Wayfinding has 3 parts
How does GPS work? There are about 30 satellites orbiting the Earth, each constantly broadcasting a radio signal with their own name and the exact current time, "I am satellite #12, and the time right now is 3:15:42." (Actually, the time is way more accurate than that.)
Your phone (or GPS device) listens to several of these satellites at once. Based on the speed of light, those signals take a split second to get to your phone. By comparing how long it takes each signal to arrive, your phone can calculate how far it is from each satellite.
Each GPS satellite constantly broadcasts its precise orbital information describing its exact position over the next few hours. So your phone knows exactly where the satellite was when it broadcast the time and with distances from at least four satellites, your phone can then do the math to say, "I must be right here, at this spot on Earth."
So why does it not work sometimes? Because those satellite signals are like very faint radio whispers from 12,000 miles away. Things can interfere:
GPS by itself only tells you a set of numbers, your latitude and longitude. On its own, that isn't very useful. What makes GPS truly powerful is map data. A digital map connects those numbers to real places -- streets, intersections, buildings, and landmarks.
Map data comes from many sources, satellites, aerial images, surveys, and contributions from people on the ground. To improve accuracy, companies like Apple and Google pay people to drive cars equipped with cameras and sensors, recording road layouts, signs, and landmarks.
Some map databases are commercial -- you have to pay to use them -- while others are free, like OpenStreetMap, which is created and updated by volunteers around the world. Many GPS apps for the blind use a combination of map data sources depending on cost, coverage, and accuracy.
So why does it sometimes not work? All of the commonly used GPS apps for the blind need an internet connection to access map data. If you lose your network connection, if you are in a network dead zone for example, your GPS app can still tell you where you are on the globe, but it won't be able to tell you your current address or what's around you.
Also, for a GPS app, the source for the map data makes a huge difference in how useful it is. Points of interest -- restaurants, stores, etc -- come and go and this often is not reflected in the map databases. If your GPS app uses one of the less accurate map databases, you may get inaccurate information, especially about local points of interest.
Map data is represented internally as a graph. Intersections are points on the graph and roads between them are edges with different lengths depending on what you are measuring. For example, 2 intersections might be 500 feet apart or they might be 90 seconds apart. If you ask for the shortest route, it would use the distance. If you ask for the quickest route, it would use the time. Whatever type of value you are using, this is called the "cost". With this data, your phone can calculate the route with the lowest cost from your current location to your destination.
Your phone won't be able to calculate a route if you don't have an internet connection. Route calculations depend on the accuracy of the map data. Sometimes it is impossible to have accurate data, especially for the fastest route, because it depends on current traffic conditions.
Wayfinding apps have 2 main uses:
Examples:
Other things wayfinding apps can do for you:
Some notes on safety:
The compass app is installed by default in all iPhones.