cropping USGS DRGs referenced to NAD83

vieuxdelamontagnevieuxdelamontagne Global Mapper UserPosts: 66Trusted User
edited October 2009 in Technical Support
Apologies if this has been previously addressed; I couldn't find it.

I'm working with a bunch of collared USGS DRGs in geotiff format that have been unconventionally georeferenced to NAD 1983. GM's automatic collar trimming trims along meridians and parallels defined by NAD 1983, which of course isn't what's wanted since the images are scanned from the usual NAD 1927 printed maps. Changing the datum before trimming has no effect. I have about two hundred such maps in a mosaic and REALLY don't want to trim each one manually! Any ideas?

(The maps I'm using are served at ftp://ftp.agrc.utah.gov/DRG/24k/Collared/. Any two adjacent quadrangles ought to serve to illustrate the problem.)

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2009
    I would suggest doing a File->Batch Convert/Reproject to convert the files back to being projected as NAD27 maps. Then you can load those back in and the automatic collar cropping should work as the automatic collar cropping is done in the native datum of the layer.

    Another option would be to right-click on each layer in the Control Center and select the option to reposition, then press the Projection button on the rectification dialog and change the datum to NAD27, then press Yes to reproject the control points from NAD83 to NAD27, then press OK and crop the collar. This would allow you to skip the export step, but might be more tedious.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • vieuxdelamontagnevieuxdelamontagne Global Mapper User Posts: 66Trusted User
    edited October 2009
    Batch convert/reproject solved the problem beautifully (after two hours of clock time, however). I especially like the way GM can make sense of a wide range of file organization and naming conventions for the files to be converted. High praise for the design of this utility!

    Question: I have always regarded reprojections as a last resort because it seems to me that the quality of the reprojected image MUST be somewhat degraded with regard to the original. Am I wrong about this?

    Thanks, again, for a timely and helpful reply.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2009
    While there is some degradation anytime you resample imagery (such as when reprojecting), so long as you resample with interpolation enabled (it is by default when reprojecting) and the sample spacing is the same, you likely won't notice anything.

    If you do the approach of repositioning via the Control Center this won't be an issue, but I'd recommend the Batch Convert approach unless you just have a few to do or notice big quality issues.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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