[Proj] Potentially (theoretically) dangerous changes in Proj4 internal routines that affect the end results..

Even Rouault even.rouault at spatialys.com
Sun Nov 8 21:36:58 EST 2015

Le lundi 09 novembre 2015 00:36:45, Charles Karney a écrit :
> Dear Janne,
> I'm against given the new utm implementation a different name.  The
> whole point is that a naive user can request "utm" and get the best
> available implementation of the projection.
> Conceivably, the old version could be made available as "oldutm" or
> whatever.  But I would like to see some evidence this is really needed
> given that with the standard UTM conventions for zones, we are only
> talking about a ~ 0.1mm difference -- and this is also the round-trip
> discrepancy in the old implementation.

For what is worth, I also don't see the point in making the old 'utm' available
under any other name for sub millimeter differences.

And there are other changes/fixes that are sometimes done in other projections
that affect computations by much larger extent, hopefully towards more correctness.
For example, the results of the aeqd projection can be very different w.r.t
older versions, when going far from the center point (and for the better since the results now round-trip)

proj 4.8.0 :

$ echo "0 -45" | cs2cs +proj=longlat +datum=WGS84 +no_defs +to +proj=aeqd +lat_0=45 +lon_0=90 +datum=WGS84 +no_defs
-5504790.57	-3840359.33 0.00

$ echo "-5504790.57 -3840359.33" | cs2cs -I +proj=longlat +datum=WGS84 +no_defs +to +proj=aeqd +lat_0=45 +lon_0=90 +datum=WGS84 +no_defs
44d36'50.801"E	0d16'23.383"S 0.000

--> completely off

proj 4.9.2

$ echo "0 -45" | cs2cs +proj=longlat +datum=WGS84 +no_defs +to +proj=aeqd +lat_0=45 +lon_0=90 +datum=WGS84 +no_defs
-10900566.43	-7663668.99 0.00

$ echo "-10900566.43 -7663668.99" | cs2cs -I +proj=longlat +datum=WGS84 +no_defs +to +proj=aeqd +lat_0=45 +lon_0=90 +datum=WGS84 +no_defs
0dW	45dS 0.000

--> perfect

If people are not happy with new proj versions, by all means, they can stay with older releases,
or fork the project to fit their purposes, or ask for a refund for their license fees.


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