Global Mapper v25.0

Gaps between tiles of exported DEM data

GeraldFord
GeraldFord Global Mapper User
edited October 2014 in Elevation Data
I have 10m DEM data for an area n the west coast and I wanted to export out 'tiles' of the dem because I have a very large area to cover and the overall data size becomes unamanageble if I try to do it in one export.

In global Mapper, I use the Digitizer tool to create the tile areas as 20,000m by 20,000m (a square tile). When I export the data, Global Mapper reports the data to be approx 8.386xx meters by 10.386xxx meters in resolution. I select export out square samples, and select it to fill holes and gaps in the data.

But when I export the DEM and import it into another application (in my case Autodesk 3ds max), my tiles are not quite square, and I have gaps between the tiles. This doesnt happen when I export out raster images of ortho texture for the same area, those are perfect.

Why do these gaps occur, and is there any way I can get perfect fitting square tiles of DEM out like the textures?

Thanks for any help ;)

Comments

  • global_mapper
    global_mapper Administrator
    edited March 2012
    What format are you exporting to? If you load the tiles back into Global Mapper do they have gaps? You really don't need to create the tiles with the Digitizer Tool prior to exporting, you can simply use the Gridding tab of the export options dialog to split the export into pieces of the desired size automatically.

    One thing to check is the option to maintain export bounds over sample spacing in the Advanced Section of the General tab of the Configuration dialog. You might want to check that so you don't possibly have any issues caused by your sample spacing not being an exact divisor of your export bounds.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • GeraldFord
    GeraldFord Global Mapper User
    edited March 2012
    Hi Mike, and thanks for responding.

    Im exporting out to USGS DEM format, as it appears to be the flavor more agreeable to 3ds max 2011.

    Ill try using the gridding tab to see if it make a difference. I used the digitizer tool for each grid only because I wanted to custom name the grid cells in the control center, whereas if I used the gridding, I dont see the 'grids' in the viewport.

    I do believe it has something to do with the non-symmetrical sample spacing, honestly Im confused as to why my 10m terrain data doesnt actually come in at 10mx10m. Ill try the maintain export bounds and see if that works, ill post back and let you know.

    Thanks!
  • GeraldFord
    GeraldFord Global Mapper User
    edited March 2012
    So I found that enabling Maintain Export Bounds did not fix the issue, the gap is still present.

    When I enabled the gridding option for a 20k x 20k (breaking it into 4 sub tiles), 3ds max will not import the dem saying "improper file format". I get this problem alot when i try resampling the dem into a different resolution. It does appear to be some sort of sampling divsor problem but Im at a loss as to what to do about it.
  • global_mapper
    global_mapper Administrator
    edited March 2012
    It could be that 3DS Max only likes DEM files that have certain resolutions. For the gaps, do they show up if you load the files back into Global Mapper or are they only in 3DS Max?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • global_mapper
    global_mapper Administrator
    edited March 2012
    Oh also what projection do you have setup on the Projection tab of the Configuration dialog? You can't get an exact size in linear units like meters if using an angular projection like Geographic (lat/lon). If you switch to UTM then you can provide your sample spacing exactly as meters.
  • GeraldFord
    GeraldFord Global Mapper User
    edited March 2012
    The problem appears to be in exported .ter files intended for Terragen as well.

    We are using UTM as our projection method.

    When I load the dem back into Global Mapper, everything appears correct - there is no gap.
  • GeraldFord
    GeraldFord Global Mapper User
    edited March 2012
    I should also note that we then loaded the dem into Autodesk Map 3D and the gap is there also.

    So it leaves me wondering if global mapper is writing some unusual flavor of dem that only it understands? Or alternatively, is this some kind of common setting that is needed gto be sure its interpreted correctly (this is hard to expect acoss several different apps) but so far I have not found an app that does not show the gap.
  • global_mapper
    global_mapper Administrator
    edited March 2012
    The DEM export is actually one of the oldest in Global Mapper and hasn't been changed in many years. It was originally written for the USGS to match their specifications, but I've found many applications have different interpretations of the DEM specification. I'm guessing these other applications are creating a mesh from the actual elevation samples in the DEM file rather than treating each sample as a grid cell with the elevation at the center, which would result in a half-sample less coverage than Global Mapper which treats the elevations as the grid cell center and renders them with a half-sample spacing of data of that elevation around each cell.

    To get around the different interpretation of a DEM grid surface you could use the Overlap section on the Gridding tab and specify 1 pixel of overlap. Another option would be to export a 3D DXF Face file which actually creates a full rectangular face for each grid cell so then it covers the cell how Autodesk products treat elevation surfaces.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • GeraldFord
    GeraldFord Global Mapper User
    edited March 2012
    Hi Mike,

    I tried the 3D DXF Face File export, but again the gap is still there if I try to export each tile individually..

    When I try using the gridding options, the resulting DEM files are unreadable by 3ds max. However the gridding option in combination with 3D DXF Face File and 0.1 percent overlap does appear to create tile which butt up against each other. However they are not square which confused me a bit (since my original tile is 20000m by 20000m).
  • global_mapper
    global_mapper Administrator
    edited March 2012
    What projection are you using for your export? You set that up on the Projection tab of the Configuration dialog prior to export. Something that is 20000m x 20000m in one projection might not actually be square on the ground due to the inherent distortion in projections. I'm not sure what type of reprojection support that 3ds max has.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ai3d
    ai3d Global Mapper User Trusted User
    edited October 2014
    I have been experiencing the similar error message when importing DEMs to max "improperfileformat". Now when i find a tile that doesnt import properly i open it and save it in 3DEM and it imports fine into Max. This suggests that the problem could be a Global Mapper issue.

    I can send the 2 different DEM files over if you want, one that works and the other that doesnt.


    Also the problem seems to occur most at the last tile in a row across.