[GRASS-dev] PROJ 5 support in trunk

Markus Metz markus.metz.giswork at gmail.com
Mon Mar 26 12:21:02 PDT 2018

There is an important difference between the PROJ4 and the PROJ5+

The old PROJ4 API uses latlong WGS84 as pivot datum for coordinate
transformations like

projection1 -> latlong WGS84 -> projection2
or in '+to' syntax
projection1 +to projection2

The new PROJ5+ API no longer uses a pivot datum. The advantage is that you
can directly convert from one datum to another, without going through
WGS84. The disadvantage is that the user/software using the new API has to
make sure that either a common pivot datum is used or coordinates are
correctly transformed from one datum to another, e.g. with a Helmert

Both GDAL and GRASS have implemented the new API as a simple 2-step
pipeline like

1. projection1 -> some latlong
2. some latlong -> projection2

or in pipeline syntax

+step +inv projection1
+step projection2

Even Rouault has done some tests and found sometimes subtle differences in
the results between the old and new API/syntax. Further on, there is no
mechanism (yet) in place to validate the pipeline, most importantly that
the output of step 1 conforms to the required input for step 2.

IMHO, the implementation of the new PROJ5+ API/syntax in GDAL and GRASS
should be regarded as experimental and testing by as many people as
possible would be a huge benefit to PROJ/GDAL/GRASS and to all applications
using these projects.

Markus M

On Sun, Mar 25, 2018 at 10:47 PM, Helmut Kudrnovsky <hellik at web.de> wrote:
> >Now (last related commit is trunk r72522) it's finished. I have
> a new GRASS API that handles both PROJ 4 and PROJ >5, consisting of
> Thanks for this Update!
> Fyi See
> https://lists.osgeo.org/pipermail/osgeo4w-dev/2018-March/003557.html
> -------
> [...]
> The API from PROJ 4 lives on in PROJ 5, so GRASS 7.4 should be able to use
> PROJ 5 as well. We’ve been carefull not to breaking anything with this
> release.
> That comes with PROJ 6 and 7. Of course there might be implementation
> details
> in GRASS that I am unaware of that makes using PROJ 5 impossible.
> --------
> -----
> best regards
> Helmut
> --
> Sent from: http://osgeo-org.1560.x6.nabble.com/Grass-Dev-f3991897.html
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