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

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Hello,

since the difference will be in sub millimeter region .. I might also 
not to be too much worried about the change .. especially if it is also 
not much slower and clearly notified. But just to remain in some cases 
it IS VERY important to get repeatable results (even if wrong) with the 
library .. but this change could also be handled just mentioning that 
the new utm formula will change the computation so and so much. And as 
such the programmer (not the end user) should NOT apply the new library 
without testing the results (so to say "keep the old version if you need 
to be absolutely sure that the computations will be repeatable in the 
future also..")

Janne.

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Even Rouault kirjoitti 09.11.2015 04:36:
> 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.
> 
> Even

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