Global Mapper v25.0

Elevation Export Issues

sshak
sshak Global Mapper User
edited March 2012 in Technical Support
When I export a section of an elevation data file (gridfloat) some kind of interpolation and skewing is occurring such that if I load the exported data as an other layer in the view, a visible box appears about the smaller section and the subset appears filtered. I've looked for every option I could find to disable any automatic interpolation, filtering, I've even done the best I can to ensure that my export bounds lie on data-points in the original file and not in between them. I am keeping the data format in GEO WGS84.
I have attached two screenshots from within globalmapper, elev_original is just the original dataset, zoomed in on a corner of the exported data bounds. elev_with_exported has the exported file overlaid on the original dataset. You can see the lines about the edge, and the slight additional fuzziness within the dataset.
elev_original.jpgelev_with_exported.jpg
Thanks for anyone's help in correcting this, as I need the data to maintain alignment with the original.

Comments

  • global_mapper
    global_mapper Administrator
    edited March 2012
    The edge is likely a render effect and not a change in the actual data. The smoothing and shading only occurs within a layer and doesn't cross layer boundaries, so you will see an edge in hilly areas where the angle of the terrain changes due to hitting the edge of the layer. You could do a combine terrain operation (File->Combine Terrain) to make sure there aren't any actual differences.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
  • sshak
    sshak Global Mapper User
    edited March 2012
    I get the impression based on what work I do after exporting the files that some change is going on. I would prefer it not to smooth any of the data. Here's the result from doing the combine terrain, which is showing up to +/- 3 m alterations going on.difference.jpg
  • global_mapper
    global_mapper Administrator
    edited March 2012
    If the bounds of your exported area do not line up perfectly with the original samples then you will get an interpolated value based on where the new grid cell centers actually do end up. You can make your exports automatically align with the original samples if possible (i.e. no reproject or resampling, just a shift) by checking the option in the Advanced Section of the General tab of the Configuration dialog labelled 'Export: Snap Export Bounds to Pixel Boundary if Possible'. You can also disable any type of resampling on the Options dialog for the elevation layer prior to export. Just set the resampling to 'Nearest Neighbor'.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • sshak
    sshak Global Mapper User
    edited March 2012
    Thanks for your help with this matter. Those options have mostly corrected the issue. Is there a way to not have the layer shift at all? While for extracting a small area these options should allow it to work, there's a related bit of work that I just retested with these options and am having a similar issue.
    If I take a set of elevation data and grid it into smaller files, I get a seam where each edge is at. I have also tried with a bit of overlap (1%), but it just moves the seam to the offset position. I've tried to make sure that my grid divisions would end on a data value ant not in between, but that doesn't seem to make a difference.
    You can see this a as a vertical line in the image, almost centered halfway.
    seam.jpg
    seam.jpg 544.7K
  • global_mapper
    global_mapper Administrator
    edited March 2012
    Even if your output aligns directly on sample, because the hill shading and display smoothing in Global Mapper don't cross layer boundaries perfectly you can still see some edge effects like that at tile edges. It has nothing to do with the data itself but is just a render effect because the hill shading doesn't cross tile boundaries. If you turn off hill shading and resampling those lines will be gone, but of course you will then be able to see individual sample cells as you zoom in.

    Thanks,

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