Subject: Re: Geotagging app
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NNTP-Posting-Date: Mon, 06 Nov 2017 13:07:13 -0600
Subject: Re: Geotagging app
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From: Alan Browne <>
Date: Mon, 6 Nov 2017 14:07:13 -0500
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On 2017-11-06 11:48, nospam wrote:
> In article <otq33j$o72$>, Mayayana 
><mayayana@invalid.nospam> wrote:
>> | iphones have a gps, which can show altitude if it's tracking 4 or
>> more | gps satellites, just as any gps would. |
>> A quick look around seems to indicate that the GPS altitude used in
>> cellphones is not very accurate.
> it can be very accurate, depending on how many gps satellites are
> being tracked.
> recent iphones also have a barometer, as do many other smartphones, 
> wearables and of course, dedicated gps devices (e.g, garmin), which 
> increases accuracy, particularly indoors.
> however, nothing is perfect. my garmin dedicated gps in my car once 
> told me i was 200 feet below sea level, despite being on a bridge
> over a river and with at least 8 sats being tracked. it was wrong. it
> did have the correct road, though.

GPS elipsoid and the local geoid are not coincident all over.  The 
difference can be quite large and correction models are not all that 

The barometer is not very useful to determine altitude above sea level 
unless set to a known reference point first.  Otherwise it's pretty good 
for relative changes over a short period.

Hmm, there's a thing Apple could do: have a function to ping Apple with 
the position of the phone and receive the nearest QNH (or a triangle of 
them to interpolate from).  Then the baro altitude could be used to 
determine an accurate altitude above sea level.