Determine if an srid refers to a geodetic CRS

[Richard Huesken]

All,

Thanks for the replies and confirmation (and warning about caveats). I will
use the predicate suggested by  @Erik Wienhold

I'm on Postgis 3.3.3

"SELECT ST_IsGeographic(4326);" returns: "SQL Error [42883]: ERROR:
function st_isgeographic(integer) does not exist"

Kind regards,
Richard


Op wo 30 apr 2025 om 00:30 schreef Erik Wienhold <ewie at ewie.name>:

> On 2025-04-30 00:01 +0200, Pettycash dev wrote:
> > ### 1) **Checking if `srtext` starts with "GEOGCS"?**
> > While `srtext` for geodetic CRSs (e.g., WGS84) starts with `GEOGCS` in
> its
> > WKT definition (e.g., `GEOGCS["WGS 84", ...]`), this method has caveats:
> >    - **It’s not foolproof**: The `srtext` format can vary (e.g.,
> > whitespace, custom definitions).
> >    - **Not future-proof**: Relies on text parsing, which may break if WKT
> > conventions change.
> >
> > Example query (works in most cases, but not fully robust):
> > ```sql
> > SELECT srid, srtext
> > FROM spatial_ref_sys
> > WHERE srtext LIKE 'GEOGCS%'; -- Filters geodetic CRSs
> > ```
>
> According to annex C.4.1 of the standard [1] "geodetic CRS" covers both
> geographic and geocentric CRS.  So GEOGCS and GEOCCS are relevant.  Also
> note annex B.8 since there's also GEODCRS, GEODETICCRS, GEOGCRS,
> GEOGRAPHICCRS in new versions.  Because those are the only ones with
> prefix "GEO", you might get away with just using that predicate:
>
>     srtext LIKE 'GEO%'
>
> [1] https://docs.ogc.org/is/18-010r11/18-010r11.pdf
>
> > ### 2) **Use the built-in PostGIS function `ST_IsGeographic()`** ✅
> > PostGIS provides **`ST_IsGeographic(srid)`** to directly check if an SRID
> > is geodetic. This is the **recommended method** because:
> >    - It uses PostGIS’s internal logic (no text parsing).
> >    - Handles edge cases and custom definitions.
>
> PostGIS does not provide ST_IsGeographic to my knowledge.  Perhaps a
> separate extension?  Or just an AI hallucination?
>
> > > Le mar. 29 avr. 2025, 22:45, Richard Huesken <
> richard.huesken at gmail.com>
> > > a écrit :
> > >
> > >> I'm wondering if there is an easy way to detect if an srid refers to a
> > >> geodetic CRS, by checking the spatial_ref_sys table.
> > >>
> > >> 1) Would it be safe to assume the srid is a geodetic CRS if the value
> in
> > >> the srtext column starts with 'GEO'? It almost seems to be too good
> to be
> > >> true...
> > >>
> > >> 2) Did I overlook a Postgis function that returns this information
> (and
> > >> if so, which one would that be)?
>
> --
> Erik Wienhold
>
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