[Proj] Projection Dev - How to call other projections from a projection?

Michael P Finn mfinn at usgs.gov
Mon Oct 10 13:11:33 EST 2011

```Yes, the "same thing" is a huge confusion for many users.

I am a professional cartographer but have had a fair amount of course work
in geodesy. I prefer the term ELLIPSOID because it isn't as easily
confused with a sphere as spheroid is.

From:
"Noel Zinn \(cc\)" <ndzinn at comcast.net>
To:
<proj at lists.maptools.org>
Date:
10/09/2011 05:46 PM
Subject:
Re: [Proj] Projection Dev - How to call other projections from
aprojection?
Sent by:
proj-bounces at lists.maptools.org

"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?

Michael Speth
Landcare Research
Scientific Programmer

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