Re: [xnewmanx] The new iPhone has a barometer in it.
I don't have iPhone 6 but have some reasons to be sceptic about its barometer. When writing L/D Magic app I quickly realized that the internal GPS in iPhone is no good for freefall. (Jump from airplane and look at altitude data yourself.) When altitude changes rapidly, it shows where you were
a minute or two ago. Why is that?
Apparently, Apple is not passing raw, unfiltered data from GPS to any application that uses location services - it heavily smooths it which introduces lag. This lag may not be noticeable when changes are slow, and with freefall (and even ride to altitude) it has a "long, stretchy rubber cord" effect on data. It almost looks like the data points are lined up in a long slow queue... My best guess is that Apple uses very heavy Low Pass Filter, it probably takes just 1% of the current measurement and averages it with 99% of the previous smoothed measurement. I was able to imitate this "rubber" effect in Excel by doing LPF with 0.99 smoothing coefficient on modeled raw data.
Apple does this, probably, to make location look better - when user is standing in one place, you don't want to show the Brownian motion that raw GPS data is, constantly jiggling around, you want to show a nice steady blue dot. Unfortunately, Apple does not provide both smoothed and raw data... and once smoothing is baked in, you can't undo it.
Long story short, I don't think that what you'll see from iP6's barometer is the actual, current altitude. It will be as unusable as GPS altitude. Users are already complaining that the Health app outputs some nonsense, far from actual elevation changes.
It's better to have an external Bluetooth pressure sensor, for example,
SensorTag from Texas Instruments. With TI, you have raw data from sensors and detailed documentation for each sensor used.