[pct-l] Altitude gain/loss for the PCT, and Oregon really is flat!

[chelin at teleport.com (Sharon & Chuck Chelin)]

Good morning, Tortoise,

I have used both the mechanical barometer type of altimeter as well as a GPS
for measuring altitude; several models of each type.  All of these were of
the standard recreation type rather than the highly sophisticated surveying
instruments.  The accuracy of each type varies for different reasons.  A
barometer, however precise, depends upon atmospheric pressure, which changes
constantly in small degrees, and occasionally dramatically.  The general
recommendation is to recalibrate the barometer every 12 hours or 50 miles,
but as a hiker, I recalibrate every time I find a map altitude reference in
which I have confidence. Peaks and most mountain passes are often reliable,
as well as the water level of lakes.  If there is no reliable reference for
a day or so, I always fall back on the average standard atmospheric pressure
of 29.92 in Hg.  Of the two pocket-size barometers that I now have, the best
is graduated in 20 ft. intervals, which with care can be read to within 10
ft.  When using it, the last thing I do at night is read and record the
altitude at my camp.  Then in the morning I reset the barometer to that same
altitude.  Doing so maintains whatever calibration accuracy that I had, and
alerts me to some possible weather change overnight.  That barometer weighs
3.5 oz. with its tether string.

A GPS relies upon triangulation from NavSat signals so it does not have to
be recalibrated by the user.  The accuracy of a GPS depends upon the
instrument, of course, but more importantly upon the signal quality.  When
my GPS receives a poor signal it continues to show a solution, but the
probable accuracy is also displayed.  Under good conditions its accuracy may
be 7-8 ft. but among the trees or in a canyon the accuracy can degrade to
200-300 feet.  That applies to the X-Y map position accuracy as well as the
"Z" (altitude) accuracy.  A GPS has the useful ability to periodically
record its X-Y-Z locations, but when moving in and out of areas with poor
signal strength it could show that I have changed altitude several hundred
feet when in fact the altitude was approximately constant.  As they say
about fat pills and gas mileage, "your results may vary", but I have not
found that altitudes from a hand-held GPS are as consistent, accurate, and
reliable as those from a good barometer, provided the barometer is properly
calibrated.  However, stopping to record a barometer reading every few
minutes would be a real pain.

For maps, I generally use Natl. Geo. TOPO! software to define routes or
points, then I download the results to a GPS.  I am continually astounded by
the accuracy and usability of that set-up, but I'm an old guy with a very
low amazement level when it comes to all this modern electronic trinketry.

Steel-Eye

"A GPS is superbly accurate at telling you its batteries are dead."
- Universal Military Quotes, Comments, and Axioms




----- Original Message ----- 
From: "Tortoise" <Tortoise73@charter.net>
To: "Jerry Goller" <jerrygoller@backpackgeartest.org>
Cc: <pct-l@mailman.backcountry.net>
Sent: Friday, September 16, 2005 8:47 PM
Subject: Re: [pct-l] Altitude gain/loss for the PCT, and Oregon really is
flat!


> OK all you nerds and geeks --->
>
> Which is more accurate -- the barometer-type altitude or a good
> calibrated GPS?
> And have you compared your altimeter readings with topos to see what
> sort of relationship and reconcilement there is?
>
> Tortoise
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