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)

http://www.edumobile.org/iphone/iphone-programming-tutorials/how-to-measure-height-in-iphone/ — MCKapur, Nov 11 '12 at 09:05
For your second question see may answer here: http://stackoverflow.com/a/9432382/1127492 — Stefan, Nov 12 '12 at 08:07
Hello.. Have you found a solution for this yet? I am facing the same situation. — Vjlakshmi, May 29 '14 at 15:17

score 4 · Answer 1 · answered Nov 11 '12 at 11:35

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.

Thanks for your explanation. So bottom line, is there any way to determine floor changes in an iPhone since it doesn't have a barometer and the altitude obtained from the GPS aren't accurate inside buildings? Maybe using the accelerometer sensor? — duncanportelli, Nov 11 '12 at 14:01

score 2 · Answer 2 · answered Nov 11 '12 at 09:10

2

Like any other device that uses GPS to calculate altitude, it's measuring the user's distance from the center of the satellites' orbits.

answered Nov 11 '12 at 09:10

Michael Dautermann

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Is it possible to obtain a different altitude on different floors? – duncanportelli Nov 11 '12 at 09:25
I suspect the altitude will be higher on higher floors since you're still calculating based on the distance from the satellite. But I haven't done an altitude-based app yet so I don't know how the algorithms behave and how they compare to sea level and/or the true altitude off the ground. – Michael Dautermann Nov 11 '12 at 09:30

score 1 · Answer 3 · edited Nov 11 '12 at 09:24

1

Your smartphone uses GPS technology to calculate your altitude depending on where you are. It measures how far away you are from the satilites which is calculating this information.

edited Nov 11 '12 at 09:24

Cfr

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answered Nov 11 '12 at 09:13

LucaSpeedStack

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score 1 · Answer 4 · answered Jan 11 '17 at 02:48

Note that newer IPhones (>= 6) have a barometer sensor which in my experience is significantly more accurate than GPS. With a barometers the accuracy appears to be within a foot whereas with GPS it would bounce around by 3-4 meters even when standing still.

http://www.iphonetricks.org/iphone-6-barometer-sensor-features/

How does my iPhone get its altitude?

4 Answers4