[Proj] Some need for updated proj-4 manuals

Gerald I. Evenden geraldi.evenden at gmail.com
Sun Sep 14 10:59:12 EDT 2008

On Sunday 14 September 2008 3:58:25 am Hamish wrote:
> > Our application's manuals tell what projections and datums are. What is
> > needed is "how to use them", "how does a change of some parameter
> > affect the projection", "what are the exact names of those parameters",
> > "which way towgs84 numbers should be given", "what projections are
> > used and how in different parts of the earth", etc.
> Janne,
> with due respect, libproj's documentation responsibility ends with the
> library interface and internals. It is well beyond the scope of the project
> to supply a linking application's end-users with a textbook primer on
> geodesy. I view it as a bit ungrateful to ask them to do so for you,
> regardless of the fact that they may be experts in the field and well
> qualified to do so.

There are a lot of books out there that cover the general subject but their 
coverage of projections is minimal---usually just one of each major class of 
projection like conic, cylindrical, etc..

For specific uses like grid systems that's what the EPSG data sets are for.

Libproj4 just covers the detailed niche of the math of the cartographic 
projection.  Nothing more, nothing less.  Usually. ;-)

> "What do PROJ4's +a, +lon_0, and +k terms mean? Which convention are
> the 7-term towgs84 terms given in?" would be valid subjects of PROJ4
> documentation.

+a like +b refer to the semimajor and semiminor ellipsoid axis.  For example, 
Snyder often refers to the Clarke '66 ellipsoid in this manner.

+lon_0 is normally refers to the central meridian (can't think of anyplace it 
does not  ;-)).

+k is scale multiplier but is deprecated in favor of +k_0 like in k_0=0.9996 
for UTM.

> "What projections are used and how in different parts of the earth" is
> far far beyond the scope.
> Having said that, I am sure the members of this list could collectively
> build an excellent "For Further Reading" list. We maintain a small list
> of links to useful guides in the GRASS GIS Wiki:
>   http://grass.osgeo.org/wiki/Gis_Concepts#Geodesy_and_Cartography
>    (direct improvements to that wiki page are most welcome)
> regards,
> Hamish
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