CODEWITHSUNDEEP
python-3.xmatplotlibgeopandas
tom91
tom91

Reputation: 685

How to plot a line representing a value from a dataframe with two geometry columns?

I have the following data:

import pandas as pd
import geopandas as gpd
from shapely.geometry import Point
import matplotlib.pyplot as plt


points = gpd.GeoDataFrame([['A', Point(1.5, 1.75), Point(2, 2), 16],
                          ['B', Point(3.0,2.0), Point(3, 4), 18],
                       ['C', Point(2.5,1.25), Point(1, 1), 19]],
                    columns=['id', 'geometry', 'geometry b', 'value'], 
                    geometry='geometry')

the first geometry column represents the starting point, the second the ending point of a line which has a value corresponding to the value column.

I have tried to plot this with:

f, ax = plt.subplots(1, figsize = [12, 12])
points.plot(ax=ax, column = 'value')

However it just plots the first geometry column and colours the points corresponding to their value.

How do I produce a plot that draws the lines, colour coded to their value?

Upvotes: 1

Views: 6467

Answers (1)

swatchai
swatchai

Reputation: 18782

This working code and its output plot demonstrate how you can achieve what you need. See comments in the code for more details.

import geopandas as gpd
from shapely.geometry import Point, LineString
import matplotlib.pyplot as plt

# handle all points and create relating lines
pA1 = Point(1.5, 1.75)
pA2 = Point(2, 2)
line_A = LineString([[pA1.x, pA1.y], [pA2.x, pA2.y]])
pB1 = Point(3.0, 2.0)
pB2 = Point(3, 4)
line_B = LineString([[pB1.x, pB1.y], [pB2.x, pB2.y]])
pC1 = Point(2.5, 1.25)
pC2 = Point(1, 1)
line_C = LineString([[pC1.x, pC1.y], [pC2.x, pC2.y]])

# create a geodataframe,
# assigning the column containing `LineString` as its geometry
pts_and_lines = gpd.GeoDataFrame([['A', pA1, pA2, 16, line_A],
            ['B', pB1, pB2, 18, line_B],
            ['C', pC1, pC2, 19, line_C]],
            columns=['id', 'beg_pt', 'end_pt', 'value', 'LineString_obj'], 
            geometry='LineString_obj')  # declare LineString (last column) as the `geometry`

# make a plot of the geodataframe obtained
f, ax = plt.subplots(1, figsize = [4, 4])
pts_and_lines.plot(ax=ax, column = 'value');
plt.show()

The output plot:

enter image description here

If you prefer to build a dataframe containing from_point and to_point first, then append new column containing LineString creating from the existing points, here is an alternative code.

import geopandas as gpd
from shapely.geometry import Point, LineString
import matplotlib.pyplot as plt

# this dataframe `points_df` contains from_point, to_point for creating `lineString`.
points_df = gpd.GeoDataFrame([['A', Point(1.5, 1.75), Point(2, 2), 16],
                          ['B', Point(3.0,2.0), Point(3, 4), 18],
                       ['C', Point(2.5,1.25), Point(1, 1), 19]],
                    columns=['id', 'geometry_a', 'geometry_b', 'value'])

# add new column, `line` to the dataframe,
# this column contains `LineString` geometry.
points_df['line'] = points_df.apply(lambda x: LineString([x['geometry_a'], x['geometry_b']]), axis=1)

# assign geometry to `points_df` using the column that has `LineString` geometry
# take the result as `target_gdf`
# `target_gdf` is now capable of plotting with matplotlib 
target_gdf = gpd.GeoDataFrame(points_df, geometry=points_df['line'])

f, ax = plt.subplots(1, figsize = [4, 4])
target_gdf.plot(ax=ax, column = 'value');
plt.show()

Its output plot is the same as the previous one.

Upvotes: 1

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