Global Mapper Pro

Non-flattened spheroids

giverson Global Mapper UserTrusted User
edited September 2008 in Suggestion Box
I work in a 6356752.3142 polar radius, 0 flattening spheroid datum with a mercator projection.

There's no real easy way to implement this in Global Mapper unless I'm overlooking something.

I see that you've added a spheroid with this 6356752 spheroid datum but I'm baffled on why you put flattening on the spheroid. I could achieve the same effect by using a WGS84 based projection with a scale since the difference between 2 identically flattened mercator datums is linear from the 0, 0 coord.

What I did the last time I had to convert something to this datum was convert all my coordinates into Interplanetary, Pluto (0 flattening, 5.410002) ratio. I then used the grid naming system to export gridded rasters (png's with pgw and prj files) with a naming convention out to a folder with all my grid numbers and export bounds at a 5.410002 ratio to what it should be in my datum. I did some image hackery that I already had to do in python and when I wrote out the png's again, I rewrote the pgw files using my target resolution and locations and the prj files using a WGS84 mercator projection.

Needless to say, it took a while to figure out a work around to the limitations posed by your datum system. Chances are, if you ever find out what the flattening is on pluto, this work around will be broken but I figure I have until 2015 before that happens.

I'd like there to be a way to specify polar radius and flattening values when creating a custom datum...and at the very least, being able to base the scales on something other than WGS84, since the translation between 2 datums with different flattening values is non-linear (which means the I can't adjust pixel values on raster export to simulate a square pixel in my target datum).


  • global_mapper
    global_mapper Administrator
    edited July 2008
    The "Sphere Radius 6356752 m" ellipsoid is a sphere with both a semi-major and semi-minor axis of 6356752.3142 m, which should mean a flattening of 0, correct. The semi-major axis is the only place that this differs from the WGS84 ellipsoid.

    I'm guessing that what you really want is the special WGS84-ish datum used by Google Maps and the like that is based on a sphere but for which you should not perform an ellipsoid transformation when converting from WGS84 ECEF coordinates to ECEF coordinates in this special datum, correct? If so, the latest version of Global Mapper have built in support for this special datum. It is called the 'Google Maps' datum. Note that direct export to several map-based web formats (including automatic tiling and reprojection) such as Google Maps, MS Virtual Earth, and NASA World Wind, are already built in to the latest release under the File->Export Web Formats menu.

    Let me know if I can be of further assistance.


    Global Mapper Support
  • giverson
    giverson Global Mapper User Trusted User
    edited July 2008
    Disregard this post as a suggestion. I missed the ability to add and edit ellipsoids in the custom datums. That is all I really need. I'm still stuck using 7.2 and that feature doesn't exist in that version...only briefly used the 9.03 trial.

    As far as I am aware, the ellipsoid spheroids use flattening. The google projection doesn't. I checked the ratios for my mercator on a point to what coordinate is translated in google coordinates and the ratios for the lat and long match each other. Still not the polar radius I use but since I can add my own ellipsoid, it doesn't really matter.
  • BaBeBiLaaaa
    BaBeBiLaaaa Global Mapper User
    edited September 2008
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