Global Mapper v25.0

“Slipping” ETM+ images

Eric G
Eric G Global Mapper User
edited July 2009 in Technical Support
Good day.

I encountered several times issues with Lansat ETM+ images I use for drawing shorelines.

I work at a scale of about 1:12,000 to draw the shorelines (in GM10, with the anti-aliasing of the ETM+ image checked), and several times, when I move the area of display using the arrows to draw the next part of the shoreline, I notice that the image is misplaced from its previous position (previous screen) of 3 to 5 pixels: the section of the shoreline I just draw doesn't match anymore with the image's one. That's a problem as I don't know anymore which position of the image is the correct one and as it makes drop the resulting scale do to reliability issues.

Why ? Do I work at a too large scale which introduces some “slipping” of the image when navigating in it ?

Thanks.
Eric

Comments

  • global_mapper
    global_mapper Administrator
    edited July 2009
    Eric,

    That is an odd issue. I would actually guess that it is the shoreline vector that is "slipping". Is the vector particular large relative to your zoom level and is it being reprojected? If your vectors get huge relative to your zoom level they are cropped for display. Typically there is no visible change, but if they are being reprojected the endpoints of reprojection would be different so new apparent positions could be affected if the native and view projections are different enough.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Eric G
    Eric G Global Mapper User
    edited July 2009
    Thanks for the answer, Mike.

    I don't know what would be huge for you. In my case, this slipping appears generally even with one to two thousand vertexes, which isn't particularly big. But yes, most of the time the UTM Landsat images are reprojected as I import them in my projects which are in Lat/Lon when I work on them.

    So if I understood well, the best would be to open the ETM+ images in their original projection, draw the shores, export them and open these vectors in my project ?
    Eric
  • global_mapper
    global_mapper Administrator
    edited July 2009
    Eric,

    By huge I mean the physical size relative to the screen, not the number of vertices. The extra cropping comes into play when you are zoomed so far in on a feature that parts of it would overflow a 32-bit integer pixel coordinate. When you get close to zoomed in that far Global Mapper crops to prevent overflow and a crazy display.

    If indeed this is the problem then drawing the vectors in the native projection should help.

    If you still have trouble and can provide some data samples, I can try and reproduce the problem.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com