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changing DEM projections

paulojulpaulojul Global Mapper UserPosts: 6
edited October 2010 in Technical Support
Hello,

I have a DEM with a Transverse Mercator projection/Nad83 datum but I need it to be in UTM Zone -2 projection/Nad83 datum.

I changed its projection under Tools -> Configuration, and also under Options via the Control Center. I then exported it as a DEM under a new file name, but when I open that new file in GM it seems to keep its original projection (I compared it with a data set that uses the projection that I need it to be in).

Do you have any tips/ advice on tackling this problem?

Thanks,
Julius

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Julius,

    You only want to change the projection using the Tools->Configuration dialog. If you also change it on the Options dialog you are basically re-interpreting the coordinates rather than reprojecting them, which won't do anything to them.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulojulpaulojul Global Mapper User Posts: 6
    edited October 2010
    Hey Mike,

    Thank you for replying.

    I am still getting the same problem. Here's my process in more detail. Maybe you can see what I am doing wrong.

    1. Open DEM
    2. Go to Tools-> Configuration
    3. Change the projection to UTM -2/Nad83
    4. Export elevation grid format (as a DEM, also drew export bound box)
    5. Close all overlays
    6. Open recently saved DEM
    7. Compare with other data as a check.

    The image shows how the data sets are not lining up properly even though they are using the same projection.

    ruxkyc.jpg

    -julius
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Julius,

    Does your data line up with the other data prior to exporting, but then when you export to a new DEM file then load it back in it no longer lines up properly? Or does it maybe not align with the other data even before export, which would indicate an issue with the initial projection setup of the data prior to export?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulojulpaulojul Global Mapper User Posts: 6
    edited October 2010
    Hi Mike,

    The image shows what the 2 DEMs look like before and after the smaller one was changed. There were no visual differences. However if you check under options, GM shows that the smaller DEM (the one that was reprojected) is now in UTM (used to be transverse mercator), which is the same projection as the bigger DEM. However, they do not line up at all even if they are now both under the same projection.

    -julius
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Julius,

    If the files didn't line up to start with then the projection selected when you loaded the original DEM was not correct. Reprojecting won't change the location of the DEM on the earth, just change the coordinate to a different projection system. What projection system did you select for the mis-placed DEM when you originally loaded and are you sure that is correct?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulojulpaulojul Global Mapper User Posts: 6
    edited October 2010
    Mike,

    When I first loaded the DEM, I left it in its default projection. Checking under the options tab, that's where I found out the it was under the Transverse Mercator projection.

    -julius
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Julius,

    Were you prompted to select a projection and just accepted the default (which would just be the projection of whatever is already loaded and not necessarily the correct one for the file) or were you not prompted at all?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulojulpaulojul Global Mapper User Posts: 6
    edited October 2010
    Mike,

    I opened the DEM without anything being loaded first, and it did not prompt me to select a projection at all.

    -Julius
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Julius,

    That means that the DEM file has embedded projection information, but it's possible that information is not correct. Is it possible for you to post the DEM file here or email it to support@globalmapper.com so that I can take a look?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulojulpaulojul Global Mapper User Posts: 6
    edited October 2010
    Sure thing Mike, I'll email it to you. Thanks.
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