Accuracy

A phone’s GPS is good most of the time and wrong in ways that add up: a position that jumps across the street and back, a cloud of positions while you stand at a crossing, a guess from Wi-Fi when the sky is hidden. Geo Tracker records every position the iPhone delivers and keeps only the ones it can trust, so the file measures the walk and not the noise. The rules below were learned on real walks, our own and our testers’, and each one is in the app because a recording showed the need for it.

Which positions go into the file

• A good first fix. Nothing is recorded until a position comes within 20 m (or, among tall buildings, within 30 seconds and 50 m), so the track starts where you are and not at the phone’s last cached guess.
• No old or rough positions. A fix older than a minute when it arrives, or worse than 50 m, is left out.
• No position without a satellite height (since 3.1). A fix whose height is uncertain by more than 40 m was not placed by enough satellites: it is the iPhone placing itself from Wi-Fi or cell towers, often tens of meters off, and it is left out. Between tall buildings, under a bridge or inside a building the line waits for the satellites to return.
• No impossible jumps. A fix farther from the last one than the activity allows over the time between them (about 14 km/h walking or hiking, 32 km/h running, 108 km/h on a ride), plus both fixes’ own uncertainty, is a spike and is dropped. After five in a row the reference was the bad one, and the next fix is taken.
• No out-and-back jumps. Each fix is held for a second until the next arrives; one that jumped out to the side and straight back, faster than the phone measured you moving, is dropped. This is why the live numbers run about a second behind.
• Nothing while you stand still. While the phone measures you slower than about 1 km/h, no new point is added: a red light, a phone call or a shop draws no scribble. The first fix of the stop is taken, so the track reaches the place, and the first one that moves again connects from it.
• A fix without a speed (Wi-Fi again) is added only once it is farther from the last point than its own uncertainty, so a slow walk on such fixes leaves a point every so often instead of a cloud.
• After a pause the next fix may be anywhere, because you may have driven to the next trailhead, so the jump rules start afresh.

Distance

The distance adds up the straight lines between the points kept, within each segment, and the recording screen counts it the same way the saved file is measured, so the number you watched is the number in the list. Where the filter left a gap, the line across it counts once, which is why a stretch without satellites is measured short rather than long. See Comparing with a watch.

Heights

A GPS height is far less certain than a GPS position. Geo Tracker writes a point’s height into the file only when the iPhone puts it within 8 m; among tall buildings that can take minutes, and a point without a trustworthy height has none in the file, rather than a wrong one. Nothing in a file is invented: no height without a vertical fix, no time without a timestamp.

Climb

With Climb from the Barometer on (the default) and a barometer in the iPhone, the climb and descent come from air pressure, which measures height changes to a fraction of a meter, with a 2 m threshold so noise does not count. Otherwise they come from the GPS heights the rule above let through, with a 5 m threshold. Either way the climb shows a dash until there is a height to trust, and the file’s heights stay GPS heights, so another app computes its own climb from the same points. The barometer needs the Motion & Fitness permission; the first Start asks for it.

Pace, speed and moving time

The pace or speed on the recording screen is the speed the GPS measures from the signal itself, not from the distance between two points, so a jump in position does not show as a burst of speed. It reads a dash while you stand still or when no fix has arrived for eight seconds. Moving time counts a step only when that step itself moved, and never a gap longer than a minute, so the average pace is the pace you walked, not the pace including lunch.

Where GPS is weak

• Tall buildings reflect the signal; expect gaps in the line and, where the line is drawn, a few meters of wobble.
• Indoors, in a station, under a thick roof there is no satellite fix at all; the line waits.
• A Wi-Fi-only iPad has no GPS and places itself from nearby networks, which makes a rough track with points far apart.
• Precise Location off gives positions kilometers apart; the recording screen says so.
• A pocket, a bag, a bike frame make little difference; the phone’s antenna sees the sky through them.

Comparing with a watch

An Apple Watch, a Garmin and this app each filter the same kind of GPS in their own way and the distances differ by a few percent, most of all among buildings, where one keeps a position the other drops. On open ground with a view of the sky, a Geo Tracker walk and an Apple Watch walk sit within a few meters of each other. A recording that reads long or short can be trimmed or fixed in GPX Explore.