[Proj] Differences across v4.8 and v4.9

[Kristian Evers]

Hi Joaquin,

For specifics about changes between versions you should consult the NEWS file in the source distribution [0],
or even better, the git commit log [1].

I tried recreating your findings with the following script:

-----------------------------------------------------------------------------------------------------------------------------------------------
#!/bin/sh

proj

echo A1
echo 45.25919444444 -111.5 | cs2cs +proj=latlong +datum=NAD83 +to +proj=utm +zone=10  +datum=NAD27 -r
echo "1402285.99      5076292.42 -0.00"
echo
echo A2
echo 4.897 52.371 | proj +proj=aeqd +ellps=WGS84 +units=m
echo "384923.72       5809986.50"
echo
echo A3
echo 4.897 52.371 | proj +proj=robin +ellps=WGS84 +units=m
echo "394576.51       5571243.64"
-----------------------------------------------------------------------------------------------------------------------------------------------

And got the following output:

-----------------------------------------------------------------------------------------------------------------------------------------------
$ ./test_proj4.9.3.sh
Rel. 4.9.3, 15 August 2016
usage: proj [ -bCeEfiIlormsStTvVwW [args] ] [ +opts[=arg] ] [ files ]
A1
1402285.98	5076292.42 -0.00
1402285.99      5076292.42 -0.00

A2
384537.46	5809944.81
384923.72       5809986.50

A3
394576.53	5570940.84
394576.51       5571243.64
-----------------------------------------------------------------------------------------------------------------------------------------------

So, I am not able to reproduce the output you get in B1, although I do get an answer very similar to A1.
This discrepancy can most likely be explained by the fact that the default UTM implementation was changed
to etmerc with version 4.9.3.

For the second example I get the same difference as you. I believe this can be explained by the improved
geodesic code that was introduced 4.9.0. The aeqd code was updated when GeographicLib [2].

Your third example I can also recreate. I believe it might be caused by a change in coefficients in the
source code [3].

For all of the three examples you've found, I would say that the reason for them is that the algorithms
have been improved (although documented poorly). The examples in the doc pages should obviously be
updated to reflect the current behaviour of PROJ.4. 

/Kristian

[0] https://github.com/OSGeo/proj.4/blob/master/NEWS
[1] https://github.com/OSGeo/proj.4/commits/master
[2] https://github.com/OSGeo/proj.4/commit/de8d6779aaabe0c46c06b9047bc6a6a978cc0758
[3] https://github.com/OSGeo/proj.4/commit/d856016ca1a178cc81c4474c0a6bbe78d106d012

________________________________________
Fra: proj-bounces at lists.maptools.org [proj-bounces at lists.maptools.org] på vegne af Joaquim Luis [jmfluis at gmail.com]
Sendt: 28. august 2017 18:46
Til: PROJ.4 and general Projections Discussions
Emne: [Proj] Differences across v4.8 and v4.9

Hi,

I have found these differences between proj.4 versions that I don't find
any documentation referring them. There is one FAQ
http://proj4.org/faq.html#why-do-i-get-different-results-with-4-5-0-and-4-6-0
entry but is about differences between 4.5.0 and 4.6.0, whilst this is
between 4.8 and 4.9.

The first one (between A1 and B1) is specially more troublesome because is
the example provide in the cs2cs doc page
(http://proj4.org/apps/cs2cs.html) that agrees with 4.8.0 and not with
4.9.2

Joaquim

proj
Rel. 4.8.0, 6 March 2012

A1
echo 45.25919444444 -111.5 | cs2cs +proj=latlong +datum=NAD83 +to
+proj=utm +zone=10  +datum=NAD27 -r
1402285.99      5076292.42 -0.00

A2
echo 4.897 52.371 | proj +proj=aeqd +ellps=WGS84 +units=m
384923.72       5809986.50

A3
echo 4.897 52.371 | proj +proj=robin +ellps=WGS84 +units=m
394576.51       5571243.64

-------------------------------------------------------------------------------
proj
Rel. 4.9.3, dd Month yyyy

B1
echo 45.25919444444 -111.5 | cs2cs +proj=latlong +datum=NAD83 +to
+proj=utm +zone=10  +datum=NAD27 -r
1402292.27      5076292.65 -0.00

B2
echo 4.897 52.371 | proj +proj=aeqd +ellps=WGS84 +units=m
384537.46       5809944.81

B3
echo 4.897 52.371 | proj +proj=robin +ellps=WGS84 +units=m
394576.53       5570940.84
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