[GRASS-dev] [EXTERNAL] Re: PROJ6 support in GRASS

[Newcomb, Doug]

Markus,
Thank you for working on this!

 Perhaps a flag to default to the transformation with the least error ( via
something like the  projinfo  -s x -t y --summary  query and with details
captured about the option selected  in the metadata)?
https://media.ccc.de/v/bucharest-198-revamp-of-coordinate-reference-system-management-in-the-osgeo-c-c-stack-with-proj-and-gdal#t=666
around
10:33 minutes in.

Looking at the reprojection options in QGIS 3.8.2 the menu for picking the
reprojection option lists the accuracy, the number of contributing
stations, and a preferred alternative. If this data can be gleaned from a
projinfo query, a preferred alternative can be established.  I would not
make that the default.

A sample workflow would be that someone tries r.proj with no flags. If
there are multiple alternatives, it does not proceed and gives a message
along the lines of " Multiple projection options detected, please rerun
with --info flag to see alternatives.

User reruns with --info flag ( which supersedes all other flags )  and sees
5 alternatives with accuracy and number of stations listed with an asterisk
next to the "preferred" alternative

User runs r.proj with a --prefer flag that takes the option with the least
error and most stations.  Alternatively, --file flag that points to a text
file with a proj6 pipeline to enforce exactly what is desired.

The preferred option may not always be the same over time, but if the
pipeline is captured in the metadata you will have a record of how it was
done.

Doug



On Tue, Sep 3, 2019 at 10:31 PM Anna Petrášová <kratochanna at gmail.com>
wrote:

> Hi,
>
>
> On Tue, Sep 3, 2019 at 4:22 AM Markus Metz <markus.metz.giswork at gmail.com>
> wrote:
>
>> Hi all,
>>
>> there is a new PR for PROJ6 support in GRASS
>> https://github.com/OSGeo/grass/pull/118
>>
>> There are two important changes when using PROJ6:
>>
>> First, reprojection with v.proj and r.proj is no longer always possible
>> without the user making informed decisions. The reason is that there can be
>> several different operations available to reproject coordinates from one
>> CRS to another CRS. These different operations are listed and the user has
>> to provide the appropriate operation with the pipeline option, taking care
>> of any axisswap.
>>
>
> first, thanks for all the work. Second, I don't see how most users are
> supposed to know what to pick. Is there perhaps a way to pick a good
> default? I just can't imagine not having r.import/v.import...
>
> Anna
>
>>
>> Second, axis order is no longer always easting, northing, e.g. for
>> EPSG:4326 it is northing, easting, and an axisswap might need to be removed
>> from operations provided by PROJ.
>>
>> There are many more changes (see details in the PR), but these are the
>> two most important ones.
>>
>> Feedback welcome!
>> _______________________________________________
>> grass-dev mailing list
>> grass-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/grass-dev
>
>

-- 
Doug Newcomb - Cartographer
USFWS
551F Pylon Dr
Raleigh, NC
919-856-4520 ext. 14 doug_newcomb at fws.gov
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