[Tagging] GPS Altitude Re: Topographic Prominence for Peaks

Bill Ricker bill.n1vux at gmail.com
Thu Sep 27 16:41:41 UTC 2018

On Thu, Sep 27, 2018 at 3:20 AM Colin Smale <colin.smale at xs4all.nl> wrote:

> On 2018-09-27 07:17, Graeme Fitzpatrick wrote:
> & when you say survey with GPS, is that accurate enough for an altitude
> reading? With my Garmin GPS (which admittedly is 10 - 15 years old, but
> *wasn't* a cheap one!), I can calibrate it in the back yard at 6m ASL, go
> for a day trip & when I get home, it displays the exact same spot as
> anything between -5 & +30m ASL :-( When out driving, I've also seen the
> altitude display change by 100s of m's instantly, when the road is
> virtually flat.
> GPS is not very accurate in the vertical direction, due to ...

and the (unclassified) GPS algorithm is intentionally optimized against
altitude accuracy, to favor horizontal accuracy, since (a) friendly
aircraft have very accurate altimeters and unfriendly projectiles don't,
and (b) other users can generally assume they are in contact with the
surface, so Altitude is the least important output, can and should be least

If one takes a long-term average solution  of GPS posits, especially with
pro WAAF input, the GPS altitude will be approximately right eventually
(and the L/L will be spot on); this is what modern surveyors' Total
Stations and GPS Stations do. (That's ok, as the incoming discount reentry
vehicle can't take a long term average to calculate its fusing height.)

Which is why Garmin units with a Barometer option (e.g. 78sc) allow
calibrating the altimeter altitude at either a known height or a known
sea-level barometer (same as aeroplane analog altimeters are recalibrated
for each airport, done before takeoff and upon making radio contact with
destination Approach Control).
This calibration is good for several hours and maybe a hundred miles or
two, but is not good for the adjacent air-mass, whether the front moves
over the house or the car moves down the interstate.

If you calibrate a Garmin GPS Altimeter to the known height of the peak
when standing on it, it should give vaguely accurate values if you hike to
a col within sight (provided the weather doesn't drastically change in the
meantime ... in which case you should have other priorities anyway), but
you'd still get a better value counting isocline contours on a topo map, or
using a Brunton Pocket Transit to shoot the angle of site [stet] and use
old school trig with the GPS horizontal baseline.

Air Pressure reported by a GPS with barometer in a moving car is prone to
wildly noisy fluctuation besides the slow change in altitude, as Graeme
Window open/closed, Air conditioning High/Low, truck swooshes past, into
the tunnel.
With the car parked and windows open a crack, it will be as protected and
stable as a Weather Service barometer in a protected instrument shed, but
not when driving.
(Unless the car is painted white, the temperature reading will not be as
accurate :-) )

A Garmin calibratable for altitude will give better Altitude numbers than a
pure GPS using only the satellites, but only if used within its
Moving/sealed vehicle altitude readings are not a reliable mode!
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