I have a PostGIS road network layer (all features are linestrings) which I am accessing in Python and would like to get the geometry for each feature as a list of points. Something like [(lat1,lon1),(lat2,lon2),(lat3,lon3)...] would be ideal. I am currently accessing the geometry by using the ST_AsText(geom) function, and then parsing the resulting string into points. But this seems clunky, and since this seems like a pretty common/basic operation I would expect there to be a standard way to do it, but I am not seeing anything in the documentation. Is there a function or good way to do it out there that I am just missing?
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1Use something like Fiona or Shapely and get an array or points directly?John Powell– John Powell2016-05-03 23:01:49 +00:00Commented May 3, 2016 at 23:01
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1ST_PointN will work, as will ST_StartPoint/ST_EndPoint if there are only two vertices.Vince– Vince2016-05-04 00:06:28 +00:00Commented May 4, 2016 at 0:06
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1 Answer
I use psycopg2 and shapely to read PostGIS geometries into Python, like this:
import psycopg2
from shapely import wkb
conn = psycopg2.connect(...)
curs = conn.cursor()
curs.execute('SELECT geom FROM line_table WHERE gid=%s', (23,)) # one geometry
geom = wkb.loads(curs.fetchone()[0], hex=True)
geom.coords[:] # [(x1, y1), (x2, y2)]
If you have MultiLineString geometries instead, then each geometry has a .coords property, e.g. the first part is geom.geoms[0].coords[:].
Note that the coordinates are Cartesian (x y) which is the norm in GIS software, not (lat lon) the human readable norm. If you need the later, there are a few tricks that can be used on either the server or client side.
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This solution has worked really well but just want to add that anyone accessing polygon coordinates will want to change geom.coords[:] to geom.exterior.coords[:] or geom.interior.coords[:] depending on whether you want the outer or inner ring of a polygonwmebane– wmebane2017-01-05 22:48:49 +00:00Commented Jan 5, 2017 at 22:48