Hello all
Following on the previous blog post about performance comparison, the
second part of Proj.4 - Apache SIS comparison (about accuracy) is
available at [1]. We have not been able to perform an extensive
comparison yet because of lack of time. But there is some talks for an
OGC test bed on map projection libraries accuracy. If such test bed
happens, much more extensive tests would hopefully happen in the
upcoming year. In the meantime, some key points in this blog post are:
* Apache SIS and Proj.4 are in close agreement for all tested
coordinate /conversions/ (this category include map projections) on
Earth.
* Apache SIS and Proj.4 are sometime in disagreement by 1 or 2 meters
for coordinate /transformations/ (this category include datum shifts).
o In the two cases were a disagreement is observed, Apache SIS is
conform to the parameters specified by the EPSG geodetic dataset.
o In one case, Proj.4 results are wrong by 0.9 ± 0.4 meter. This
is not a bug that could be easily fixed, but a Proj.4
architectural problem. More details are given in the blog post.
o In the other case, one can argue that Proj.4 is not wrong
provided that the difference between Apache SIS and Proj.4
results are smaller than the stochastic errors associated to the
tested transformations. Apache SIS provides this information,
but not Proj.4. This lack of information makes difficult to
determine if the difference is acceptable or not.
* For /Cylindrical Equal Area/ ("cea") projection//on Jupiter (tested
because it is a more flattened planet than Earth):
o If Jupiter had the size of Earth (for easier distance
comparison), average Proj.4 error would be 90 meters.
o Apache SIS does not have this error; it keeps centimetric precision.
Those errors are not significant to everyone. Unfortunately, there is
nothing in current implementation telling to user if Proj.4 is
delivering the accuracy that (s)he need, or if (s)he got the operation
for the right geographic area. Apache SIS on the other hand provides
this information, notably through ISO 19162 (WKT 2) formatting. A path
for adding WKT 2 support to Proj.4 is proposed in the blog.
Martin
[1]
https://www.geomatys.com/wordpress/index.php/2017/09/20/proj-4-versus-apache-sis-an-accuracy-comparison/?lang=en
