[Proj] WGS84 to Sphere Inconsistency Between Proj Versions

[Clifford J Mugnier]

When didling with spherical projections, the concept of "DATUM" is entirely inappropriate.
 
Cliff Mugnier
LOUISIANA STATE UNIVERSITY

________________________________

From: proj-bounces at lists.maptools.org on behalf of Frank Warmerdam
Sent: Thu 21-Aug-08 22:38
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] WGS84 to Sphere Inconsistency Between Proj Versions



Faron Anslow wrote:
> I just ran this:
>
> cs2cs +proj=latlong +datum=WGS84 +to +proj=lcc +lat_1=50 +lat_2=50
> +lat_0=50 +lon_0=-107 +a=6371200.0000000000 +es=0.0 +f=0.0
> +towgs84=0,0,0 -r
>
> on:
> 70.933 -8.667
>
> and got:
> 2873633.37    4593659.18 -12148.43
>
> which is my original matlab answer and that from the old proj4.4.  Now,
> the question is if forcing the datum shift with +towgs=0,0,0  is
> appropriate? Doesn't +towgs48 conflict with/override the spherical
> projection definition?

Faron,

I can't imagine any situation other than an effort to match old answers
where it makes sense to apply a plain lat/long on a sphere to lat/long on an
ellipsoid conversion the way this is doing.

If you want the lat/long values computed from the lcc projection based on
a sphere to treated as WGS84, then use +nadgrids=@null (or in 4.6.0 just
omit a datum specifier for the lcc projection).  This is *likely* want
you want.

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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