Doc: document oddity related to identification of CRS from ESRI WKT

Or more generally formulations that don't have an explicit axis order.
Refs https://github.com/pyproj4/pyproj/issues/475

projinfo 'GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]'
returns EPSG:4326 with 100% confidence.
But its axis order is not the same as EPSG:4326.

I've pondered about this, like decreasing the confidence of the match,
but this would have downstream effects on GDAL (shapefiles with the
above content in a .prj would no longer be identified as EPSG:4326).
So for now, document that oddity.

1 files changed, 18 insertions, 6 deletions

diff --git a/src/iso19111/c_api.cpp b/src/iso19111/c_api.cpp
index f3badb51..5f34a22b 100644
--- a/src/iso19111/c_api.cpp
+++ b/src/iso19111/c_api.cpp
@@ -2158,14 +2158,26 @@ PJ *proj_get_target_crs(PJ_CONTEXT *ctx, const PJ *obj) {
  * The method returns a list of matching reference CRS, and the percentage
  * (0-100) of confidence in the match. The list is sorted by decreasing
  * confidence.
- *
- * 100% means that the name of the reference entry
+ * <ul>
+ * <li>100% means that the name of the reference entry
  * perfectly matches the CRS name, and both are equivalent. In which case a
  * single result is returned.
- * 90% means that CRS are equivalent, but the names are not exactly the same.
- * 70% means that CRS are equivalent), but the names do not match at all.
- * 25% means that the CRS are not equivalent, but there is some similarity in
- * the names.
+ * Note: in the case of a GeographicCRS whose axis
+ * order is implicit in the input definition (for example ESRI WKT), then axis
+ * order is ignored for the purpose of identification. That is the CRS built
+ * from
+ * GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],
+ * PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
+ * will be identified to EPSG:4326, but will not pass a
+ * isEquivalentTo(EPSG_4326, util::IComparable::Criterion::EQUIVALENT) test,
+ * but rather isEquivalentTo(EPSG_4326,
+ * util::IComparable::Criterion::EQUIVALENT_EXCEPT_AXIS_ORDER_GEOGCRS)
+ * </li>
+ * <li>90% means that CRS are equivalent, but the names are not exactly the same.</li>
+ * <li>70% means that CRS are equivalent), but the names do not match at all.</li>
+ * <li>25% means that the CRS are not equivalent, but there is some similarity in
+ * the names.</li>
+ * </ul>
  * Other confidence values may be returned by some specialized implementations.
  *
  * This is implemented for GeodeticCRS, ProjectedCRS, VerticalCRS and