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Im trying to define the srs EPSG 3857 inside Terrasolid sofware. I ran into a problem because the definition of the reference ellipsoid is given with a major axis and inverse flatening, not via both axis. Since flatening of a sphere is 0, inverse of a zero is infinite and i cant input that. Doesnt work with a 0, 1 or some very high value either. Any thoughts on how to define it, and if its even possible?


The issue is, a sphere (reference body for EPSG 3857) is a kind of ellipsoid that has no flattening, hence you cannot define 1/flatening. Im looking for a workaround in this particular software, or just a confirmation that its in fact impossible to do at this point, since i cannot define datum that involves a sphere as a reference body.

Heres a screenshot of the datum input screen: Screenshot

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Could you please be more specific about the way(s) in which the software "doesn't work"? What precisely do you do and how does it exhibit failure in response? – whuber Dec 27 '12 at 16:03
up vote 1 down vote accepted

The EPSG has defined the code 3857 here

official values are:

Semi major axis (a)     6378137     metre

Inverse flattening  298.257223563   unity
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I feel so stupid, reference ellipsode is not a sphere, duuuuuh :) Thanks for pointing out the obvious – U2ros Dec 28 '12 at 9:54
I have to correct myself again. It is in fact using a sphere, not wgs84 ellipsoid. The definition in the link you sent is wrong, theres nothing more to it. Also, i Terrasolid doesn't support it, i ve done transformation of lidar data via FME, where you can define a projection with a sphere – U2ros Dec 28 '12 at 12:27
The EPSG definition is correct. The projection algorithm uses the semimajor axis value for the radius of the sphere, and ignores the flattening (which is what Google and Bing/Microsoft do too). – mkennedy Dec 28 '12 at 17:30
So the fault is in the projection, not in the ellipsoid. Here is some further reading:… – AndreJ Dec 28 '12 at 19:40

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