Importing elevation data without "webbed-feet"-like extrapolations.

sktsesktse Global Mapper UserPosts: 31Trusted User
edited August 2013 in Elevation Data
I've been trying to import a large amount of data that does not have a rectangular bounding area - it follows a winding watercourse.

This means that at the edge of the surface data, there is an acute angle, and in this acute angle, Global Mapper will try to extrapolate heights in between these areas, resulting in a "webbing" of sorts of highly inaccurate data. I have some coarser data that I would like to merge with this high-density dataset, but unfortunately, this webbing will overwrite the data. I have a picture of an example of what I mean, but unfortunately, I am having trouble uploading pictures to the forum.

I've tried to play with the "Tight/Loose" settings on the DEM import, but if I make the setting too tight, then the points in the DEM won't be interpolated in each other.

Is it possible at all to remove this feature from Global Mapper, so that the points within the coverage area are interpolated, but outside the coverage area Global Mapper does not try to extrapolate them? Unfortunately due to the sheer size of the data, it is very difficult for me to do additional processing like building bounding boxes without Global Mapper crashing (perhaps due to lack of memory).

Thanks very much for your help!

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2013
    If you have an area boundary, first grid the data as it is now with the 'webbing', then simply crop that grid on export to a new file (like a GMG) to the area boundary. Then you can load in your coarse data and the cropped GMG on top.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • sktsesktse Global Mapper User Posts: 31Trusted User
    edited August 2013
    Unfortunately I don't have an area boundary, and I can only determine the boundary based on the points in the coverage area. When i try to load the data as points to create a coverage area, it will produce the same "webbing" where on the interior of an acute angle, so unfortunately I'm not able to use that feature to generate a coverage. Is there another method that I could try?

    Thanks!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2013
    There is a new and improved gap-filling binned gridding algorithm in the Lidar toolkit in the upcoming v15 release (beta very soon). That gap filling if set low enough should fill the interior gaps very well but not fill in the external areas except very acute angles where you just really can't tell automatically if it's a bend in the data or just spaced-out data. As an added benefit the new gridding is extremely fast (almost instant for even a couple million points).

    If you want to provide your data file I can see what it does with the new gridding.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • sktsesktse Global Mapper User Posts: 31Trusted User
    edited August 2013
    Thanks so much for helping out. Is there a place that I can send the file? It's approximately 2.98 MB.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2013
    That's pretty small, you can just zip it up and post it here or email to geohelp@bluemarblegeo.com.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2013
    I got the file and took a look. In the file that you sent if you just slight the Tight/Loose slider nearly all the way to the left (less than 2) then you get just the area covered by the points and nothing outside of that. Was the problem maybe only apparent in larger files or something?

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • sktsesktse Global Mapper User Posts: 31Trusted User
    edited August 2013
    There are some other terrain datasets that we work with where the points are not spaced so regularly in the interior of the DEM. I was wondering if there was a way that one could only interpolate the interior of the polygon but not the exterior areas (i.e. outside of the boundary of the area)
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2013
    The problem is in an acute area it's pretty difficult to tell in an automated way if something is outside a data set or there are just sparse points in the area. For example in a U-shaped bend there would be points up and to the right and left of a point, just not straight up, but in a sparse data set that might be ok. If you have one of those (just the points around the bend) I can see if the new gap-filling algorithm for Lidar handles it better, although very near the bend you will still get points. The gap filling is much smarter though and uses an inverse weighted distance algorithm.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
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