[osgeo4w-dev] proj 5 update?

[Helmut Kudrnovsky]

Kristian Evers-2 wrote
>> On 25 Mar 2018, at 21:15, Helmut Kudrnovsky <

> hellik@

> > wrote:
>> 
>> Kristian Evers-2 wrote
>>> There’s already a ticket for this:
>>> https://trac.osgeo.org/osgeo4w/ticket/558
>>> 
>>> The bug you are referring to is not fixed in PROJ 5.0.0 so an update
>>> will
>>> not really help you in that regard. Best option for that is to use
>>> 4.9.3.
>>> A
>>> bug fix release is on its way though, so when that is out it should be
>>> safe
>>> to update the OSGeo4W PROJ package and have it as the default.
>>> 
>>> /Kristian
>> 
>> for the record, GRASS trunk is PROJ 5 ready:
> 
> Very cool! I wasn’t aware of that. 
> 
>> 
>> https://lists.osgeo.org/pipermail/grass-user/2018-March/078023.html
>> 
>> https://lists.osgeo.org/pipermail/grass-commit/2018-March/043391.html
>> to
>> https://lists.osgeo.org/pipermail/grass-commit/2018-March/043408.html
>> 
>> Log:
>> libproj: +new GRASS API for coordinate transformation
>> 
>> this isn't backported to the 7.4.-release branch, not sure it will ever
>> be.
>> 
>> so it could be that GRASS 7.4.-release branch has to live with 4.9.3 for
>> a
>> while.
> 
> 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
>> --
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See

https://lists.osgeo.org/pipermail/grass-dev/2018-March/088123.html

-----
There is an important difference between the PROJ4 and the PROJ5+
API/syntax:

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
Transform.

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

+proj=pipeline
+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
-------------------



-----
best regards
Helmut
--
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