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  1. #1
    Global Mapper User
    Join Date
    Dec 2008
    Posts
    6

    Default Polar Stereographic Geotiff projection

    Hello

    I have a Geotiff which tags an image as Polar Stereographic. When I loaded it into GM10 at first, it displayed at 0E/W instead of approx. 78W. By contrast, the geotiff would display in a different software package okay (geomatica freeview). I found that if I added this tag:

    ProjStraightVertPoleLongGeoKey (Double,1): -78.25

    into the geotiff then GM will display the image almost in the correct position, but still out by maybe 3mm at map scale. Is there some other geotiff tag that GM expects?

    The geotiff tags are:

    Geotiff_Information:
    Version: 1
    Key_Revision: 1.0
    Tagged_Information:
    ModelTiepointTag (2,3):
    0 0 0
    -32183.4933 -930858.353 0
    ModelPixelScaleTag (1,3):
    8.46666667 8.46666667 0
    End_Of_Tags.
    Keyed_Information:
    GTModelTypeGeoKey (Short,1): ModelTypeProjected
    GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
    GeographicTypeGeoKey (Short,1): GCS_NAD27
    ProjectedCSTypeGeoKey (Short,1): User-Defined
    ProjectionGeoKey (Short,1): User-Defined
    ProjCoordTransGeoKey (Short,1): CT_PolarStereographic
    ProjLinearUnitsGeoKey (Short,1): Linear_Meter
    ProjNatOriginLongGeoKey (Double,1): -78.25
    ProjNatOriginLatGeoKey (Double,1): 78
    ProjFalseEastingGeoKey (Double,1): 0
    ProjFalseNorthingGeoKey (Double,1): 0
    ProjScaleAtNatOriginGeoKey (Double,1): 1
    ProjStraightVertPoleLongGeoKey (Double,1): -78.25
    End_Of_Keys.
    End_Of_Geotiff.

    I also saved a .prj after I'd loaded the geotiff into GM10. The .prj contains:

    PROJCS["Polar_Stereographic",GEOGCS["Geographic Coordinate System",DATUM["NAD27",SPHEROID["Clarke 1866",6378206.4,294.97
    86982139006]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Polar_Stereographic"],PARAMETER["long
    itude_of_origin",-78.25],PARAMETER["standard_parallel_1",78],PARAMETER["false_easting",0],PARAMETER["false_northing",0],
    UNIT["Meter",1]]

    I would like to give my client a geotiff that will display well in their copy of GM. I also need to generate KML using GM; I need to resolve this issue to be able to do that.

    Thanks for any help!

  2. #2
    Administrator
    Join Date
    Jun 2007
    Posts
    16,451

    Default

    The ProjStraightVertPoleLongGeoKey was the one expected by Global Mapper, but I have been able to update the GeoTIFF importer to support other central longitude keys in case the ProjStraightVertPoleLongGeoKey key is missing. I have placed a new build of Global Mapper 10 with the change at http://www.globalmapper.com/global_mapper10.zip . Simply download that file and extract the contents over your existing v10.xx installation to give it a try.

    Regarding the small position difference, I noticed the use of the NAD27 datum, which seems unusual for polar data. Are you certain that is the appropriate datum?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

  3. #3
    Global Mapper User
    Join Date
    Dec 2008
    Posts
    6

    Default Thanks

    Thanks Mike, I'm going to download the zip and try it.

    NAD27: this is quite an old data set so yes NAD27 is correct. Thanks for pointing that out t

  4. #4
    Global Mapper User
    Join Date
    Dec 2008
    Posts
    6

    Default All well

    Hello again Mike

    Well I was going to write back to say the new .exe shows the same problem of position being out by maybe 3mm to lower-right. Then I switched from the hand tool to the ruler (cross) tool to measure the offset. That's when I realised that the cross cursor lines up very well to my map grid. The hand tool position relates to the top-left of the hand, whereas I'd supposed it relates to the center of the hand !!

    So all is well, and that's good to know that other longitude keys are in there too now. Thanks for the excellent support

  5. #5
    Administrator
    Join Date
    Jun 2007
    Posts
    16,451

    Default

    Do you know which NAD27 incarnation is used (i.e. NAD27 - Canada, or perhaps one of the Alaska ones)? I would not expect NAD27 (CONUS) to be used as it is only for the continental US. Perhaps if you pick the correct incarnation of NAD27 manually in Global Mapper you will get a better conversion. Normally the plain NAD27 will do this automatically, but there isn't a specialization that I know of used at the North Pole.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

 

 

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