Fundamentals of Data Visualization

Chapter 15. Visualizing Geospatial Data

Many datasets contain information linked to locations in the physical world. For example, in an ecological study, a dataset may list where specific plants or animals have been found. Similarly, in a socioeconomic or political context, a dataset may contain information about where people with specific attributes (such as income, age, or educational attainment) live, or where man-made objects (e.g., bridges, roads, buildings) have been constructed. In all these cases, it can be helpful to visualize the data in their proper geospatial context, i.e., to show the data on a realistic map or alternatively as a map-like diagram.

Maps tend to be intuitive to readers, but they can be challenging to design. We need to think about concepts such as map projections and whether for our specific application the accurate representation of angles or areas is more critical. A common mapping technique, the choropleth map, consists of representing data values as differently colored spatial areas. Choropleth maps can at times be very useful and at other times quite misleading. As an alternative, we can construct map-like diagrams called cartograms, which may purposefully distort map areas or represent them in stylized form, for example as equal-sized squares.

Projections

The earth is approximately a sphere (Figure 15-1), and more precisely an oblate spheroid that is slightly flattened along its axis of rotation. The two locations where the axis of rotation intersects ...