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Comment by cryptoz | Hacker News Reader

Curious that the Sensors list does not include a barometer. It used to be the case that the A9 with coprocessor carried a barometer inside it. Has this changed for this phone? That would be incredibly sad if they removed sensors. Did the M9 iteration of the coprocessor remove the barometer?

http://www.apple.com/iphone-se/specs/

Edit: Interestingly, the new iPad Pro does have a barometer in the Sensor list: http://www.apple.com/ipad-pro/specs/

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Out of curiosity what use did people find in a barometer?

Apple themselves use it to count the number of flights of stairs you climb. Google uses it in Android phones for faster GPS and for indoor location mapping.

I'm using it to forecast the weather by crowdsourcing billions of live atmospheric pressure measurements at Sunshine (https://thesunshine.co).

Other use cases are for migraine headache prediction/prevention and other health-related issues. The primary and most exciting use case however is weather forecasting!

Edit: Basically, barometers are useful for any altitude-related task, as their error margins for relative altitude changes are very small. Additionally, if you can filter out the altitude data as noise, you're left with atmospheric data, which has immense applications when crowdsourced at scale.

I was really excited about your app but it's not available in the UK app store yet...

Coming to the UK soon! We're constrained by geography in part due to where we run our weather models, but the UK is coming soon I think!

It's also used for emergency phone calls. When 911 calls are made, the barometric pressure is also relayed to give the emergency operators an idea of how high someone is, which is especially useful if the call is made from inside a building.

Is it really? That's awesome.

Indeed, it really is.

How does it work? Isn't 911 just a voice call?

E911, or Enhanced 911, is much more than just a voice call. For some time, E911 calls have sent sensor data from smartphones to the 911 dispatch, such as latitude and longitude. Soon, the FCC is going to mandate all barometer-carrying smartphones to send the barometric pressure when an E911 call is made.

https://apps.fcc.gov/edocs_public/attachmatch/FCC-15-9A1.pdf

What, really? I would figure GPS coords+altitude would be more useful than raw barometric pressure.

Below is the FCC hearing on the matter. The barometric pressure is used to determine the altitude.

https://apps.fcc.gov/edocs_public/attachmatch/FCC-15-9A1.pdf

Your app looks great on paper so I downloaded it from the app store.

It requested me to allow location access, even while in the background.

This is a deal breaker for me for battery life and privacy, so I will not be able to use it.

Edit: Upon checking in settings, I can choose "while using the app only". I understand you probably prompted with "while in the background" to improve your crowdsource data.

I'm glad I will still be able to use it, thanks for allowing this!

How does a physicist use a barometer to compute altitude?

A: drop it from the top of the building and count the number of seconds it takes to hit the ground.

You take it up to the roof, drop it, and use d=0.5 g*t^2 to calculate the height of the building.

GPS is bad at figuring height, can be off by 30-100ft. Think 911 trying to locate your cellphone in a multi-floor building. Barometer data is very good at figuring height.

Yes, but is there any data about the ambient pressure inside a building VS outside? There's a whole lot of horsepower dedicated to HVAC units, do they really hit a zero equilibrium?

I would be surprised and I'm curious too. I always assumed a barometer was how the iPhone measures flights of stairs climbed without expensive GPS calculations.

I think you can pretty easily distinguish stairs climbed using just the accelerometer, but I could be way off. I'd be impressed if a barometer had enough resolution to indicate smaller (~10ft) altitude changes.

The barometers in Android and iOS devices can usually detect altitude changes as small as a few centimeters! Although I wouldn't say that deltas that small are reliable. I think you can get a reliable, trusted altitude delta as small as about half a meter.

That's how the Fitbit Charge HR works.

Could it be that they moved the barometer to that 3D touch sensor block, which isn't included in the SE? Do digital barometers take up significant space?

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