Bring SRID in from_postgis()

[adamboche]

I have a geometry in postgis and query it with df = gpd.GeoDataFrame.from_postgis(). I then want to use df.to_crs(), but df doesn't seem to have a crs already, so it says ValueError: Cannot transform naive geometries. Please set a crs on the object first.

I can set df.crs = ..., but that seems fragile. Is it possible to make geopandas get the srid from postgis when it makes the query, and automatically set the crs?

[kuanb]

I'd like to second this. In my workflows, I tend to have a parse_wkt function that performs the following step:

    srid = None
    if geo_data.startswith('SRID'):
        semi_idx = geo_data.index(';')
        srid = int(geo_data[5:semi_idx])
        geo_data = geo_data[semi_idx + 1:]

    geo = shapely.wkt.loads(geo_data)

The srid is then held and passed along downstream to create a crs dictionary. Seeing as this logic is repeated consistently, and a common use case is to manipulate spatial query results from PG in memory, this would be a useful addition.

[emiliom]

Yeah, I ran into this last November, with the current release.

Note that you can be pre-emptive and pass the crs as an argument to the from_postgis call:

seas = GeoDataFrame.from_postgis("select * from world_seas", conn, 
                                 geom_col='geometry', crs={'init': 'epsg:4326'}, 
                                 coerce_float=False)

But that's also fragile.

It would also be nice if geopandas could guess the geom_col; if only one geometry-type column is pulled by the query, it should be unambiguous.

I may be able to look into this and provide a PR over the next month or so, specially for the crs/SRID part, and specially if I can get some pointers from the developers. So far I see the relevant code (read_postgis) is at https://github.com/geopandas/geopandas/blob/master/geopandas/io/sql.py#L9, and to a lesser extent at https://github.com/geopandas/geopandas/blob/master/geopandas/geodataframe.py#L195 (the from_postgis warapper to read_postgis).

[emiliom]

This need was noted as far back as August 2014, in #161; linking here to that issue b/c there are additional details and discussions there that may be helpful when trying to add this capability.

[jdmcbr]

Closed by #627.