Why GPS position drifts or jumps outdoors

The measurement

Every location fix has a margin of uncertainty.

A receiver estimates its position from the travel time of signals sent by navigation satellites. It needs signals from several satellites to solve for three-dimensional position and correct its own clock. The calculation is repeated as you move and as the available signals change.

The coordinate shown on screen is therefore a best estimate. Apple defines horizontal accuracy as a radius of uncertainty around the reported latitude and longitude. A larger radius means less confidence in the point. Consumer devices may hide that number, but the uncertainty still exists.

Outdoor conditions

Obstruction and reflection cause most visible errors.

• Cliffs and deep valleysRock can block much of the sky, leaving the receiver with fewer useful satellites or a poor spread of them.
• Dense tree coverLeaves, branches and moisture weaken signals. Accuracy often changes as the canopy opens and closes.
• Buildings and metal surfacesSignals can reflect before reaching the device. This multipath effect makes the measured path longer and can place the fix on the wrong side of a path or street.
• The device itselfA body, pack or vehicle can obstruct the antenna. Receiver design, power settings and the location services available to the operating system also affect the result.

GPS.gov reports that a typical smartphone can be accurate to about 4.9 metres under open sky. That is not a promise for a wooded ravine, a city street or every individual fix.

Before relying on it

Give the receiver better conditions.

1. Move

   Find a wider view of the sky.

   Step away from a wall, overhang, vehicle or dense stand of trees when it is safe to do so. Even a modest change in position can remove a strong reflection.

2. Wait

   Allow the position to settle.

   Open the navigation or recording view before setting off. A short pause gives the device time to replace an initial coarse location with a stronger fix.

3. Check

   Compare the screen with the ground.

   Use paths, junctions, contours and visible features. If the marker conflicts with several clear landmarks, treat the marker as uncertain.

4. Keep

   Carry the device where signals can reach it.

   A phone near the top of a pocket or pack generally has a better chance than one buried under equipment. Keep the device secure and protected from weather.

Recorded routes

A bad point can affect distance, shape and elevation.

A route recording is built from successive location fixes. A sideways jump may add distance that was never travelled. A cluster of uncertain points can cut a corner, leave the path or create a spike. Software may filter or smooth some of this noise, but no filter can reconstruct every missing detail with certainty. How apps smooth GPS tracks explains the trade between reducing noise and preserving real movement.

Start with a settled position, avoid covering the device and keep location access enabled for the recording app. When reviewing the result, consider the overall route rather than treating one displaced point as proof that you left the path. How GPS route recording works explains how those fixes become a saved line.

Use GPS as one source of evidence. The route line, terrain, signs, conditions and your own judgement still matter.