[PROJ] Questions about declaring a CRS

[Floris Vanderhaeghe]

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.
 2. 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.
 3. 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'). 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.
 4. For backward compatibility reasons, PROJ strings with '+towgs84' or
    '+datum', although deprecated, will remain supported by PROJ and GDAL.
 5. 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.

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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