Underground map: find your way with no GPS or network
Updated October 3, 2026
A few metres below the surface, a phone loses GPS and network. Its usual map becomes a blank page. Finding your way in a gallery calls for another approach: keep a record of the path, drop landmarks, know at every moment in which direction and how far the exit is. That is what Lidark does, entirely offline.
Why GPS stops underground
GPS relies on very weak satellite signals, which cross neither rock nor concrete. The mobile network meets the same fate as soon as you leave an opening behind. Even an offline map does not place you, because it needs a position to show a dot: without one, it stays a still image.
What remains is what the phone can do without satellites: measure its movements with its sensors, see with its camera and, on iPhone Pro, measure distances with its LiDAR scanner.
A compass app has a similar weakness near steel, reinforced concrete and electrical equipment, which is why a method built on vision and motion is more dependable underground than one built on magnetic north.
How Lidark positions you without GPS
Lidark relies on ARKit, Apple’s layer that fuses camera and motion sensors to track a device’s position in space, to the centimetre over short distances. The starting point is the entrance: everything is then located relative to it, in a local frame. There are no geographic coordinates, no network, no account: the position is relative.
Meanwhile the LiDAR measures walls and floor, and the app derives a minimap that draws itself as you advance.
None of this is new: cavers, miners and divers have always navigated by tracking distance, direction and landmarks rather than absolute position. Lidark automates the bookkeeping, so your attention stays on the terrain.
The thread back: return along the path you walked
The classic trap on the way back is “as the crow flies”: an arrow to the entrance that crosses solid rock is useless. Lidark instead computes the real distance along your path, and shows the next stretch to follow. If you have closed a loop, it spots the shortcut. It is the principle of Ariadne’s thread, built without any string.
Habits that keep you from getting lost
No app excuses you from good practice. A few rules from underground exploration:
- Start the survey at the entrance, before going deeper.
- Drop a landmark at every junction and at every notable point (shaft, danger, change of level).
- Look behind you regularly: a corridor is hard to recognise when you walk it the other way.
- Set a turn-back rule: battery, elapsed time, fatigue.
- Carry two light sources independent of the phone and a power bank.
- Tell someone where you are going and when you will be back.
Night vision and legibility
In the dark, a white screen dazzles. Lidark is pure black by default, with a single colour of light and huge digits readable at arm’s length. A moon button turns everything red: text, plan, 3D and camera. Buttons are 52 points tall so they remain usable with gloves.
Screen brightness is also battery. Pure black pixels are off on OLED iPhones, so a mostly black interface lowers consumption, which matters when the same battery powers the camera, the LiDAR and the torch for a long outing.
What this method does not do
Lidark has no absolute positioning: it does not tell you where you are on a map of the region, only where you are relative to the entrance and the path. Over long distances, drift can build up. Tracking is visual: it needs a little light and walls with relief. Finally, it is an aid, not a guarantee: it replaces neither a guide, nor a rope, nor a prior reconnaissance.
Respect restricted access and private property. For concrete uses, read the guides to urbex in authorised places, quarries and caving. And for data: everything stays on your device, see the privacy policy.