The GPS Toolkit: Compass, Altitude and Speed

What each readout is genuinely good for, and exactly how much to trust it.

Most people never open the tools section of a maps app, which is fair — a compass and a speedometer sound like things your phone already has. What makes them worth knowing is not the individual readouts but the fact that they sit next to a map, so a number and a place are on the same screen.

Here is what each one is genuinely good for, and how much to trust it.

The compass

The most useful and the most misunderstood. A phone compass reads the Earth's magnetic field, which means it points at magnetic north rather than the north on your map, and the difference between the two varies depending on where you are standing.

For everyday orientation that difference does not matter. For anything precise it does, and the fix is to know which one you are reading.

What a compass beside a map is actually for: working out which way you are facing when you emerge from a station and every direction looks identical. Point the phone, match it to the map, walk.

It is easily confused. Metal, magnets, phone cases with clasps, cars and reinforced concrete all pull the needle. If the reading swings or feels wrong, step away from whatever you are near and move the phone in a figure of eight to recalibrate.

Altitude

The number people trust most and should trust least, because phones derive it two different ways.

From GPS, which is good horizontally and noticeably weaker vertically. Errors of tens of metres are normal.

From the barometer, which measures air pressure and infers height. This is more responsive and drifts with the weather, since a passing weather system changes the pressure without changing your altitude.

So treat a displayed altitude as approximate. It is excellent for relative change — knowing you have climbed two hundred metres since the car park — and less good as an absolute number to quote.

The speedometer

GPS speed rather than wheel speed, which makes it genuinely different from a car's dashboard reading.

A car speedometer is required to never read low, so most read slightly high on purpose. A GPS speed is calculated from your actual movement across the ground, which is usually closer to the truth. Comparing the two on a motorway is how a lot of people discover their car reads a few per cent over.

Where it falls down is anywhere the sky is blocked — tunnels, urban canyons, dense forest. Fewer satellites means a worse fix, and the number becomes unreliable exactly when you are moving fast between tall buildings.

It is not a substitute for looking at your dashboard, and no reading on a phone is a defence for anything.

Using altitude with the map, which is the actual point

A number on its own is trivia. A number beside terrain is planning.

Look up a hike before you go and read the altitude at the start and at the top. The difference is the climb, and the climb is the thing that decides whether a walk is an afternoon or a day — far more than the distance does.

The same reading answers a question people ask badly. "Is this hotel high up?" usually means "will it be cold, and will I be out of breath?" Fifteen hundred metres in the Alps in October is a different holiday from six hundred, and the map will tell you which one you have booked.

It also explains snow. Two resorts on the same mountain, one at eight hundred metres and one at eighteen hundred, have entirely different seasons. Checking the base altitude before the brochure is the single most useful thing you can do with this number.

Nearby places and weather

The two everyday ones.

Nearby places answers "what is around me" without a specific search — restaurants, parks, attractions, points of interest within reach. Most useful in a city you do not know, in the twenty minutes when you have not decided anything yet.

Live weather sits alongside the map rather than in a separate app, which sounds trivial and is not. Checking the weather where you are about to travel, while looking at the terrain you are about to travel into, is a different thought from checking a forecast on its own.

Questions people ask

Why does my compass point the wrong way?

Usually magnetic interference. Move away from metal, cars and speakers, then wave the phone in a figure of eight. If it is consistently off by a fixed amount, that is the magnetic-versus-true north difference rather than a fault.

Is the altitude reading accurate enough for hiking?

For relative height gain, yes. For an exact summit elevation, no — use it as a guide and trust a marked map or a sign for the real figure.

Does the speedometer work without signal?

It needs a GPS fix, not a data connection. It will usually work with no mobile signal in open country and fail in a tunnel with full signal.

Do these drain the battery?

Continuous location use costs more than browsing does. Close the tools when you are finished with them rather than leaving them running.

Learn the compass and ignore the rest until you need them. It is the one you will use every trip.