Hi Milan, > Dewey, you mentioned current writers using inline strings - what are they > inlining ? are they inlining projjsons or authority:identifiers ?
The writers are writing the CRS representation they receive, which for Arrow C++ and arrow-rs comes from the geoarrow.wkb extension type metadata [1]. This is usually PROJJSON but is permitted to be a string (including authority:code). If you run pyarrow.parquet.write_table(pyarrow.table(geopandas_geof.to_arrow())) today, you will get a Parquet file with an inlined PROJJSON CRS, because that is how GeoPandas encodes CRSes when converting to Arrow. > reality of current implementations is such > that most implementations do write `authorithy:identifier`, spec should be > written so that at least it doesn't look like thats invalid. The reality of current implementations is that they are writing PROJJSON, although I would also happily support a rewording that adds authority:code to the recommended options list. > Arent EPSG:<number> also understood to map directly to > corresponding PROJJSON definition ? They can be mapped to a PROJJSON definition (or a number of other less friendly export formats) using a database with the licensing ambiguity Jia mentioned. Conversely, PROJJSON can be mapped to authority:code with some minimal JSON parsing (we do this in Arrow C++ and arrow-rs to canonically remove CRS definitions that correspond to lon/lat to produce more universally consumable Parquet files). Cheers, -dewey [1] https://geoarrow.org/extension-types.html#extension-metadata On Fri, Mar 27, 2026 at 3:52 PM Milan Stefanovic <[email protected]> wrote: > > Thanks Jia and Dewey, > > Dewey, you mentioned current writers using inline strings - what are they > inlining ? are they inlining projjsons or authority:identifiers ? > Given that current implementations avoided using srid:<number> and > projjson:<field_ref> perhaps we should remove these examples from spec as > they seem to bring some confusion. > > @Jia Yu <[email protected]>, you mentioned that OGC:CRS84 are understood to > map directly to its corresponding PROJJSON definition. > Arent EPSG:<number> also understood to map directly to > corresponding PROJJSON definition ? > > Also I'm fine with not being explicit about `authorithy:identifier` if that > was the prior consensus, but if reality of current implementations is such > that most implementations do write `authorithy:identifier`, spec should be > written so that at least it doesn't look like thats invalid. > > What are your thoughts? > > Milan > > On Wed, 25 Mar 2026 at 15:53, Dewey Dunnington <[email protected]> > wrote: > > > Hi Milan, > > > > A short answer is that the current language of the spec does not > > forbid writing "OGC:CRS84" to the CRS field (which is "just a string" > > as far as thrift is concerned). All existing readers that I know about > > (DuckDB, arrow-rs, Arrow C++, GDAL) will accept that string and > > interpret it unambiguously on read (for example, > > `GeoPandas.from_arrow(pyarrow.parquet.read_table(...))` works). There > > is also an example file in parquet-testing that covers this case > > (arbitrary string that is neither of the recommended options) [1]. I > > put together a small example script to demonstrate the read path for > > the tools I mentioned [2]. > > > > Jia is correct that the GeoParquet community will require writing an > > inline PROJJSON string in the forthcoming 2.0 version of the > > specification [3]. This was a pragmatic decision that reflects the > > needs of existing GeoParquet users because: > > > > - srid does not explicitly name the EPSG database, so any code written > > there does not have an unambiguous interpretation (even if it did it > > would place ambiguous licencing and/or dependency requirements on > > consumers) > > - projjson:some_field was not pragmatic to implement on the write side > > for either of the implementations I was involved in (C++ and Rust). > > Implementations just don't expose the global key/value metadata when > > converting types and doing so would have required breaking changes in > > the APIs. There are also ambiguities with respect to existing > > propagation of schema metadata (i.e., the projjson schema key is often > > propagated in unexpected ways into pyarrow and beyond, including being > > written into the key/value metadata of a resulting Parquet file). > > > > As a result, most of the tools that can write GEOMETRY and GEOGRAPHY > > (Arrow C++, GDAL, arrow-rs are currently writing inline strings > > (because inline strings are what is available in the representation > > passed to Arrow-based writers and this was better than omitting CRS > > information). For all the implementations I was involved in, we also > > try to explicitly omit the CRS when we detect that the string we were > > passed is lon/lat (i.e., if they see "OGC:CRS84", they write an > > omitted CRS to minimize the need for consumers to be CRS aware). > > > > I'll echo Jia's comment that none of us are keen to reopen a CRS > > discussion but I also agree that the current language of the spec is > > vague and doesn't reflect the reality of the ecosystem as it has > > evolved. I'm happy to review any PRs to improve the language or > > implementations :) > > > > Cheers, > > > > -dewey > > > > [1] > > https://github.com/apache/parquet-testing/tree/master/data/geospatial#geospatial-test-files > > [2] https://gist.github.com/paleolimbot/7759e58bf1f98ecf8f2c459367bbdeda > > [3] > > https://github.com/opengeospatial/geoparquet/blob/main/format-specs/geoparquet.md#crs-parquet-property > > > > On Wed, Mar 25, 2026 at 12:49 AM Jia Yu <[email protected]> wrote: > > > > > > Hi Milan, > > > > > > The authority:identifier pattern was explicitly rejected in prior > > > community discussions. The core concern is that it forces query > > > engines to rely on external registries to resolve CRS definitions, > > > which breaks the goal of self-contained data. More importantly, the > > > most widely used authority, the EPSG database, comes with licensing > > > terms that are not particularly open-source friendly: > > > https://epsg.org/terms-of-use.html > > > > > > As a result, the community has leaned toward requiring data writers to > > > use a fully self-contained CRS representation such as PROJJSON. In > > > that model, a reference like OGC:CRS84 is understood to map directly > > > to its corresponding PROJJSON definition, as outlined in the > > > GeoParquet specification: > > > > > https://github.com/opengeospatial/geoparquet/blob/main/format-specs/geoparquet.md#ogccrs84-details > > > > > > That said, this expectation is not clearly spelled out in the Parquet > > > and Iceberg specifications, which leaves some ambiguity in practice. > > > > > > I don’t have a strong stance either way. In fact, I can see the case > > > for allowing authority:identifier. But it’s worth noting that > > > introducing it now would likely reopen a fairly contentious discussion > > > in the community. > > > > > > Jia > > > > > > On Tue, Mar 24, 2026 at 10:09 AM Milan Stefanovic > > > <[email protected]> wrote: > > > > > > > > Hi everyone, > > > > > > > > I’m looking for some clarification (and potentially a small spec > > update) > > > > regarding the Geospatial Physical Types documentation - > > > > https://parquet.apache.org/docs/file-format/types/geospatial/, > > specifically > > > > the CRS Customization section. > > > > > > > > 1) The Confusion > > > > > > > > Currently, the spec states that custom CRS values should follow the > > > > `type:identifier` format, where type is either `srid` or `projjson` - > > > > (e.g., `srid:4326` or `projjson:property_name`). The spec also defines > > the > > > > default CRS as `OGC:CRS84`. > > > > > > > > Depending on how the specification is read, the reader may consider as > > > > valid CRS definition to be only strings of the form `srid:<some > > number>` or > > > > `projjson:<property name>`, which implies that `OGC:CRS84` does not > > adhere > > > > to the rules defined in the customization section. This creates > > confusion > > > > for implementers: should the type string always be parsed as a strict > > > > "custom" format which necessitates the srid: prefix? > > > > > > > > 2) The Suggestion > > > > > > > > I suggest we update the language to be explicit about allowed formats > > for > > > > CRS, and my suggestion is that we break it down like this: > > > > - Standard CRS: Any string from a known authority in a format of > > > > `<authority>:<identifier>` (e.g., `EPSG:4326`, `OGC:CRS84`, > > `ESRI:102100`) > > > > is accepted. > > > > - Custom CRS: in the format of `type:identifier` > > > > - `srid:1234`: The definition resides in a local/database > > spatial > > > > reference table. > > > > - `projjson:key`: The definition is stored in Parquet > > file/table > > > > metadata. > > > > > > > > This would validate `OGC:CRS84` as a first-class string while > > providing a > > > > clear "escape hatch" for custom definitions. > > > > > > > > What are your thoughts ? > > > > > > > > Kind regards, > > > > Milan > >
