The qibla away from home
Travelling breaks a qibla compass in ways that standing in your own living room does not: the building is unfamiliar, the phone may have no signal, and sometimes the floor itself is moving.
Hotel rooms
Work it out by the window, not by the bed. Modern hotels are steel frames wrapped in concrete with wiring in every wall, and a magnetometer in the middle of one can be several tens of degrees out without any warning. A window, a balcony or the street outside gives the sensor a far cleaner field.
Then bring the answer back inside by eye: note which corner of the room, or which edge of the bed, the line runs towards. That reference is stable for the whole stay, and it does not need the phone again.
Lifts, air conditioning units and the metal frame of a mirror will all pull the needle. If the reading changes as you walk around the room, believe the one you took by the window.
No signal, no data
Location does not need a data connection — GPS is a signal your phone receives, not one it asks for, and it works in flight mode. What it does need is a view of the sky, so a windowless room may take a minute or fail entirely.
This site keeps working offline once you have opened it once, and if you know you are travelling somewhere without coverage it is worth loading it before you go. If GPS will not fix at all, type a postcode instead — or use the bearing for the nearest city from the table on the front page, which is accurate to a fraction of a degree over any distance you are likely to be from it.
Trains and cars
A vehicle is a steel box, and inside one a magnetometer is measuring the vehicle rather than the Earth. There is no calibration trick that fixes this. If you have to work out the qibla during a journey, do it at a stop, outside, away from the train or the car.
Aircraft
At altitude the compass is useless — the airframe, the engines and the seat frames see to that — and the bearing changes continuously as you fly, sometimes by a great deal over a long flight. The honest answer is that a phone in a cabin cannot tell you the qibla.
The established position among the jurists is that you face the qibla if you reasonably can and pray in the direction of travel if you cannot; on a scheduled flight the second case is the usual one. Many aircraft moving maps show the aircraft’s heading, which is the practical way to relate the direction of travel to the cabin.
Somewhere new, on foot
Step outside, take the reading in the open, then pick something on the horizon that sits on the line — a mast, a tower, a hill. A landmark cannot be demagnetised, and it keeps working for the rest of the trip.
If nothing is visible and the compass is behaving oddly, the sun and a shadow need no equipment at all.