Choosing a Projected Coordinate System
Geographic coordinate systems, like latitude and longitude pairs from WGS84, are inconvenient for calculating distance between points or area of a polygon. You normally need to convert from your GCS (geographic coordinate system) to a PCS (projected coordinate system) to make useful calculations.
By dragging on the map and clicking on polygons, you can guide your choice of a projected coordinate system. Each PCS is only useful within certain bounds, because points outside become too distorted to be useful.
This tool uses EPSG area polygons, filtered for small, non-deprecated area polygons with units in meters or feet. It's mostly possible because of FlatGeoBuf's MapLibre example, which was released under the BSD 2-Clause "Simplified" license.
There are known polygon coverage issues, especially in Central Africa, Mongolia, Brazil, and the open ocean. In this areas, you should consider using a UTM projection system, which covers any location on the Earth.