1. The "Multipath" Error (Bouncing Signals)
Your iPhone receives signals from satellites orbiting 12,000 miles above Earth. If you are in a city with tall buildings (the "urban canyon" effect) or under dense trees, the satellite signal bounces off the buildings before reaching your phone.
Because the signal took a longer, bouncing path, your phone calculates that the satellite is further away than it really is, resulting in your location jumping across the street.
2. You have Wi-Fi turned off
Smartphones don't just use satellites. They use a system called Assisted GPS (A-GPS). Apple and Google maintain massive databases of Wi-Fi router locations. If you have Wi-Fi turned on (even if you aren't connected to a network), your phone scans nearby routers and instantly knows your location down to a few meters.
3. The Datum Shift Error (Advanced)
If you are trying to match your iPhone's coordinates to a paper map or a local government property boundary, they might not line up at all. This is usually due to a Datum Shift.
Your iPhone outputs coordinates in a global standard called WGS84. However, many local maps use regional systems (like NAD83 in the USA or OSGB36 in the UK). If you don't convert the coordinates mathematically, your location will appear to be off by several meters.
Using your phone for property lines? STOP.
Using an iPhone to determine a property line or fence boundary is legally dangerous. A 5-meter iPhone error can result in building on your neighbor's land, costing thousands in legal fees.