Mapping Hacks

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Chapter 1, Mapping Your Life

#7 Will the Kids Barf?

HACK

I originally developed my own little index of road curviness that I called the

“quasi-fractal dimension” of a route. As it turns out, the professionals

already have their own measure for this property, and our technical reviewer

Edward Mac Gillavry set me straight.

In spatial analysis, the network deviousness is the discrepancy between the

lengths of the actual routes in a network and the straight-line distance

between the places linked up. For any pair of places on the network it can be

measured by the detour index. The detour index is a measure of how directly

movement may be made on a network. It is calculated as the ratio of the

shortest actual route distance between a given pair of nodes and the direct,

straight-line or geodesic distance between the same two points.

Detour index = shortest distance on a network between two points /

direct distance * 100.

The minimum value of the index is 100, representing a direct route with no

detour. High ratios suggest a weakly connected network but may also reflect

the indirectness or deviousness of the individual routes connecting the

nodes. The detour index is also referred to as the index of circuity or as the

route factor.

So if the straight-line distance between two points is 20 miles, but the road

distance is 25 miles, we get a detour index of 25/20 * 100 = 125. The road

isn’t curvy, honey; it’s ...