How does my iPhone get its altitude?

Question

I made a simple app which displays the altitude according to the GPS position on iPhone.

• How is the altitude obtained?

• Does it get the altitude of the land at the coordinates I am or does it get the actual altitude? (I tried the same application in the same building; once at ground floor and once on the third floor but the same altitude is displayed)

• Is it possible to obtain a different altitude on different floors? (Like my android phone has a barometer sensor and I calculate the elevation by the atmospheric pressure; but none of the iPhones has a barometer sensor)

Answer 1

The altitude as read by the gps receiver depends on the positions of the satellites over the horizon, relative to each other. It is possible then for the altitude reading to be unreliable when either there are not enough satellites in the sky or they are too close together. This then depends on the time of day, or date.

The signals from the satellites are very weak once they have reached the Earth's surface, and may not be received correctly inside buildings, or where there are other tall buildings nearby, such as a city centre like Manhattan.

Barometer readings will vary according to the weather and so an absolute reading can't be applied to a specific floor. However relative readings may be reliable enough for floor to floor changes, e.g. a lower value means going up.