[PROJ] Questions about declaring a CRS

[Alan Snow]

This may be a useful reference on the topic:
https://pyproj4.github.io/pyproj/stable/build_crs.html

On Mon, Jun 28, 2021 at 9:47 AM Floris Vanderhaeghe via PROJ <
proj at lists.osgeo.org> wrote:

> Hi,
>
> Below are some questions about the usage of 'PROJ strings' (as opposed to
> using WKT or AUTHORITY:CODE) as an option to declare the CRS of
> coordinates. The context is geospatial work in R with PROJ >= 6, the fact
> that users *can* still input PROJ strings to declare an object's CRS, and
> the question what to advise to users. We already had some discussions on
> this in the R-spatial community, which lead me to posing some questions
> here.
>
> After reading through several PROJ documentation pages, and observing
> output from 'projinfo':
>
>    1. It is clear that currently, support for representing a CRS by using
>    a PROJ string is only partial, especially because the datum can often not
>    be specified. ==> *QUESTION*: *Is specifying a CRS with a PROJ string
>    going to receive continued support, or will it eventually be dropped? And
>    related with that, **[QUESTION:] should we still advertise and use it
>    to represent a CRS?* The current state may appear somewhat unclear:
>       - There is support for '+type = crs' (to declare a CRS)
>          - e.g. projinfo does accept a PROJ string as input to represent
>          a CRS, by inclusion of '+type=crs'
>          - There is (still) support for '+towgs84' (to declare a
>       BOUNDCRS, hence not defining the datum directly)
>          - Cf. mention of '+towgs84' in the context of a pipeline, in
>          https://proj.org/operations/operations_computation.html#when-the-source-or-target-crs-is-a-boundcrs
>          - '+towgs84' can be still used in a PROJ string that represents
>          just a CRS (combining '+towgs84=' and '+type=crs')
>       - However, PROJ strings are now generally described in the
>       documentation as representing a coordinate operation, including datum
>       *shifts* when using a pipeline string. That is, the pipeline itself
>       does not define input and output datums, but may contain a transformation
>       step to perform a datum shift.
>       - In several places the *deprecation* of '+towgs84' is referred -->
>       *QUESTION*: *will '+towgs84' be dropped eventually? *
>       - Although using '+type = crs', the datum cannot be specified: it
>       is WGS84 when no '+ellps' is provided, and else 'unknown'. But with an
>       unknown datum, the CRS is incomplete. *QUESTION*: *Will** '+type =
>       crs' get continued support, or is it intended to eventually end support for
>       '+type=crs'?*
>       - A comment in the second code chunk under
>       https://proj.org/development/migration.html#code-example clearly
>       disrecommends PROJ strings to define a CRS (as opposed to WKT or
>       AUTHORITY:CODE), but generally this advice seems rather sparse in the
>       documentation. In fact, the still-supported use of a PROJ string to define
>       a CRS is only touched under proj_create()
>       <https://proj.org/development/reference/functions.html#c.proj_create>
>       and discouraged.
>    2. *If PROJ strings should not be used to define a CRS, then: *[
>    *QUESTION*:] *is it possible, with currently advised methods, to
>    define a **custom** CRS in a relatively short way*, i.e. not having to
>    write out a full WKT? Perhaps having the possibility to refer an authority
>    CRS (just using its code or name), and a way to alter specific (WKT?)
>    parameters, could work, although I don't know whether specification of the
>    second part can be standardised easily.
>
> Thanks in advance! I am not an experienced PROJ user.
>
> With regards,
>
> Floris
>
> *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
> www.inbo.be
>
> ///////////////////////////////////////////////////////////////////////////////////////////
>
>
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-- 
Alan Snow
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