any insights would be appreciated. 

Thanks!
-ek
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  • bmg_bob
    bmg_bob Global Mapper Programmer
    Answer ✓
    Hello,

    It sounds like you are exporting an elevation grid.  Is this correct?  What file format are you exporting? Does the exported file show the terraced effect when you import back into Global Mapper?  

    During the export, be sure to select the "Elevation (32 bit floating point samples)".  Choosing the option for 16-bit samples will create the terraced effect that you are seeing.

    Cheers,

    Bob
  • emkleb
    emkleb
    Yes, I am exporting an elevation grid using Geotiff. The file does not show the terraced effect in Global Mapper. I tried your suggestion and it works! Thank you. Is it using the float points that makes the difference or the number of bits?
  • bmg_bob
    bmg_bob Global Mapper Programmer
    Answer ✓
    emkleb said:
    Yes, I am exporting an elevation grid using Geotiff. The file does not show the terraced effect in Global Mapper. I tried your suggestion and it works! Thank you. Is it using the float points that makes the difference or the number of bits?
    It is the user of floating point values that makes the difference.  When you use the 16-bit integer values, elevations are rounded to the nearest full unit, which can produce the terraced effect depending on the unit being used (i.e., if the vertical unit is meters, then you will see a terraced output, but if the vertical unit is millimeters, you would probably not notice any.)  Floating point allows for storage of the exact elevation, without rounding, at the cost of producing a larger file.

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