Problem with 'phone apps' is people chuck it in the bag, pocket etc. It really needs to be as high up as possible so you are not blocking line of site to the satellites. Many Apps don't make use of the other functions on a phone, compass, accelerometer to help out. Probably a bit to complicated to use direction to double check GPS position to position or see if you are in motion or going up or down hill and at what angle.
Garmins are a bit more evolved and cost a lot more for what they do.
No doubt as the phone power increases and Garmin/TomTom etc move over they can use the many years and extra sensors with better GPS or similar dedicated chipsets. Build them robust and waterproof too. Things might improve. Of course tag a Bluetooth magnetic sensor to it and expand away.. Rear and front wheel and cranks would be all easy to add.
To my mind, there seems to be a couple of factors for gps reception. Smartphones tend to use other things (network towers, data access to servers to assist in locating sats, potentially WiFi locating data), and also do tend to have things like accelerometers and "compass" functionality to assist / augment in location. That said, whether all sports tracking apps make use of them (and as an additional example, I know Viewranger downloads some data to assist with altitude.
My only other experience of sat navs has been automotive ones, and whilst they used various things to help with positioning and ephemeris data, and tended to have improved chipsets, sensitivity and reception over time, smartphones do seem to have the edge in terms of filling the gap (and after all, WiFi positioning and NFC are technologies and schemes mooted to help with indoor positioning - eg large malls, say, in the US).
So I can't help but think that decent smartphones, at present at least, probably have the edge in acquiring and maintaining a decent perspective of position. That said, when it gets coarse grained, it probably looks more unreliable that dedicated GPS devices that wouldn't do such a thing.
Decent GPS / sat nav devices where supposed to use acclerometers and digital compasses for dead-reckoning when losing or suffering unreliable GPS reception. Can't say as I've ever used any, though, that did it. I'd like to think they'd thought of, and were using that sort of idea with smartphones - but I fear not - I suspect it just goes to a rather more coarse-grained "generalised" location, based on the other location techniques that can be used - such a shame, or at least that there appears no option, because they look to have all the hardware devices needed.
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