[pct-l] Altitude gain/loss

[ukstoveman at hotmail.com (David Toms)]

On the subject of barometer vs GPS and the accuracy of maps, I found the 
following:

1) The databook and PCT guidebook maps cannot be relied upon to be accurate 
beyond +/- 300ft elevation. That's not to say all the points were that far 
off, but some were. This isn't about absolute accuracy, but relative, ie you 
can hike in one hour from some point in the guide, stated to be at say 
4500ft, to another point at 3000ft, but your altimeter will record only 
1200ft of drop.  This happened many many times. Coming from the UK this was 
a bit of a shock - our country is small enough and densely populated enough 
that it is very accurately mapped.

2) As Jerry says, major passes and lakes tend to be more accurate, but even 
moving between these I would get some decent 1-200ft errors. Of course, that 
could be because there was 100ft of snow in the Sierra when we were there 
:-)

3) I seem to recall a post in the past by one of the guidebook authors 
indicating that the accuracy of the USGS base maps left something to be 
desired, and that the guidebook showed the PCT where it actually fitted the 
topography rather than necessarily in a completely accurate location.

Now it gets geeky:
Whilst a GPS triangulates off satellites and is immune to barometric 
pressure changes, the technical problem is that the satellites need to be 
well distributed for good triangulation (ie if you triangulated from three 
satellites that were all clustered together, your location would be very 
inaccurate). In the X-Y dimension this is fine - the satellites are very 
well scattered across the sky. However, in the Z dimension there is less 
variation, ie the satellites are at similar distances from your GPS on the 
earth's surface.  This makes altitude triangulation inaccurate. It gets 
worse in moutainous environments because the mountains restrict the 
satellites you can reach to those that are clustered overhead, with little Z 
distance variation from you. I only trusted my GPS altitudes when it could 
get a good view of at least 8 satellites.

So, given that you have no good reference point (the maps/databook aren't 
reliable enough), and can't rely on technology to help you out, even my 
elevation change stated to 3 significant figures probably overestimates the 
real accuracy.

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