[Proj] Roussilhe Projection

Gerald I. Evenden gerald.evenden at verizon.net
Mon Oct 24 13:11:56 EDT 2005

OK, OK, now I am getting confused---or, at least, more confused than I was
reading this thread before.

On Monday 24 October 2005 11:28 am, Clifford J Mugnier wrote:
> The Roussilhe Stereographic Projection was published in the early 1900s by
> the Hydrographer of the French Navy, Roussilhe.  Although his formulae,
> adopted by several European Powers, are computationally unique; that has
> absolutely NOTHING to do with a datum shift.  The various "block
> interpolations" are just variations on the implementation of NADCON or
> NTv02 algorithms shifting from one coordinate reference frame to another -
> not projections.

Is the "correction" polynomial (PASCAL code) a datum shift or a
correction to a projection???  If it is a datum correction than I will not
code it for inclusion in libproj4 (see earlier debate over projection
and datum shift software) as it belongs in datum shift library.

> Of use? Certainly.  However, I sense that there is some confusion between
> the implementation of an analytic function for a map projection and the
> implementation of a datum shift.
> I recommend that you keep the two concepts separate rather than combining
> the topics under the subject of the Roussilhe Stereographic Projection.
> (Note there is ALSO the Hristow Stereographic Projection as used in Romania
> as the "Stereo 70.")  Roussilhe (Russell), was French; Hristow was
> Bulgarian and he published in German.

Hmm.  Now to figure out where to get a copy of the math for Hristow.
> Does this mean that there is an intention to incorporate ALL available grid
> corrections into the code of libproj4?
> The Dutch cadastre provides upon request NLGEO2004 correction data for the
> RD, the Australians have National Transformation Grids for AGD66 and AGD84,
> the French IGN has several grid corrections available, and undoubtedly
> there
> are many more countries with freely or for minimal charge available data
> sets and procedures.
> All in all this could add 100s of MB to the libproj tarball. Please make
> sure you also make a version without grid data available!

Good point.  I was totally unaware that "corrections" were such a major
part of some grid systems.

I think I'll back off of this subject before it gets out of hand.  I do NOT
have the resources, time nor inclination to get into this activity.

Jerry and the low riders:Daisy May and Joshua
"The being cannot be termed rational or virtuous,
who obeys any authority, but that of reason."
---Mary Wollstonecraft 1792

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