Rectification of tiff map - distortion

KirstyKirsty Global Mapper UserPosts: 6
Hi

I am trying to rectify a tiff image of a 1:250,000 geological map. The map is using a transverse mercator projection (WSG84). I have added at least 10 control points to the image, but the resulting rectified image is still distorted around the edges. Does anyone have any suggestions about how to fix this or how to do the rectification properly so there is no distortion?

Thanks

Kirsty

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2009
    Kirsty,

    What version of Global Mapper are you using? There was similar problem to this in some older versions.

    If you already have the latest version, the other thing that you can do is change from the default triangulated (piecewise-affine) rectification for that many control points to a polynomial rectification. To do this, use the Options menu on the rectification window and change your rectification method to Polynomial.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KirstyKirsty Global Mapper User Posts: 6
    edited March 2009
    Hi Mike

    We have just purchased the lastest version of Global Mapper. I have already changed the rectification to polynomial but still no success. I am unsure if the problem is that we are trying to 'flatten' something that has already been rectified, if that makes sense. We are trying to mosaic all the geological maps into one large file and then re-rectify them together at once.

    Thanks

    Kirsty
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2009
    Kirsty,

    Can you provide a screenshot of what you are seeing?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KirstyKirsty Global Mapper User Posts: 6
    edited March 2009
    Hi

    I have attached a word document with several screen shots of the rectification problem.

    Thanks

    Kirsty
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2009
    Kirsty,

    The slight offset I might expect from using a polynomial rectification as it is a over-solved equation that only needs 4 points, so when you have more than 4 point a least-squares fit is used. What did it look like before when you used triangulated rectification. I would have expected it to fit then.

    Another note, typically the native projection of these maps aren't lat/lon, so you might get better results if after entering your control points, change the projection on the rectification dialog to something like UTM (assuming you don't know the actual native projection) with an appropriate zone and select Yes to Reproject, then see if the results are better.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KirstyKirsty Global Mapper User Posts: 6
    edited March 2009
    Hi Mike

    I had tried triangulated rectification but with fewer GCP's, now that I have more GCP's and changed back to triangulated the rectification works very well.

    Thanks for your help.

    Kirsty :)
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