PROPOSAL: to_crs method to accept an optional transformer kwarg

Follow up from: https://github.com/geopandas/geopandas/issues/1142#issuecomment-537634477

Currently there are several options for transforming geometries. The the current method is to use pyproj + shapely as it is quite performant. However, it does not fit all use cases at the moment (e.g. antimeridian). As such, it might be useful to allow for alternate transformation methods in to_crs().

Some defaults could potentially be added to geopandas.transformers or something.

This would be similar to what we do in geocube to allow users to rasterize their points: https://corteva.github.io/geocube/html/examples/rasterize_point_data.html

The initial supported ones could be based on current dependencies:

1. pyproj + shapely transform(default)
2. fiona’s transform_geom (based on https://github.com/geopandas/geopandas/issues/448#issuecomment-539778719)

It could look something like:

def to_crs(..., transform_maker=None):
    ...
    if transform_maker is None:
        transform_maker = pyproj_transform_maker

    transformer = partial(transform_maker, src_crs=self.crs, dst_crs=crs)
    result = self.apply(transformer)

And the transform maker would use functools.partial to return a transformer based in the src_crs and dst_crs that would accept a geometry to transform.

Then, could have different transform makers in geopandas.transformers that could be extended based on various use cases.

Thoughts?

[jorisvandenbossche]

Another use case for such a “transform” method is where you might want to use a different transformation pipeline from pyproj, instead of the default “best” one:

trans_group = pyproj.transformer.TransformerGroup("EPSG:4326", "EPSG:31370", always_xy=True)
# in case you don't want the first
transformer = trans_group.transformers[1]

from shapely.ops import transform
gdf.geometry.apply(lambda geom: transform(transformer.transform, geom))

For this specific case, it might still be worth adding a transformer keyword to to_crs that would only accept a pyproj Transformer, and with which you could override the default Transformer.from_crs(self.crs, crs, always_xy=True)

[jorisvandenbossche]

Alternative might be a method name like geom_transform() ?