[pct-l] Question about Halfdmile App

[Dan Welch]

In November I "tested" the Halfmile App on my iPhone on the AT in a location
where there was no cell service.  I wanted to be sure it would work just
based on the phone GPS function alone. It did great at pinpointing my
location on my USGS map and I think it did well at telling me how far and
which direction to the PCT.  (1932 miles?! Really?  It was probably
correct...) 

One anomaly I noticed was that the elevation was frequently off by 100 feet
or so...  which wouldn't have seemed so odd to me except that the
geolocation was so accurate.  I know the geolocation was accurate because I
checked it at definitive locations (where the trail crossed a stream or
road; where the stream crossed a road; at the top of a saddle or peak; etc)
It compared very favorably to my USGS map...  except the elevation. It seems
like elevation is frequently off by 50 to 150' in elevation.  Anyone else
noticing that same variation?

Which got me wondering - how does it determine elevation?  I don't think
there are any barometric sensors on the iPhone, so I ruled that out.  Do the
GPS satellites allow an estimate of elevation?  Or is it in comparison to
some topo map data?  (This seems unlikely since it would have to store map
data... a ton of it.)

So my guess is that the GPS function is doing both coordinates and
elevation.  Can anyone confirm or deny that?

My second question - I'm due to get a new phone before I start my Thru next
year and I am all but certain that I will switch to the Galaxy S4.  Can any
S4 users confirm that the Halfmile app works well with that phone?  I expect
that it will, but if there are any issues, I'd like to know it BEFORE I make
the switch.

Thanks all!
Timberline