I noticed tracks on my iPhone 4S don't have elevation unless the app has built in topos. Tracks without elev suddenly get them when I upload to a server – presumably they combine lon-lat track data with a topo.
If this is true – I haven't found independent confirmation – then one should be careful about any elev data off the iPhone. The reason is that the x-y points are strongly affected by whether you are on a steep slope or not. The signal tends to get drawn in and up the hillside by the multipath reflections and ground plane conduction. If the x-y is off by even ten meters in such a case, it could easily place your elev way off since it derives the elev from a topo. Note also that the topos are not super-super accurate either.
A great example of this is when I cross the Golden Gate bridge – 250 feet over the water, but my GPX track from the phone has me at sea level.
When I look closely at iPhone tracks on Google Earth, I see they do not exactly follow the clearly visible trail I walked. I know GE is often off, but it is off by a fixed vector over a small enough area. I find that north and south slope tracks "suck up the hill" rather than being both off in the same direction. The hidden effect of this is again in elevation.
I use elevation info (along with fall line heading) when orienteering from a topo and I'm not sure if the distant peaks are what I think they are. I have found that my baro altimeter is more accurate than the iPhone elevation once I take into consideration weather variance (recal at known spots).
On a side note, my electronic altimeter, which is spec'd to 10%, reports exactly 10% shorter elevation change than was actually travelled. So if I recal at Road's End (5035) and hike up Copper Creek to the benchmark at Lower Tent (7825), the unit shows 7550 = 10% short of my known location. Just the opposite is true going down – it reports a net 2500 feet, ending up at the correct recal elevation. I sure wish the unit would take a correction factor input, but at least I can keep myself occupied with the math.
My GPS is an ancient Garmin eMap, so I wouldn't use it for any comparison with what may be available today. Has anyone else compared the elevations in the GPX files between smartphone and a dedicated GPS?