# Node elevations and edge grades¶

API usage limits: 50 requests per second, 2500 free requests per day, and 512 locations per request. So that's 2500 * 512 = 1.28 million total locations theoretically possible per day. But, the request URL is limited to 8192 characters, so OSMnx rounds coordinates to 5 decimal places (approx 1 meter) to fit 350 locations in a batch (thus ~875,000 nodes are possible per day).

Note that there is some spatial inaccuracy given Google's dataset's resolution. For example, in San Francisco (where the resolution is 19 meters) a couple of edges in hilly parks have a 50+ percent grade because Google assigns one of their nodes the elevation of a hill adjacent to the street.

In [1]:
from keys import google_elevation_api_key #replace this with your own API key
import osmnx as ox, networkx as nx, numpy as np
ox.config(log_console=True, use_cache=True)

In [2]:
# get the street network for san francisco
place = 'San Francisco, California'
G = ox.graph_from_place(place, network_type='drive')

In [3]:
# add elevation to each of the nodes, using the google elevation API, then calculate edge grades


## Calculate some summary stats¶

Use an undirected representation of the network so we don't overcount two-way streets (because they have reciprocal edges pointing in each direction). We use the absolute value of edge grade because we're interested in steepness, not directionality.

In [4]:
edge_grades = [data['grade_abs'] for u, v, k, data in ox.get_undirected(G).edges(keys=True, data=True)]

In [5]:
avg_grade = np.mean(edge_grades)


Average street grade in San Francisco, California is 4.0%
Median street grade in San Francisco, California is 2.6%


## Plot the nodes by elevation¶

Plot them colored from low (violet) to high (yellow).

In [6]:
# project the street network to UTM
G_proj = ox.project_graph(G)

In [7]:
# get one color for each node, by elevation, then plot the network
nc = ox.get_node_colors_by_attr(G_proj, 'elevation', cmap='plasma')
fig, ax = ox.plot_graph(G_proj, fig_height=10, node_color=nc, node_size=20, node_zorder=2, edge_color='#dddddd')


## Plot the edges by grade¶

Grade is the ratio of elevation change to edge length. Plot edges colored from low/flat (violet) to high/steep (yellow).

In [8]:
# get a color for each edge, by grade, then plot the network
ec = ox.get_edge_colors_by_attr(G_proj, 'grade_abs', cmap='plasma', num_bins=100)
fig, ax = ox.plot_graph(G_proj, fig_height=10, edge_color=ec, edge_linewidth=1.5, node_size=0)