[OSRS-PROJ] prime meridians

Frank Warmerdam warmerdam at pobox.com
Mon Dec 9 17:21:59 EST 2002

Gerald Evenden wrote:
> First, and I think a very important point, PROJ does not know of the
> existance of a "Prime Meridian."  Input longitude is always relative to
> a user define origin and may be offset with the built in "lon_0" option.
> I do not see where specifying a prime meridian is anything more than
> redefining lon_0.


As Cliff points out, the primary reason for tacking on a distinct
concept of prime meridian is so that coordinate system to coordinate
system conversions can be done within the API.  This is outside the original
scope but a part of what I am trying to accomplish with pj_transform() ...
which is intended to have the ability to translate between any two
coordinate systems (with lots of limitations of course).

Now it is  practical to translate coordinates between coordinate systems with
different prime meridians.  You can dispute whether coordinate system
to coordinate system translation belongs in PROJ.4, but at this point
it is part of my goals for the library, and the least I can do is try
and do it well.

> As for radian input, that applies to the library calls that emulate other
> math function calls that employ radian input (ie. sin, cos, tan, ...).  Program
> proj allows for conversion of variants of DMS to and form radians as
> well as binary radian I/O.

This is correct of course, but when I talk about capabilities I am mainly
looking at the API level which is what is used in the various environments
like GRASS, MapServer, OGDI and OGR.  The commandline programs are useful but
not the way that most interaction with "PROJ.4" occurs.

Best regards,

I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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