paleolimbot commented on code in PR #8801:
URL: https://github.com/apache/arrow-rs/pull/8801#discussion_r2505093123
##########
parquet-geospatial/src/crs.rs:
##########
@@ -0,0 +1,97 @@
+use std::{collections::HashMap, sync::Arc};
+
+use arrow_schema::{Schema, SchemaBuilder};
+use serde_json::{Value, json};
+
+#[derive(Debug)]
+pub enum Crs {
+ Projjson(serde_json::Value),
+ Srid(u64),
+ Other(String),
+}
+
+impl Crs {
+ // TODO: make fallible
+ fn try_from_parquet_str(crs: &str, metadata: &HashMap<String, String>) ->
Self {
+ let de: Value = serde_json::from_str(crs).unwrap();
+
+ // A CRS that does not exist or is empty defaults to 4326
+ // TODO: http link to parquet geospatial doc
+ let Some(crs) = de["crs"].as_str() else {
+ return Crs::Srid(4326);
+ };
Review Comment:
If you could guarantee that an "srid" value in either Parquet or GeoArrow
referred to an EPSG identifier, it would be safe to make that assumption;
however, neither make any guarantees about SRIDs and so writing a GeoArrow CRS
as `{"crs": "4326", "crs_type": "srid"}` actually won't get interpreted as
lon/lat in most cases, and there is no guarantee that a Parquet file with a crs
of `srid:4326` will be interpreted correctly (although it probably will be).
The canonical way to write "lon/lat" crses to Parquet is `None` (i.e., not
present); the canonical way to write "lon/lat" to GeoArrow is either `{"crs":
"OGC:CRS84", "crs_type": "authority_code"}` or the 4 KB PROJJSON string with
the full definition (although there are a few other ways it could possibly show
up in GeoArrow):
https://github.com/apache/sedona-db/blob/9cecb5b42e7ed2e5c234d61c24ea286ba3d58245/rust/sedona-schema/src/crs.rs#L318
Arrow C++ tries as ever hard as it possibly can to detect a GeoArrow CRS
that is lon/lat to ensure that the Parquet representation is `None`, and
converts a `None` Parquet crs to GeoArrow metadata of `{"crs": "OGC:CRS84",
"crs_type": "authority_code"}` because I didn't feel like explaining to
non-spatial reviewers why we needed 4 KB of JSON to solve that case.
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