Georectifying Greenland Figures

EDHEDH Global Mapper UserPosts: 6
edited March 2013 in Technical Support
I have been trying to georeference topographic maps of Greenland, but always end up with figures that are stretched along one axis. I have tried all of the coordinate systems that I think might apply, but nothing works. Is there some special setting(s) for projections so far north?

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    I would try using a UTM projection with an appropriate zone, or perhaps just an Orthographic centered near the center of your map. You can still enter the lat/lon points and let the rectification dialog convert them to your selected projection.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • EDHEDH Global Mapper User Posts: 6
    edited March 2013
    Mike, thanks for your suggestions. I got the figure rectified, but have been unable to open ASCII files with sample locations (two columns with decimal lat and long - this format has worked for me before). I have tried saving the excel file as CSV, CSV(MSDOS), Text(TAB delimited) and a few others, but the files won't load. Any suggestions? Thanks again.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    Do you still have the file open in Excel? Excel holds the files open so Global Mapper can't open them, so you have to close the file in Excel first before anything else can use them.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • EDHEDH Global Mapper User Posts: 6
    edited March 2013
    Mike, again thanks. I listened to the video tutorial a few times, but missed the obvious. I successfully populated the rectified figure with the sample locations, but noticed that the northing values on the figure are incorrect. When I rectified the figure, I used two east-west oriented points. The figure completed, but I did not check the displayed northing values closely enough to notice the error. The easting values are what they should be, but the northing values are off by half a degree. I am out of ideas, do you have any other suggestions?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    You should definitely use more than 2 points to rectify the image, and if only 2 you should put them in corners if possible. If they are along the east-west axis it will be very easy to get large errors in the Y axis. I would suggest using 4 or more points in general.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • EDHEDH Global Mapper User Posts: 6
    edited March 2013
    Therein lies the problem. When I input a 2-dimensional set of points (box-shaped)GM_Created File.jpgGM_Original Data.jpg, the resulting figure is always distorted either north or south. I used all 6 points in the accompanying "original" figure and got a skewed result.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    What projection do you have selected in the rectification dialog? I would use all 6 of the known corner points, or at least the 4 most separated ones, although if you have any control points around the edges those are better. Getting a projection selected that is at least close to what the map is originally in will allow you to get much better results with fewer control points.

    Sometimes you might also want to open the Options menu on the rectification dialog and force the rectification method to be Polynomial rather than switching to Triangulated (Piecewise Affine) as the latter can sometime look strange outside of the known control points if there is a lot of distortion.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • EDHEDH Global Mapper User Posts: 6
    edited March 2013
    I tried;
    Projection = Geographic
    Datum = WGS84
    Planar Units = ARC DEGREES
    Rectification Method = all 6 listed in "options"
    Resampling Method = all combinations of the 2 listed in "options", with the rectification methods

    Nothing seems to work.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    Try switching your projection to UTM or maybe an Orthographic centered near your map, or maybe even Mercator. Geographic will be extremely distorted at that latitude so it is very unlikely the original map is in that projection. You can still enter your control points as lat/lon. Global Mapper will detect that and prompt you to reproject to the new selected projection.

    Actually looking at your screenshot maybe first try Mercator. The lat/lon lines look orthogonal as they would be for Mercator, but they certainly aren't sized the same in each direction as they would be for Geographic.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • EDHEDH Global Mapper User Posts: 6
    edited March 2013
    Mike, thanks again for your help. I eventually got a useable figure using "Mercator". Longitudinal distances remain constant, but latitudinal distaces vary from the equator to the poles. Why is the distortion that I have been trying to overcome not a problem when using a "Geographical" projection in Nevada, for example? Thanks again.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    The distortion in longitude varies with the inverse of the cosine of the latitude. So there is none at the equator, then it starts slowly gets worse as you move from the poles. As you approach the poles (i.e. in Greenland) the distortion quickly becomes severe as it approaches infinite distortion. At the latitude of Nevada it's not bad, at Greenland it is quite severe.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
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