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  1. #1
    Global Mapper User
    Join Date
    Nov 2007
    Posts
    5

    Default Questions on working with elevation data and raster data from different sources

    Hi all,

    I am using GlobalMapper to generate raster data (in png format) to build
    scenery for the Glider simulator "Condor The Competition Soaring Simulator".
    The tools used to do scenery development have there own issues, but
    discussion of them don't belong here.

    This involves using SRTM3 .HGT files for elevation data from
    ftp://e0srp01u.ecs.nasa.gov/srtm/version2/ and raster data from
    a variety of sources.

    So far, I've used been successful combining Landsat false color
    images from https://zulu.ssc.nasa.gov/mrsid/.

    I can load the .SID files and the .HGT files into GlobalMapper.
    The raster data and elevation data line up perfectly.

    On the other hand, the Canadian government provides complete landsat data
    as in GEOTIFF format including good old RGB (seperate images for each channel)
    http://geogratis.cgdi.gc.ca/download...o/geotiff/utm/.

    When I load the geotiff files with the .HGT files into GlobalMapper
    they don't line up. I think the issue is that the geotiff files are
    in UTM NAD83 projection (to be honest, I don't know what that means).
    On the other hand, the raster data from https://zulu.ssc.nasa.gov/mrsid/
    are UTM WGS84.

    So, here is the question. How can I change the projection, or whatever I have
    to do to get the HGT files and the GEOTIFF files to line up.

    Thanks for your help!!!

  2. #2
    Administrator
    Join Date
    Jun 2007
    Posts
    15,865

    Default

    The projection should not be an issue. Global Mapper takes care of that automatically. In any case, WGS84 and NAD83 are coincident to well under a meter, so that is no difference.

    Which data were you loading from the Canadian site? I downloaded some TIFFs from http://geogratis.cgdi.gc.ca/download...ff/utm/005024/ and they automatically loaded and lined up perfectly with other data, so they are correctly positioned. Perhaps there is something different with the files that you downloaded though.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

  3. #3
    Global Mapper User
    Join Date
    Nov 2007
    Posts
    5

    Default

    The files I am having trouble with are:

    N43W075.hgt
    N43W074.hgt
    N44W075.hgt
    N44W074.hgt

    from ftp://e0srp01u.ecs.nasa.gov/srtm/ver...North_America/

    and

    014029_0100_010608_l7_08_utm18.tiff

    from http://geogratis.cgdi.gc.ca/download...ff/utm/014029/

    These two data sources align well near the northwest corner of the tiff
    file but they become gradually mis-aligned the further southeast
    you go.

    On the other hand, these hgt files align almost perfectly with:

    N-18-40_2000.sid

    from https://zulu.ssc.nasa.gov/mrsid/


    When I try to work with the geotiff and hgt files, I simply load the hgt files first
    then load the geotiff files. Is there something else I need to do?

    The area I am interested in is around Lake Placid (plus or minus 35 kilometers).
    The coordinates of Lake Placid are 44.282, -73.983.

    I just tried loading the .sid and .tiff files together and setting the .tiff to
    50% transparency. I can clearly see the difference between the two.

    Thanks again!!

  4. #4
    Administrator
    Join Date
    Jun 2007
    Posts
    15,865

    Default

    I took a look and could indeed see the offset in the Southern part of the map from the Canada site. The portion of the map in Canada looks fine though. I'm guessing that the Canadian site used some datum grid shift file for Canada when they generated their data, and that file did not have a useful conversion outside of Canada, resulting in the data becoming inaccurate outside of Canadian borders.

    If you are just looking for imagery of Lake Placid, I would suggest using the File->Download Online Imagery menu command in Global Mapper and using the TerraServer DOQ source. It should be of much higher resolution than the Landsat imagery anyway.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

  5. #5
    Global Mapper User
    Join Date
    Nov 2007
    Posts
    5

    Default

    Thanks for your help, again.

    Can you tell me if there are any restrictions on the non-commercial use
    of these images available for download from within GlobalMapper?

  6. #6
    Administrator
    Join Date
    Jun 2007
    Posts
    15,865

    Default

    There are not any restrictions on use of the DOQ imagery that you can download. It is public domain USGS stuff.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

 

 

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