[Proj] New ellipsoid

Gerald I. Evenden gerald.evenden at verizon.net
Tue Jul 4 22:14:29 EDT 2006

With any level of the proj4 software you can change the ellipsoid to any value 
you wish within "reasonable" limits.  Major axis is not a problem in any case 
as internally the projections are evaluated for a "unit ellipsoid" and the 
results multiplied by the real major axis.  A problem may occur if the 
eccentricity increases significantly over the range of the earth's value 
because many calculations are approximations that assume a value of e^2 in 
the range of 0.006.  I would suspect that for e^2 < 0.01 would work but I 
would be careful beyond that range.  Smaller e should never be a problem.

Note: in most cases the computational factor involving eccentricity is in the 
form of e^2 or e (eccentricity) so that is what I must referred to here.  To 
phrase the range in terms of flattening, minor axis, etc. you will need to do 
a little math with the equations given in the documentation relating them to 

On Friday 30 June 2006 3:33 pm, Fabrizio Bernardini wrote:
> I am new in this list and I do not know if this topic has been discussed
> yet. I tried to search for it but got no useful results.
> My question is:
> How can I change ellipsoid and specify a completely
> new one, for instance for a planet which is NOT Earth.
> Will the projection library still work in this case?
> Did anybody ever discussed this issue?
> Is there a reference to that anywhere?
> Thanks,
> Fabrizio
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