Cartography. Vizualizing speed of movement with color scale on map in Python

Question

I have 3 main values (longitude, latitude and speed) in csv. Using Folium library i can map the location with lon and lat degree.my current code is:

import pandas as pd
from geopy.geocoders import Nominatim
import folium
from pandas import *
import numpy as np
from matplotlib import cm
import folium

df = DataFrame({'lon': [9.899193, 9.899236, 9.899292, 9.899331, 9.899372, 9.899411, 9.89946, 9.899518, 9.899393, 9.899409, 9.899456, 9.899498, 9.89952, 9.899558, 9.8996, 9.899625, 9.899646, 9.899659, 9.899678, 9.899707, 9.899745, 9.899778], 'lat': [48.849231, 48.849201, 48.849163, 48.849128, 48.84909, 48.849056, 48.84901, 48.848968, 48.849018, 48.849014, 48.848979, 48.848949, 48.848926, 48.84888, 48.848831, 48.848789,48.848762, 48.848735, 48.848712, 48.848686,48.848655, 48.848632 ], 'speed' : [15, 25, 32, 45, 50, 75, 64, 32, 13, 23, 42, 13, 23, 42, 64, 32, 13, 23, 42, 13, 23, 42]})
ave_lt = sum(df['lat'])/len(df)
ave_lg = sum(df['lon'])/len(df)
points = zip(df['lat'], df['lon'])
points = list(points)
my_map = folium.Map(location=[ave_lt, ave_lg], zoom_start=14) 
folium.PolyLine(points, color="red", weight=2.5, opacity=1).add_to(my_map)
my_map

till here it works well and gives me output like this: but now i want also put the speed attribute from csv to generate color scale (based on speed values) on current map. for example if the speed is between 0-20 the that part of line is red, if speed is between 20-60 the yellow, if the speed is higher than 60 then the line is green. is it possible to do it in python? Can anybody help me with this?I would like to get output like this :

Im sorry for this question but im new in python and i really need to do this, Thanks in advance!! I'd really appreciate your help!

Answer 1

Try this:

First, write the following two functions

def draw_polylines(points, speeds, map):
    colors = [speed_color(x) for x in speeds]
    n = len(colors)
    # Need to have a corresponding color for each point
    if n != len(points):
        raise ValueError
    i = 0
    j = 1
    curr = colors[0]
    while i < n and j < n:
        if colors[i] != colors[j]:
            line = folium.PolyLine(points[i:j], color=curr, weight=2.5, opacity=1)
            line.add_to(map)
            curr = colors[j]
            i = j
        j += 1
    if i < j:
        folium.PolyLine(points[i:j], color=curr, weight=2.5, opacity=1).add_to(map)


def speed_color(speed):
    if speed < 0:
        raise ValueError
    elif speed >= 0 and speed < 20:
        return 'red'
    elif speed >= 20 and speed < 60:
        return 'yellow'
    else:
        return 'green'

Then, after the line

my_map = folium.Map(location=[ave_lt, ave_lg], zoom_start=14) 

make this call

draw_polylines(points, df['speed'], my_map)

I tried it on your given data and it seemed to work. Definitely review it for yourself.

Essentially, the line segment colors are decided point by point, using the corresponding speed for each one. If there isn't a speed for every point, or there is a negative speed (write more extensive error testing; data is usually not clean!) this code will throw an error.