[PROJ] Relevant descrepancies between Sinusoidal and Homolosine

[Luí­s Moreira de Sousa]

Thank you Even for following up on this. To summarise:

1. The current implementation of the Homolosine is strictly spherical and should stay like so at least until an ellipsoidal implementation of the Mollweide pops up.

2. The Sinusoidal is implemented both in spherical and ellipsoidal forms, but doubts remain on the accuracy of the latter.

3. Differences in area however are relatively small, I will not worry with them for now.

While differences between the spherical and the ellipsoidal implementations are expected, in this case they run up to dozens of kilometres. I could be wrong, but this looks too wide for comfort. I will keep investigating this.

Regards.

--
Luís

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, March 10, 2020 4:22 PM, Even Rouault <even.rouault at spatialys.com> wrote:

> On mardi 10 mars 2020 14:49:37 CET Luí­s Moreira de Sousa wrote:
>
> > Hi again Even, apologies for insisting. In the .ods I sent earlier you can
> > verify that the Homolosine is preserving the northing-latitude identity,
> > even with the ellipsoid. And also that it does not match the Sinusoidal.
> > My guess is that the Homolosine is already spherical-spherical as it is.
>
> Ah indeed... This P->es = 0 thing wasn't used for the instanciation of the
>
> sinusoidal, so yes my PR was completely useless. Sorry for the noise.
>
> So, igh preserves the identity because it does force es = 0 to the underlying
> sinu... Whereas sinusoidal standalone will use an ellipsoidal formulation for
> an ellipsoid.
>
> As far as whether sinusoidal for the ellipsoidal case is correctly implemented
> or not, I've no idea. I'm not sure your CSV file clearly demonstrates the loss
> of the equal-area property(at least, this is not obvious to me), since it
> probably compares spherical vs ellipsoidal formulations of different
> projections.
>
> Even
>
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