I used to be studying the feedback on a time-lapse map of energy outages within the US posted on Reddit yesterday. Lots of the feedback centered on the sizes of U.S. counties, significantly noting how counties seem to get noticeably bigger as you journey westward throughout the nation.
I due to this fact determined to take a better take a look at the measurement of U.S. counties throughout the nation, finishing up some spatial evaluation utilizing MapLibre GL JS and Turf.js.
My interactive map County Stripes lets you discover county sizes in two methods:
- County Coloring Mode – every county is coloured based mostly on its particular person land space.
- Longitude Band Mode – the map divides the nation into vertical slices (longitude bands), calculates the typical county measurement inside every band, and colours the bands accordingly.
Understanding the Longitude Bands
U.S. counties range dramatically in measurement from east to west. Jap states have smaller, densely packed counties, whereas western states are characterised by massive, sprawling counties.
To seize this sample, I divided the nation into longitude bands. For instance, one band would possibly span –112° to –110° longitude. Inside every band, turf.js is used to calculate the typical space of all counties whose centroids fall in that band. This enables the map to indicate how county sizes step by step change throughout the nation, revealing the stark distinction between small japanese counties and huge western counties.
By toggling to the Longitude Band Mode, customers can see vertical bands coloured in keeping with these averages, making it instantly clear the place counties are largest and smallest.
Calculating County Areas with Turf.js
The map makes use of Turf.js, a strong geospatial evaluation library for JavaScript, to calculate the land space of every county. The important thing operate used is:
turf.space(function)This operate accepts a GeoJSON Polygon representing a county and returns the realm in sq. meters. We then convert the end result to sq. miles for simpler interpretation:
const areaSqMiles = turf.space(countyFeature) / 2_589_988.11;Western counties are bigger than these within the east largely attributable to historical past. Jap counties have been created when populations have been dense, so smaller, extra manageable divisions made sense. Western territories, have been settled later and have been initially sparsely populated, requiring a lot bigger counties to cowl these wide-spread much less dense populations.
