Converting Matplotlib Contour Objects to Shapely Objects in Python

matplotlibpythonshapely

I would like to convert a Matplotlib contour collection to a Shapely geometry collection, in order to calculate the area contained within closed contour lines.

it seems that all the information is there in the Matplotlib contour collection to create Shapely polygon collection, based on these stackoverflow answers on extracting data from matplotlib contours:
https://stackoverflow.com/questions/5666056/matplotlib-extracting-data-from-contour-lines and https://stackoverflow.com/questions/17051131/matplotlib-extracting-values-from-contour-lines/17056513#17056513

Has someone already done this?

Best Answer

If I use your first example matplotlib - extracting data from contour lines

import matplotlib.pyplot as plt
x = [1,2,3,4]
y = [1,2,3,4]
m = [[15,14,13,12],[14,12,10,8],[13,10,7,4],[12,8,4,0]]
cs = plt.contour(x,y,m)

The result is:

enter image description here

The number of elements (lines) is given by:

len(cs.collection)
7

and the result you want is the area of one of the polygons (with contourf(): 7 polygons)

enter image description here

In fact, the xy list determined by: 

p = cs.collections[0].get_paths()[0]
v = p.vertices
x = v[:,0]
y = v[:,1]

are the coordinates of the exterior LinearRings of the coloured Polygons. So

from shapely.geometry import polygon
for i in range(len(cs.collections)):
    p = cs.collections[i].get_paths()[0]
    v = p.vertices
    x = v[:,0]
    y = v[:,1]
    poly = Polygon([(i[0], i[1]) for i in zip(x,y)])
    print i, poly
    0 POLYGON ((4 3.5, 4 4, 3.5 4, 4 3.5))
    1 POLYGON ((4 3, 4 3, 4 3.5, 3.5 4, 3 4, 3 4, 3 4, 4 3, 4 3))
    2 POLYGON ((4 2.5, 4 3, 4 3, 3 4, 3 4, 2.5 4, 3 3.333333333333333, 3.333333333333333 3, 4 2.5))
    3 POLYGON ((4 2, 4 2, 4 2.5, 3.333333333333333 3, 3 3.333333333333333, 2.5 4, 2 4, 2 4, 2 4, 2.666666666666667 3, 3 2.666666666666667, 4 2, 4 2))
    4 POLYGON ((3 2, 4 1.5, 4 2, 4 2, 3 2.666666666666667, 2.666666666666667 3, 2 4, 2 4, 1.5 4, 2 3, 2 3, 3 2, 3 2))
    5 POLYGON ((4 1, 4 1, 4 1.5, 3 2, 3 2, 2 3, 2 3, 1.5 4, 1 4, 1 4, 1.333333333333333 3, 2 2, 2 2, 3 1.333333333333333, 4 1))
    6 POLYGON ((2 1, 2 1, 3 1, 4 1, 3 1.333333333333333, 2 2, 2 2, 1.333333333333333 3, 1 4, 1 3, 1 2, 1 2, 2 1))
    7 POLYGON ((1 1, 2 1, 1 2, 1 1))

Plot of the Polygon 4

enter image description here

and the result is given by poly.area

But there are other solutions as in matplotlib - users: pyplot: Extract contourset without plotting or stackoverflow : Python: find contour lines from matplotlib.pyplot.contour() with the undocumented module matplotlib._cntr without plotting anything.

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