Extracting SRID from metadata CRS

[sunng87]

I'm trying to get the SRID from geoarrow metadata, for postgis.

Reading the docs from CRS struct, I saw it's a wrapper of PROJJSON. I learned PROJJSON structure may look like this:

{
  "type": "GeographicCRS",
  "name": "WGS 84",
  "id": {
    "authority": "EPSG",
    "code": 4326
  },
  ...
}

But from the test cases of CRS module it seems we could have other representations fo SRID.

So my question is:

• What would be the standard way to extract SRID from the CRS metadata?
• Is it possible to add something srid(&self) -> Option<u64> to CRS?

[kylebarron]

As of GeoArrow v0.2, the CRS can be either PROJJSON or WKT2 or Authority Code or SRID.

When receiving data, you don't know in advance what CRS format you'll be receiving. Some file formats like FlatGeobuf only support WKT2 / Authority code / SRID. GeoParquet 1.1 only supports PROJJSON.

So my question is:

• What would be the standard way to extract SRID from the CRS metadata?

I think the standard way would be to use PROJ APIs to convert the generic input into an SRID. This is the intention of the CrsTransform trait, to allow implementors to generically specify how to convert CRS representations

geoarrow-rs/rust/geoarrow-schema/src/crs.rs

Lines 165 to 206 in b3e473e

	/// CRS transforms used for writing GeoArrow data to file formats that require different CRS
	/// representations.
	pub trait CrsTransform: Debug {
	/// Convert the CRS contained in this Metadata to a PROJJSON object.
	///
	/// Users should prefer calling `extract_projjson`, which will first unwrap the underlying
	/// array metadata if it's already PROJJSON.
	fn _convert_to_projjson(&self, crs: &Crs) -> GeoArrowResult<Option<Value>>;
	/// Convert the CRS contained in this Metadata to a WKT string.
	///
	/// Users should prefer calling `extract_wkt`, which will first unwrap the underlying
	/// array metadata if it's already PROJJSON.
	fn _convert_to_wkt(&self, crs: &Crs) -> GeoArrowResult<Option<String>>;
	/// Extract PROJJSON from the provided metadata.
	///
	/// If the CRS is already stored as PROJJSON, this will return that. Otherwise it will call
	/// [`Self::_convert_to_projjson`].
	fn extract_projjson(&self, crs: &Crs) -> GeoArrowResult<Option<Value>> {
	match crs.crs_type() {
	Some(CrsType::Projjson) => Ok(crs.crs_value().cloned()),
	_ => self._convert_to_projjson(crs),
	}
	}
	/// Extract WKT from the provided metadata.
	///
	/// If the CRS is already stored as WKT, this will return that. Otherwise it will call
	/// [`Self::_convert_to_wkt`].
	fn extract_wkt(&self, crs: &Crs) -> GeoArrowResult<Option<String>> {
	if let (Some(crs), Some(crs_type)) = (crs.crs_value(), crs.crs_type()) {
	if crs_type == CrsType::Wkt2_2019 {
	if let Value::String(inner) = crs {
	return Ok::<_, GeoArrowError>(Some(inner.clone()));
	}
	}
	}
	self._convert_to_wkt(crs)
	}
	}

• Is it possible to add something srid(&self) -> Option<u64> to CRS?

I think it would be great to have a geoarrow-proj crate that implements this. It would be much too difficult to try to implement this ourselves. For some simple PROJJSON values you can inspect the SRID easily, but for any generic solution you'd need PROJ.