[Proj] Robinson discrepancy btwn proj and cs2cs

Frank Warmerdam fwarmerdam at gmail.com
Wed Aug 17 15:10:24 EDT 2005

On 8/17/05, William K <woklist at charter.net> wrote:
> (forgot about the list change - Mac OS X Mail address 'memory'
> feature had the old address, now removed)
> I thought it might be some spheroid transformation thing, from some
> past cs2cs questions on the list.  My brain shorts out a bit when I
> start to think about ellipsoids, spheroids and datums - so, which one
> is giving the correct value?


What is "correct"?  That is almost a metaphysical question in an
underdetermined context.  That means, I don't know but none are
exactly right or wrong.

> I'm guessing that MAPublisher is not a good comparison, since it
> tends to dumb down projection stuff (never sure what it does
> sometimes, especially with ellipsoid transformations, and no sphere
> selection, you have to enter major/minor ellipsoid axes).
> I thought I had looked at this before, but using the sphere ellipse
> in proj 4.4.9 gives:
> Sumomo:~ williamk$ proj +proj=robin +ellps=sphere
> 0 45
> 0.00    4799694.61
> ^C
> Sumomo:~ williamk$ cs2cs +proj=longlat +ellps=sphere +to +proj=robin
> +ellps=sphere
> 0 45
> 0.00    4799694.61 0.00
> ^C
> They're the same now, but different from either ellipsoid case.
> Closer to the proj ellipsoid.  I could only read off coordinates from
> a clikc in a GRASS window, so they're not exact, but at a large
> enough zoom, they're very close to the cs2cs case.
> I guess it's just habit to use WGS84 for small-scale stuff (regions/
> continents and world), I should probably update my habits, huh?
> Remove those transformation issues from the equation (literally).
> I'll check this with GRASS and MAPublisher when I'm feeling better
> and back at work (hopefully tomorrow).
> Oh, and what is that 3rd number in the cs2cs output?  The man page
> didn't say.

The third value is elevation from the ellipsoid.   It changed dramatically 
in the original robinson case because of the large vertical difference
between the sphere and the ellipsoid. 

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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