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 <e...@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.hues...@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 >
