Shapely Length Attribute – Understanding the Unit of Shapely Length Attribute

lengthlinepythonshapelyunits

I'm doing a very simple calculation of the length of a Polyline using shapely:

from shapely.geometry import LineString

... 
xy_list = [map(float, e) for e in xy_intm]
line = LineString(xy_list)
s = '%s,%s,%s' % (fr, to, line.length)

My coordinates are in WGS84. I can't seem to find any information about Shapely's length attribute. What is the unit of the length attribute? Is there an easy way to convert to to km or meters?

Best Answer

Coordinate Systems

[...] Shapely does not support coordinate system transformations. All operations on two or more features presume that the features exist in the same Cartesian plane.

Source: http://toblerity.org/shapely/manual.html#coordinate-systems

shapely is completely agnostic in reference to SRS. Therefore, the length attribute is expressed in the same unit of coordinates of your linestring, i.e. degrees. In fact:

>>> from shapely.geometry import LineString
>>> line = LineString([(0, 0), (1, 1)])
>>> line.length
1.4142135623730951

Instead, if you want to express length in meters, you have to transform your geometries from WGS84 to a projected SRS using pyproj (or, better, execute geodesic distance calculation, see Gene's answer). In detail, since version 1.2.18 (shapely.__version__), shapely supports the geometry transform functions ( http://toblerity.org/shapely/shapely.html#module-shapely.ops) that we can use it in conjunction with pyproj. Here's a quick example:

from shapely.geometry import LineString
from shapely.ops import transform
from functools import partial
import pyproj

line1 = LineString([(15.799406, 40.636069), (15.810173,40.640246)])
print(str(line1.length) + " degrees")
# 0.0115488362184 degrees

# Geometry transform function based on pyproj.transform
project = partial(
    pyproj.transform,
    pyproj.Proj('EPSG:4326'),
    pyproj.Proj('EPSG:32633'))

line2 = transform(project, line1)
print(str(line2.length) + " meters")
# 1021.77585965 meters
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