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

Michael Speth spethm at landcareresearch.co.nz
Mon Oct 10 18:00:46 EST 2011

```Well, there has been very interesting discussion on the terms of Sphere,
Spheroid, and Ellipsoid.  I'm new to the GIS world so thank you for
helping me realize the importantance of terminology :)

As for my problem, I figured out a solution.  Instead of calling the
#define'd function within a #define'd function, I just created a new
static function that both the FORWARD and INVERSE functions call.

Thank for helping me think though the problem.

Regards

On 10/10/11 13:27, Noel Zinn (cc) wrote:
> Thanks.  Please clarify what you mean by "implementing"?  You're looking for
> the Proj.4 commands that will pull this off?  Or you're coding this as an
> additional projection in your instance of Proj.4?  Or you are coding this
> projection in some language other than Python independent of Proj.4 and
> thought the members of this forum could offer some guidance.  Pardon me if
> I've missed the obvious.  -Noel
>
> 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 6:56 PM
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] Projection Dev - How to call other projections
> fromaprojection?
>
> 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
>
> Thank you for your help!
>
> 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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