Changing the elevation of a DEM in a map catalog.

chris11chris11 Global Mapper UserPosts: 3
edited November 2013 in Technical Support
I have about 100 DEMs with elevation data in cm that I am putting in a map catalog. Unfortunately I am not getting a good result. Global Mapper 14 assumes most of the DEMS have elevation data in meters. This would not be a problem, but it puts the elevation data of one or two DEMs in cm. So the result gives a catalog that has a drastic drop in elevation for one base DEM.

I would like to change the elevation for one single DEM in the catalog to match the others, but I can't seem to do this. How is this done?

Comments

  • tjhbtjhb Global Mapper User Posts: 454Trusted User
    edited November 2013
    That might be possible, I don't know. (Maybe by editing the catalog file itself, in a text editor?)

    But maybe it would be a more promising strategy to find the source of the problem. Why does GM assume metres—and only in most cases?

    To figure that out, it would be necessary to know what format the source DEMs are in.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2013
    If you go to the Alter Elevations tab of the Raster Options dialog for the map catalog you should be able to override the elevation units interpretation. However Tim is right, it might be best to figure out why the elevation units were wrong to start with. What format are the files?

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • chris11chris11 Global Mapper User Posts: 3
    edited November 2013
    Thanks, I did notice the option to change the elevation, but it did not help me, since I think that changes the elevation for every single DEM. I think it is a data problem. I loaded up the problem DEM, and Global Mapper assumed it is in meters. But the elevation scale looks correct for the county. I then loaded up another neighboring DEM, and it is assumed to be in meters too. This time the elevation data is actually in centimeters, so the scale tops out at around 25,000m. The readme files both say elevation is in centimeters. So my guess is that one DEM's elevation values was accidentally put into meters instead of centimeters.

    The DEM format is Erdas Image File, .img.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2013
    Are you prompted for a projection when loading the centimeter Erdas .img file? If so that dialog should also have a prompt for elevation units. The .img file itself unfortunately doesn't store elevation units in a standard way, so if anything unusual is used it won't come in automatically. Is it possible to maybe batch convert the files to something like a Global Mapper Grid format where the units are stored so you can correct them once, then build a map catalog of that?

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • chris11chris11 Global Mapper User Posts: 3
    edited November 2013
    GlobalMapper only prompted me for a projection on about two of the one hundred files. Neither one was the problem .img file. What I ended up doing was loading the problem DEM, and used the elevation options and scale factor to change the units to centimeters. I then loaded up a neighboring DEM to make sure that the elevations were being interpreted correctly and exported the DEMs to a new image file. That seemed to work. I was able to use that in the map catalog, and the elevation units look uniform.
Sign In or Register to comment.