Spherical Transverse Mercator

MykleMykle Global Mapper UserPosts: 381Trusted User
edited April 2014 in Projection Questions
Note: the following is not a critical issue. I'm just playing with a map that was recently
released, and it is not part of a project that requires an immediate solution.

A "new" USGS "Magnetic Anomaly Map" covers North America from almost the equator to almost the pole.
Parameters included on the map:
- scale 1:10 000 000
- projection: Spherical Transverse Mercator
- central meridian: 100 W
- base latitude: 0
- scale factor: 0.926
- earth radius: 6,371,204m

PDF: U.S. Geological Survey Special Map

Are any of the native Global Mapper projections appropriate, not finding one by this name?

I have created control points for every grid intersection (10 degree grid),
and most projections display a very distorted image (over 100 gcp's).
Bonne is about the closest, but it looks like we should be able to do better.
Now that I have entertained myself for awhile playing with different projections
(and apparently mucking up the system with too many conversions from projection to projection)
it is time to ask for guidance.

Second question:
In the rectification dialog, when rectifying a map for the first time, the map is shown with
constant pixel scale for X and Y directions. When re-rectifying, the current projection is used,
and most of the time this is the best image to use. However, this can be a very distorted image.
Do we have a method by which the existing control points can be retained, but display the image
like a new import with constant pixel scales?

Thanks,
Mykle

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2014
    I would suggest using the normal Transverse Mercator, but create a custom datum that is based on an ellipsoid that is a sphere radius 6,371,204m. The other parameters should map directly to what is shown in the definition.

    There isn't currently a way to display the map in its original unrectified form from the re-rectify command. You would have to save the GCPs to a file, then load the file back in from disk and load the GCP files to see the original form.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • MykleMykle Global Mapper User Posts: 381Trusted User
    edited April 2014
    That works very nicely, thank you! I used a dozen control points, just to see what I'd get. The results are essentially perfect, so I could probably do with less. That's lots better than using every one of the 10 degree marks (over 100). At the very least, this was a good exercise about projections!
    Thanks,
    Mykle
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