# [Proj] Projection Dev - How to call other projections from aprojection?

Michael Speth spethm at landcareresearch.co.nz
Sun Oct 9 18:56:48 EST 2011

```I meant ellipsoid and sphere; I apologize for confusing those terms.

I am trying to implement the HEALPix projection from a python
implementation from one of our team members.  The python version has
methods for projecting onto an ellipsoid as well as a sphere.  The
python version's forward and inverse ellipsoid functions call the
forward and inverse sphere functions.

We will also be implementing a new projection called an rHEALPix which
is documented here:
http://code.scenzgrid.org/index.php/p/scenzgrid-py/source/tree/master/docs/scenzgrid_math.pdf

On 10/10/11 12:43, Noel Zinn (cc) wrote:
> "Spheroid" and "ellipsoid" are synonyms for the same thing in geodesy and
> cartography.  See:
>
> http://en.wikipedia.org/wiki/Spheroid
>
> Do you mean that you have an ellipsoid and a sphere, maybe a conformal
> sphere intermediate between the ellipsoid and the plane?
>
> What "new" projection are you implementing?  Does it have a name or is it
> something you invented?  Is it conformal?
>
> Noel Zinn, Principal, Hydrometronics LLC
> +1-832-539-1472 (office), +1-281-221-0051 (cell)
> noel.zinn at hydrometronics.com (email)
> http://www.hydrometronics.com (website)
>
> -----Original Message-----
> From: Michael Speth
> Sent: Sunday, October 09, 2011 4:06 PM
> To: proj at lists.maptools.org
> Subject: [Proj] Projection Dev - How to call other projections from
> aprojection?
>
> Greetings,
>     I am working on implementing a new projection and stumped on how to
> call a projection from within a projection.
>
> For example, the projection has both a spheroid and ellipsoid.  The
> function for determine the projection of the ellipsoid (both forward and
> inverse) use their respective spheroid projections during calculation.
> Unfortunately, I'm not sure if the ENTRY function can be used because
> for the ellipsoid calculation, but the spheroid function is only an
> intermediate step int the calculation and there exists additional
> computations after the spheroid function is called.
>
> Consider this example:
>
> INVERSE(e_inverse);
>      lp = INVERSE(p_inverse);
>      lp.phi = auth_lat(...);
>      return lp;
>
> Does this make sense?  If so, is there any way of calling the inverse
> function for sphere within the inverse function for ellipsoid?
>
> Thank you for your help
> Michael Speth
> Landcare Research
> Scientific Programmer
>
>
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```