[Proj] datum matters

Richard Greenwood richard.greenwood at gmail.com
Sat Mar 28 19:09:36 EST 2009

I made a post to this list earlier this week that offended at least
one member of the list. I’ll try here to clarify my position and
hopefully not start a flame war. I know that 90% of the subscribers to
this list have already forgotten more than I will ever know about
projections and datums, so please be tolerant of my simplistic

In this era we must think in terms coordinate systems, not just
projections, despite the name of the list and library. I’m using
“coordinate system” as a projection + datum, or lat/long + datum. To
tie a position to the earth you need both. Many users, both C
programmers incorporating the library in their own work, and end users
accessing the library thru the various OSGEO stack applications that
use the library, do not distinguish between a “projection” and a
“coordinate system”.

Two subtle, but important, changes that I have noticed in the library
are: (1) No longer assuming a datum of WGS84 when either a source or
destination system lacks an explicit datum (2) depredicating pj_fwd()
and pj_inv() in favor of pj_transform(). These changes encourage users
to think in terms of the coordinate system as a whole and may keep
people out of trouble. Thanks to Frank or who ever deserves credit for
these changes.

Richard Greenwood
richard.greenwood at gmail.com

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