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I am trying to calculate the distance (in meters) between 2 lon/lng points of type shapely.geometry.point.Point using Python and Shapely. These 2 points are returned from PostGIS by SQLAlchemy/Geoalchemy2 as geoalchemy2.elements.WKBElement, which is then converted to shapely's shapely.geometry.point.Point using geoalchemy2.

However, when I try to use the distance function to calculate the distance in meters, the result is expected to be around 1490 m, but the code calculated it to be 0.013402616901217364.

What went wrong with our code?

Note: Distance calculations will not exceed 30 km

from geoalchemy2.shape import to_shape

users = db_session().query(User).limit(2).all()
foo, bar = users[0], users[1]

print(type(foo.coordinates))               # <class 'geoalchemy2.elements.WKBElement'>
print(type(to_shape(foo.coordinates)))     # <class 'shapely.geometry.point.Point'>

distance = to_shape(foo.coordinates).distance(to_shape(bar.coordinates))
print(distance)                            # 0.013402616901217364
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  • Look at What units are used by GeoPandas/Shapely area and distance functions?
    – gene
    Commented Jan 23, 2021 at 15:25
  • @gene Thanks, if I understand it correctly it appears that Shapely calculates the Euclidean distance, which is does not suitable for lat/lng data which requires WGS84 distance for it to be in the unit of meters. In this case, is it easier/better to work with the WKBElement originally returned by Geoalchemy?
    – Nyxynyx
    Commented Jan 23, 2021 at 16:50
  • Simply change the projection of the geometry
    – gene
    Commented Jan 23, 2021 at 17:33
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    @Nyxynyx, You're right about shapely returning the euclidean distance. If you want to do operations that involve CRS information, you'll probably want to use either geopandas or pyproj (which is what geopandas uses). For example pyproj.Geod.inv will solve your problem, I think.
    – Jeremiah England
    Commented Feb 3, 2021 at 22:56

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