Plotting Geographical Data using Basemap

This article by Sandro Tosi is dedicated to Basemap, a Matplotlib toolkit to draw geographical data.

We can use Matplotlib to draw on geographical map projections using the Basemap external toolkit. Basemap provides an efficient way to draw Matplotlib plots over real world maps.

Basemap is a Matplotlib toolkit, a collection of application-specific functions that extends Matplotlib functionalities, and its complete documentation is available at http://matplotlib.sourceforge.net/basemap/doc/html/index.html.

Toolkits are not present in the default Matplotlib installation (in fact, they also have a different namespace, mpl_toolkits), so we have to install Basemap separately. We can download it from http://sourceforge.net/projects/matplotlib/, under the matplotlib-toolkits menu of the download section, and then install it following the instructions in the documentation link mentioned previously.

Basemap is useful for scientists such as oceanographers and meteorologists, but other users may also find it interesting. For example, we could parse the Apache log and draw a point on a map using GeoIP localization for each connection.

We use the 0.99.3 version of Basemap for our examples.

First example

Let's start playing with the library. It contains a lot of things that are very specific, so we're going to just give an introduction to the basic functions of Basemap.

# pyplot module import
import matplotlib.pyplot as plt
# basemap import
from mpl_toolkits.basemap import Basemap
# Numpy import
import numpy as np

These are the usual imports along with the basemap module.

# Lambert Conformal map of USA lower 48 states
m = Basemap(llcrnrlon=-119, llcrnrlat=22, urcrnrlon=-64,
  urcrnrlat=49, projection='lcc', lat_1=33, lat_2=45,
  lon_0=-95, resolution='h', area_thresh=10000)

Here, we initialize a Basemap object, and we can see it has several parameters depending upon the projection chosen.

Let's see what a projection is: In order to represent the curved surface of the Earth on a two-dimensional map, a map projection is needed.

This conversion cannot be done without distortion. Therefore, there are many map projections available in Basemap, each with its own advantages and disadvantages. Specifically, a projection can be:

• equal-area (the area of features is preserved)
• conformal (the shape of features is preserved)

No projection can be both (equal-area and conformal) at the same time.

In this example, we have used a Lambert Conformal map. This projection requires additional parameters to work with. In this case, they are lat_1, lat_2, and lon_0.

Along with the projection, we have to provide the information about the portion of the Earth surface that the map projection will describe. This is done with the help of the following arguments:

The last two arguments are: