Does turning off Cellular Data affect GPS tracking on a hike?

The short answer

Turning off Cellular Data does not pause, gap or degrade a GPS recording already running, and it does not stop a new recording from starting. The GPS receiver in an iPhone works out a position by listening to satellites, not by exchanging anything with a mobile network, so a setting that only controls what apps can send and receive over that network has no route to affect it.

What the Cellular Data toggle actually controls

Settings → Cellular → Cellular Data is a single switch for whether apps can use the mobile network to send or receive data at all. With it off, calls and text messages over the cellular network keep working as normal — this toggle is about data, not the phone's ability to make calls — but nothing on the phone can browse, sync, stream or download over cellular until it is switched back on or the phone joins a Wi-Fi network instead. People reach for it mainly to avoid a roaming bill while travelling, or to keep a limited data plan from running out before the month does.

That makes it a broader switch than it might sound: unlike a single app's own setting, it applies to every app on the phone at once, cellular only, and it says nothing about Wi-Fi one way or the other.

Why GPS tracking does not run through that path

A recorded route is built entirely on the phone: satellite signals reach the receiver, the receiver works out a position, and that position is written to the track being built up on the device. No part of that chain is a request sent out over the mobile network or a reply waited on. Cellular Data governs exactly that kind of back-and-forth — an app asking a server for something and the server answering — and a GPS fix was never that kind of exchange to begin with, cellular data on or off.

How this differs from Airplane Mode and Low Data Mode

It is easy to lump these three together because none of them affect GPS tracking, but they are not doing the same thing to get there. Airplane Mode turns off cellular, Wi-Fi and Bluetooth together, which means it can also slow down how quickly a phone gets its very first fix of the day if there is no Wi-Fi to fall back on for downloading current satellite orbit data. Turning off just Cellular Data leaves Wi-Fi and Bluetooth completely untouched, so if a phone is on a known Wi-Fi network — a hotel, a car park with a hotspot, a visitor centre — that assist data can still come through Wi-Fi even with cellular data switched off, and the first fix is no slower for it. Low Data Mode, for its part, does not switch anything off at all; it quietly holds back background traffic on a chosen network while leaving foreground use alone. Cellular Data off is the blunter of the three: no cellular data moves at all, for any app, until it is turned back on.

Where a setting like this can still cost something

The parts of a hiking app that do depend on a network are the parts this toggle can genuinely hold back — specifically, anything that would have used cellular data to do it. Syncing a finished route to the cloud is the clearest example: with Cellular Data off and no Wi-Fi in range, that sync simply waits, the same way it waits with no signal at all, and completes once the phone reaches a network it is allowed to use. None of this touches the track itself. The points, timestamps and altitude readings already saved on the device are exactly the same either way; the only thing pending is whether they have been sent anywhere yet.

Things that trip people up

• Assuming "Cellular Data" and "Airplane Mode" are two names for the same switch — they are not. Airplane Mode also drops Wi-Fi and Bluetooth; Cellular Data off leaves both alone.
• Turning Cellular Data back on mid-hike to try to fix a track that looks off — it will not, because the setting was never touching the recording. The usual cause is ordinary GPS noise under tree cover or near cliffs.
• Expecting a cloud-synced route to show up on another device right away with Cellular Data off and no Wi-Fi nearby — the route on the phone that recorded it is already complete; only the copy elsewhere is waiting on a network.
• Leaving Cellular Data off for an entire international trip expecting it to save battery the way Low Power Mode does — the two settings target different resources, data versus charge, and neither one changes how the GPS receiver behaves.

Where TrailCam fits in

TrailCam records a route from GPS in the background and does not need a data connection to do it, so a hike started with Cellular Data switched off, on a trip abroad or anywhere else, records exactly the same track as one started with it on. The only part of the app that needs a network at all is the cloud backup included with the optional TrailCam Pro upgrade, which simply catches up once the phone reaches Wi-Fi or cellular data is switched back on. TrailCam is free to download.