Lines of Population

County Stripes

Population Density Shown as Longitude and Latitude Stripes 

 

🗺️ County Stripes – Exploring America’s Data in Stripes

The theme for the second day of the #30DayMapChallenge is Lines. For this theme I've created a map that colors longitude and latitude lines based on population density.

County Stripes is an interactive map that visualizes U.S. county data not just by county boundaries, but also as smooth horizontal and vertical “bands” of aggregated data. The map lets you switch between county shading, longitude bands, and latitude bands, to reveal how different metrics - such as population, land area, or population density - vary across the country’s geography. 

🔍 What the Map Shows

In its default mode, County Bands colors each U.S. county by a chosen statistic:

• Area - the size of each county in square miles 
• Population - the total number of residents 
• Population Density - people per square mile 

But perhaps the more interesting view is the stripes options. With a single click, you can switch from viewing individual counties to:

• Longitude bands - vertical stripes that group counties by their position east or west 
• Latitude bands - horizontal stripes that group counties by their position north or south 

Each band is shaded according to the average of the selected metric across all counties within that band. This gives a much smoother, more generalized picture of how America’s population and geography change from coast to coast or from the southern border to the northern plains. 

For example: 

• In longitude mode, you can clearly see how population density peaks along the East Coast, thins through the Great Plains, and rises again on the Pacific coast.  
• In latitude mode, you can observe how population and density tend to cluster around certain climate and economic zones - denser in the temperate mid-latitudes, and sparser in the far south and north. 

⚙️ How the Map Works 

County Stripes is built using MapLibre GL JS, an open-source mapping library compatible with Mapbox styles. It loads county geometries from a GeoJSON file that includes population data for each county. Behind the scenes, the map uses Turf.js to calculate the area, centroid, and population density of each county.