What is geomagnetism?

Geomagnetism is a Swift toolkit that tells you the magnetic field characteristics for any spot on Earth. If you're building an iOS app and need to know things like magnetic declination (the difference between true north and magnetic north), field strength, or inclination at a given location, this library gives you those numbers. You create an object with a longitude and latitude, and optionally an altitude and date, then read off properties like declination. The idea is that compass needles don't point to true north everywhere, the gap varies depending on where you are and shifts slowly over time. This library models those variations so your app can correct for them. The main use case is navigation or mapping apps. If you're displaying a compass, routing hikers, or working on any feature where a user's phone compass needs to align with real-world directions, you need magnetic declination to convert between what the phone's magnetometer sees and actual geographic north. A pilot app, a hiking tracker, or a marine navigation tool would all need this kind of correction. Researchers or hobbyists working with geomagnetic data might also find it useful. The README doesn't go into detail about the underlying model, accuracy, or which magnetic model it uses (such as WMM or IGRF), so you'd need to check the source to understand how precise the calculations are and whether they'll stay valid over time. Installation is straightforward: you can pull it in as a Swift package or just drop the single source file into your project. It's a small, focused tool that does one thing, give you geomagnetic values for a coordinate, without any extra dependencies.