Fwd: [Proj] Robinson discrepancy btwn proj and cs2cs

[Frank Warmerdam]

On 8/17/05, William K <woklist at charter.net> wrote:
>  From proj 4.4.7 to 4.4.8 something changed so I get different
> projected coordinates for the Robinson projection now.  I didn't
> think to check cs2cs before - I noticed it in GRASS.  But after
> checking cs2cs, I see that it matches GRASS, so it doesn't appear to
> be a GRASS problem.
>
> Here's what I get with proj 4.4.7:
>
> Sumomo:~ williamk$ proj +proj=robin +ellps=WGS84
> 0 45
> 0.00    4805073.65
> ^C
> Sumomo:~ williamk$ cs2cs +proj=longlat +ellps=WGS84 +to +proj=robin
> +ellps=WGS84
> 0 45
> 0.00    4805073.65 0.00
> ^C
>
> And here's what I get with proj 4.4.9:
>
> Sumomo:~ williamk$ proj +proj=robin +ellps=WGS84
> 0 45
> 0.00    4805073.65
> ^C
> Sumomo:~ williamk$ cs2cs +proj=longlat +ellps=WGS84 +to +proj=robin
> +ellps=WGS84
> 0 45
> 0.00    4785120.02 -10647.46
> ^C
>
> There is a ~20000m Y difference.  (X coordinates are not affected)
>
> The only other software I can check against is MAPublisher 5 for
> Illustrator.  It produces the same results as the proj command.  I
> had other problems with Robinson in ArcGIS, so I don't know what that
> does. (That was Arc 8, I'll have our GIS guy try it again in Arc 9
> when he gets back.)
>
> Which is right?  How do I tell - are there some reference coordinates
> to check?

William,

I believe the issue is that Robinson is a spherical-only projection.
So internally the ellipsoid is changed from WGS84 to a sphere.
The cs2cs command notices that the two coordinate systems have
different earth models, and applies a sphere to ellipsoid conversion
as part of the operation.

The ellipsoid to ellipsoid conversion approach changed in a
fairly recent PROJ.4 library release, hence the dfference.  In
the past, if there was no datum shifting information, lat/long values
were treated unchanged when going from raw ellipse to raw ellipse.

I am considering changing the "new" behavior, since it seems to
different from what most other systems do, and what people expect.
Feedback on this idea is welcome.

PS. I have rerouted this email to proj at lists.maptools.org, and I have
completely removed the proj list from xserve.flids.com. 

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


-- 
---------------------------------------+--------------------------------------
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