[Gdal-dev] Double question: Spheres and Ellipsoids, Central Latitude

[Frank Warmerdam]

On 10/21/05, Bobby Kennedy <kennedy at vrco.com> wrote:
>
> Hello all,
>
> Two questions, just figured I'd roll it into one email....  The little
> main() below illustrates both questions (I'm using gdal 1.2.3, and proj
> 4.4.8).
>
> 1) With regards to proj.4 and the projections that only support
> spherical solutions (e.g. Equirectangular), I saw the recent post "eqc
> warping problem" where you mention a possible work around for the z
> changes, do you have plans on implementing that?  Or even better, do you
> know if there are plans to add support for ellipsoids to the these
> projections (it's mentioned on the proj.4 site under 'Future plans', any
> idea of the time frame)?

Bobby,

You would need to ask Gerald Evenden about this.  The math is far beyond me,
so such things are done as he chooses.

> 2) Second question is regarding the code below and the central latitude
> for my equirectangular projection. It doesn't seem to be having any
> effect.  As can be seen in the code, I do a:
> OSRSetEquirectangular(ecWGS84, 30, 110, 0, 0).  When I convert the
> geographic point (110,30,0), I would expect to see a output point of
> (0,0,whatever), but the Y is not 0, instead it appears to the meter
> location of the 30 deg lat.  Is this a fixed bug and I'm just out of
> date, or something else?  Any ideas?

With the PROJ.4 cs2cs
cs2cs +proj=latlong +datum=WGS84 +to +proj=eqc +lat_ts=30 +lon_0=110
+datum=WGS84
110 30 0
0.00    3321065.15 -5312.58

I find different values of +lat_ts (or +lat_0) have no effect.  I wonder
if I am confused about what parameters apply to this projection.

I would suggest you email Gerald, or the PROJ.4 list for an answer
on this.

Best regards,
--
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