Anyone who has mapped with a phone or handheld GPS has watched a recorded point land metres from where they stood, or a track wander across a road they never crossed. That drift is normal, and it comes from the physics of measuring position from satellites tens of thousands of kilometres away, through an atmosphere that bends signals, in environments that reflect them. Understanding the error sources lets you predict, reduce, and honestly report positional error instead of fighting it blindly.

GPS receivers calculate position by timing signals from at least four satellites and converting those times into distances. Any error in timing becomes an error in position, and several effects corrupt timing: satellite clock and orbit imperfections, signal delay in the ionosphere and troposphere, reflections off buildings and water, and the geometry of the satellites in view. Consumer devices typically achieve a few metres of accuracy in open sky, which is fine for navigation but often insufficient for mapping that others will rely on.

The ionosphere, the charged layer high in the atmosphere, slows GPS signals by an amount that varies with solar activity and time of day, and it is the largest single error source for single-frequency receivers, often contributing several metres. The troposphere adds a smaller delay related to humidity. Multipath happens when signals bounce off buildings, cliffs, vehicles, or water before reaching the antenna, making the path longer than the true distance and shifting the computed position, which is why urban canyons and forest edges produce the worst drift.

The simplest fixes cost nothing: wait for a good fix before recording, check that your device reports an accuracy estimate of a few metres, and average multiple readings at a point rather than trusting one. Stand away from buildings, vehicles, and tree canopy when recording important points, hold the device steady with a clear sky view, and revisit points at a different time of day if a location matters, since satellite geometry and ionospheric conditions change over hours.

Not every task needs centimetre accuracy, and chasing it wastes money. Navigation, asset inventories, and community mapping work fine at a few metres; property boundaries, construction setout, and engineering surveys do not, and using consumer GPS for them creates legal and safety risks. Match the tool to the tolerance: phone GPS for rough inventories, mapping-grade GNSS for utility and environmental mapping, and survey-grade equipment with proper control for anything that defines rights or structures.