When traveling across a landscape, the shortest point from A to B is rarely “as the crow flies” due to barriers or twists and turns in the route. Likewise, the uniform distance or travel time from a particular point (say five miles from Point A in any direction) can rarely be measure by a circular buffer zone. Instead, isodistance and isochrone calculations are used to show bands of equal distances and travel times from a specific point respectively (typically represented as polygons although the calculations are actually performed along routes, i.e. lines. In these maps, the outer edges of each polygon represents the isoline).
Isodistances represent the calculation of specific distance intervals from one point extending out along every possible route. The word is a concatenation of iso, from the Greek word isos which means ‘equal’ and distance. Isodistances can be used to calculate bands of walking or travel times. For example, one might want to calculate the 1/4, 1/2, and 1 mile walkable routes from a local school so parents can understand how far their house is from the nearest route to school.
There are several examples of the use of isodistance. MapBox recently posted about the technical background behind calculating isodistances using its platform. The post, entitled “Dive deeper into isodistances” includes an embedded demonstration map made using the Directions API that users can click on to calculate the 1, 2, 3, 4, and 5 mile distances from any point within Washington, D.C.
Closely related to isodistance, isochrone is a concatenation of iso and chrone from the Greek word chrónos for ‘time’. Isochrone maps show lines of equal travel time across a geographic area. Created by Stefan Wehrmeyer, Magnificent shows calculations of equal travel times using public transportation for select major cities in the United States, Europe, Australia, and New Zealand plus Tel Aviv, Israel, São Paulo, Brazil, and Kuala Lumpur, Malaysia. To use, select a city from the list and adjust the time slider to uncover the geographic extent a traveler could theoretically access using public transportation. Here’s how far you might be able to travel in 8 minutes in Halifax, Canada:
Not just a recent function of GIS, Isochrone maps have been in use since at least the early 1880s. Francis Galton published a piece in the November 1881 issue of the Proceedings of the Royal Geographical Society and Monthly Record of Geography entitled “On the Construction of Isochronic Passage-Charts” in which he produced a map showing travel times from London, England around the world.
Galton, F.. (1881). On the Construction of Isochronic Passage-Charts. Proceedings of the Royal Geographical Society and Monthly Record of Geography, 3(11), 657–658. http://doi.org/10.2307/1800138