[Proj] Some questions

Frank Warmerdam warmerdam at pobox.com
Wed Apr 1 09:22:02 EST 2009

support.mn at elisanet.fi wrote:
> Hello,
> here are some questions to the proj.4 list.
> 1) What I have understood, the LibProj is an updated projection package?
> Is there any plan to integrate this into the Proj.4 or is it too different for that
> purpose. End users would of coures like to have more and more projections
> and inverses available. Maybe some time in the future?


libproj4 is fairly similar to PROJ.4, and with a little work it is
possible to port projection algorithms from one to the other.  I have
carried a couple of items from libproj4 to PROJ.4 in the last few years
however, I have not made any comprehensive effort to keep things in sync.

There is no immediate plan to re-engineer PROJ.4 to use libproj4 as it's
underlying projections library though that has been discussed as a

> 2) The ellipsoid question. How was the double ellipsoid case solved.. or
> will it be of any error source? I am assuming that if the datum shift ellipsoid
> is different than the original one, the error will be anyway too small to have
> any practical meaning? And so it does not matter which ellipsoid is used
> for the datum shift since all the ellipsoids are almost the same... but sure
> the numbers will differ.

The double ellipsoid case was not really resolved.  It is still
necessary in some cases to use tricks like +nadgrids=@null to
effectively treat a non-WGS84 ellipsoid based dataset as if it were
based on WGS84 as a datum.

The differences in ellipsoid can be very significant.  The most
common situation this comes up is with sphere based projections
(like google spherical mercator) where the projection is on a
sphere but you are expected afterwards to treat the lat/long
result as being on WGS84.  In this situation incorrect interpretation
can result in very substantial errors.

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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