Exported DEM mismatch

TibTib Global Mapper UserPosts: 9
edited August 2012 in Elevation Data
Hello,

I'm using Global Mapper with DEM data (16 bits BIL files) and I export a subpart of these data (by drawing a box) in another BIL file to make some operations on it (with another software).

The BIL file that I'm exporting does not exactly match the original file.

sshotGlobalMapper.png

I can see some gaps around the border in atlas shader (cf attached file) and height values can differ from 10 cm to nearly 1 meter from original data.


It's a problem for me because all the data are then used together and this border appear in final result.

It does not look like an offset problem, maybe this is a kind of rounding error?

Is this a normal behaviour? Any ways to avoid it?

Thank you!

Best regards

Thibaud

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    Thibaud,

    The most likely cause of the difference is if the samples/pixels in your output BIL are not aligned perfectly with those in the input BIL. There is an option in the Advanced section of the General tab of the Configuration dialog to have your output pixels snap to the same boundary as the input pixels if possible. If you check that you will likely eliminate any sub-pixel offset issues.

    Let me know if I can be of further assistance.
  • TibTib Global Mapper User Posts: 9
    edited August 2012
    Hello again,

    Thank you for the answer, it look like it's working!

    I have no more gaps between original DEM and exported one (at the border).

    But the two DEM still look like a little bit different (cf attached screenshots), is there another option that I should activate?

    Thibaud

    screenshotGM1.pngscreenshotGM2.png
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    I can visually see the differences, but that could be because the elevation shader is stretched to the loaded range so if you load a file with a different range independently it would look different even if the values are the same. To really check the differences load both layers into Global Mapper and use File->Combine Terrain to create a difference surface to see if it is different. If the output file is perfectly aligned with the input and uses the exact same resolution the difference surface should be 0 everywhere.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
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