[PROJ] Questions about declaring a CRS

[Even Rouault]

Le 29/06/2021 à 17:27, Floris Vanderhaeghe via PROJ a écrit :
>
> Thank you Even!
>
> To R users, would this be appropriate advice regarding PROJ strings? - 
> please comment or edit where needed:
>
>  1. Using a PROJ string to specify a CRS is discouraged
>     <https://proj.org/development/reference/functions.html#c.proj_create>
>     and currently not much documented by PROJ (PROJ strings are used
>     for coordinate operations instead). At least for registered CRSs,
>     always use AUTHORITY:CODE or WKT2
>     <https://docs.opengeospatial.org/is/18-010r7/18-010r7.html>, not
>     PROJ strings.
>
Yes
>
>  1. Also custom CRSs are ideally specified as WKT2
>     <https://docs.opengeospatial.org/is/18-010r7/18-010r7.html>, or
>     more conveniently as (currently not yet official) PROJJSON
>     <https://proj.org/specifications/projjson.html>, which follows the
>     WKT2 structure.
>
Yes
>
>  1. When specifying a custom CRS with a PROJ string, the WGS84
>     ensemble datum (EPSG:6326
>     <https://epsg.org/datum_6326/World-Geodetic-System-1984-ensemble.html>)
>     will automatically be adopted if no '+ellps' is specified (and
>     with '+ellps', the datum is considered as 'unknown').
>
Yes
>
>  1. Note that EPSG:6326 always refers to the latest WGS84 realization,
>     not to a fixed one. If these conditions are fine for your usecase,
>     then there's no problem.
>
No. For EPSG, EPSG:6326 is the datum ensemble WGS 84, not the latest 
realization. We have indeed discussed a number of times on this list if 
we should consider it as the latest realization in practice, but haven't 
decided so yet.
>
>  1. For backward compatibility reasons, PROJ strings with '+towgs84'
>     or '+datum', although deprecated, will remain supported by PROJ
>     and GDAL.
>
Yes
>
> 1.
>
>
>  2. When specifying a custom CRS that doesn't use the WGS84, NAD27 or
>     NAD83 datum (which can be specified with '+datum'), it is better
>     to use WKT2
>     <https://docs.opengeospatial.org/is/18-010r7/18-010r7.html> or
>     PROJJSON <https://proj.org/specifications/projjson.html> and
>     properly refer the geodetic datum. A typically less accurate
>     alternative is defining as a BOUNDCRS by specifying '+towgs84'
>     transformation parameters.
>
Yes
>
> With regards
>
> Floris
>
>
>
> mailhandtekening
>
> *Floris Vanderhaeghe*
>
> Flemish Government
> RESEARCH INSTITUTE FOR NATURE AND FOREST
> Team Biometry, Methodology and Quality Assurance
> Havenlaan 88 bus 73, 1000 Brussels
> Belgium
> floris.vanderhaeghe at inbo.be <mailto:floris.vanderhaeghe at inbo.be>
> www.inbo.be <http://www.inbo.be/>
>
> ///////////////////////////////////////////////////////////////////////////////////////////
>
>
>
>
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