New ellipsoid

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New ellipsoid

Fabrizio Bernardini
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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Re: New ellipsoid

Gerald I. Evenden-2
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
eccentricity.

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
>
>
> _______________________________________________
> Proj mailing list
> [hidden email]
> http://lists.maptools.org/mailman/listinfo/proj

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"Cogito cogito ergo cogito sum"
   Ambrose Bierce, The Devil's Dictionary
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