The thirteen degrees that move a zone
A phone compass points at magnetic north. Vastu is drawn from true north. On the US west coast the difference is a whole zone — so the app tells you which north it is using.
Hold a phone flat in Mountain View and the compass says north. It is pointing about thirteen degrees east of it.
That is magnetic declination — the angle between where a compass needle settles and where the pole actually is — and it varies with where you stand. In Delhi it is under a degree and nobody notices. In California it is thirteen. In Seattle, fifteen. The vastu mandala is drawn from true north, so a house read off an uncorrected phone compass on the west coast has every zone rotated by most of half a zone. The kitchen you think is in the south-east is straddling the south. The pooja corner is in the wrong quarter.
Every phone can correct for this; almost none do by default. The correction needs your location, because declination is a function of it, and a compass reading is only marked “true” while the phone knows where it is. Ask for a single fix and the phone hands you true north for a moment, then quietly falls back to magnetic. The dial looks perfectly healthy. It is thirteen degrees wrong.
So the scan does three things. It keeps a coarse location fix running for the whole walk — kilometre accuracy is plenty, since declination changes about a hundredth of a degree per kilometre — and uses it for exactly one purpose. It names the reference on screen: True north — corrected for your location, or, if the fix hasn’t landed yet, Magnetic north for now in copper, so you never mistake one for the other. And it writes the reading, the correction, and the accuracy into the record of the scan, so a home whose north was estimated under poor conditions is read with less confidence than one whose north was confirmed.
Nothing about your location is stored or shown; the app has no interest in where the house is, only which way it faces. The number under the dial — 95° E, ±3° — is the same one you’d read off the Compass app held in the same hand. That is deliberate. A reading you can check against something you already trust is worth more than one you have to take on faith.