Does high altitude affect GPS accuracy on a hike?

The short answer

The altitude of a GPS receiver, on its own, makes no meaningful difference to accuracy. A phone works out its position by comparing signals from satellites roughly 20,000 kilometres overhead, and the distance between standing at sea level and standing on a 4,000-metre summit is a rounding error against a number that large. The geometry a receiver depends on to get a good fix barely shifts between the two.

What people are usually noticing when a track looks rougher high on a mountain is something else entirely: standing close against a rock wall, in a narrow gully, or just under a summit block, all of which block part of the sky in the same way a canyon or a dense tree canopy does. That is a terrain effect, not an altitude effect, and it can happen at low elevation just as easily as high.

Why a few thousand extra metres barely register

GPS satellites orbit at an altitude of about 20,200 kilometres. Climbing a mountain changes your own altitude by, at most, a handful of kilometres — a fraction of a percent of the distance a signal has already travelled to reach you. The angles a receiver uses to work out where it sits relative to several satellites at once are set almost entirely by how those satellites are spread across the sky, not by the last few thousand metres of your own elevation. Nothing about being higher up meaningfully improves or worsens that geometry.

There is a small, genuine effect in the opposite direction from what intuition suggests: a satellite signal loses a little precision passing through the atmosphere, and there is less atmosphere between a receiver and space the higher that receiver sits. It is a real physical effect, but a minor one, and not something a hiker would ever notice on a recorded track.

The case that actually is different on a mountain, and it is a good one

An open summit or an exposed ridge is usually close to the best environment GPS gets. There are no trees, no buildings and no canyon walls between the receiver and most of the sky above it, which is exactly the wide-open view every accuracy figure assumes. If anything, the fix on top of a bald summit tends to be tighter than the one recorded through the forest on the way up, for reasons that have nothing to do with the extra elevation and everything to do with what is, and is not, standing between the phone and open sky.

Where mountain terrain does genuinely hurt a fix

The moments that do degrade accuracy on a mountain are the ones where the terrain itself blocks part of the sky: pressed against a rock face on an exposed scramble, working through a narrow gully with walls on both sides, or standing right under a summit block that hides the sky behind it. That is the same mechanism a deep canyon or a dense forest canopy causes — part of the sky is physically hidden, so fewer satellites are visible and the ones that remain are bunched into whatever gap is open, rather than spread wide overhead. It happens because of the shape of the ground around the receiver, and it happens just as readily on a low crag as on a high one.

What does correlate with altitude, but is not GPS accuracy

A few things really do change on a mountain and get blamed on the wrong cause. Cold temperatures at altitude make a phone battery report less charge and drain faster, which is a battery effect, not a positioning one — the GPS signal itself is entirely unaffected by how cold the device is. Weather tends to be more exposed and more changeable higher up, and cloud or wind can make a hike feel harsher, but rain and cloud cover pass through GPS radio signals without disturbing them. If a track looks worse on a summit day, the weather, the terrain, or the phone tucked away from the cold are far more likely explanations than the elevation number itself.

Things that trip people up

• Assuming a rougher track high on a mountain means GPS struggles at altitude — it almost always means terrain near the summit blocked part of the sky, not that height itself was the problem.
• Expecting a summit fix to be worse than one at the trailhead — an open summit usually has a clearer view of the sky than a trailhead surrounded by trees or valley walls, so the opposite is often true.
• Blaming a jumpy elevation reading near the top of a climb on altitude, when the more likely cause is the same vertical GPS noise that shows up everywhere, standing still or not.
• Crediting or blaming cold weather for a track's accuracy — cold affects the battery, not the satellite signal a phone is reading.

TrailCam records a route in the background the whole way up and down, exposed ridge or sheltered valley alike, and reports the elevation profile underneath the map once you are back to look at it. It is free to download.