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Given a geopandas GeoDataFrame containing a series of polygons, I would like to get the area in km sq of each feature in my list.

This is a pretty common problem, and the usual suggested solution in the past has been to use shapely and pyproj directly (e.g. here and here).

Is there a way to do this in pure geopandas?

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If the crs of the GeoDataFrame is known (EPSG:4326 unit=degree, here), you don't need Shapely, nor pyproj in your script because GeoPandas uses them).

import geopandas as gpd
test = gpd.read_file("test_wgs84.shp")
print test.crs
test.head(2)

enter image description here

Now copy your GeoDataFrame and change the projection to a Cartesian system (EPSG:3857, unit= m as in the answer of ResMar)

tost = test.copy()
tost= tost.to_crs({'init': 'epsg:3857'})
print tost.crs
tost.head(2)

enter image description here

Now the area in square kilometers

tost["area"] = tost['geometry'].area/ 10**6
tost.head(2)

enter image description here

But the surfaces in the Mercator projection are not correct, so with other projection in meters.

tost= tost.to_crs({'init': 'epsg:32633'})
tost["area"] = tost['geometry'].area/ 10**6
tost.head(2)

enter image description here

  • Your text is epsg:3857, but your code is epsg:3395, which of the two is correct? – Aleksey Bilogur Nov 20 '16 at 20:11
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    The .to_crs function gets passed to pyproj anyways. A good example of an equal area projection: proj4.org/projections/cea.html which can be passed as follows: .to_crs({'proj':'cea'}) – Swier Jan 24 '17 at 11:25
  • For the US Census Tracts shapefiles at least, I can confirm that {'proj':'cea'} produce the closest area estimations. – Polor Beer Dec 7 '17 at 20:12
2

I believe yes. The following ought to work:

gdf['geometry'].to_crs({'init': 'epsg:3395'})\
               .map(lambda p: p.area / 10**6)

This converts the geometry to an equal-area projection, fetches the shapely area (returned in m^2), and maps that to a km^2 (this last step is optional).

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