Mapping latitude and longitude to country, state, or city

Summary

Austin Z. Henley Associate Teaching Professor Carnegie Mellon University Mapping latitude and longitude to country, state, or city 6/3/2025 An app can easily check a user's location (with permission), but figuring out where that location is, is far more difficult. For example, a web app can use geolocation.getCurrentPosition() to get a user's location in the form of latitude and longitude coordinates. But how do you convert coordinates to country, state, or city? Well, you have to look it up, somehow. (I later learned this is called reverse geocoding.) At the startup I worked at, we paid several thousands of dollars per year for the Google Maps API to do a reverse lookup (that decision was made long before I joined). Given the coordinates, it gave us back a full address. We only cared about the state they were in, so we ignored the rest. If the user didn't allow us to check their location, we simply showed a state selection screen: We paid thousands of dollars a year, just for users to avoid this screen!? Yep. Shortly after I joined the company, I tried to find a library that lets you lookup the state from coordinates, but I didn't find one. It seemed like the sort of thing that should exist. There are a lot of products that want to show you the nearest store. Some products use the user's IP address to determine the location. Or we could have done a simple distance check to a set of known points. Either would have been good enough. Now that I don't have a need for such a library, I went down the rabbit hole and made coord2state based on actual US state borders for quick lookups. It is a single-file JavaScript library with no dependencies that can run client-side to determine which state a given latitude and longitude point falls in. It is 260 KB and is 99.9% accurate compared to the US Census Bureau's borderlines. You could have 100% accuracy if you were ok with 21 MB! It is on GitHub and NPM. Try it: How does it work? The US Census Bureau publishes border data for ...