Re: Bicycle Computer Altimeter Accuracy

KF wrote:
On Sep 17, 3:01 pm, "Clive George" <cl...@xxxxxxxxxxxxxxxxxx> wrote:
T'other way round - the barometric is more accurate. At a guess, a plane
doesn't really care that much about a 10m altitude difference, until it gets
close enough to the ground that the pilot can work that bit out. (fx: reads
Garmin website. It appears they don't use GPS for altitude unless the main
barometric one has died).

Not necessarily so, but let's at least define what we mean by
"accurate."
Take a GPS altimeter reading. With a decent "view" of the sky, it's
almost always within 20 feet or so of the correct altitude and NEVER
varies as the barometer fluctuates. So if you don't know where you are
and don't know what the barometric trend has been, the GPS
gives far more reliable elevation information.

On the other hand if you have just calibrated your barometric
altimeter and you climb a small hill (say, 200 feet high) the
barometric altimeter wil be able to tell you quite accurately how high
the top of the hill is, and even without calibration, can tell you how
much you have gained (the difference in elevation), probably to within
a few feet (although not all have that kind of fine resolution). But
try to measure the top of the hill again tomorrow without
recalibrating and you could easily be off by a couple hundred feet
just due to normal atmospheric changes. The GPS will always be
subject to the +/- 20 feet at any one location but will very rarely be
off more than that. So while the barometric altimeter under ideal
conditions may tell you that the hill is 200 +/- 5 feet high, under
unknown barometric changes it may really only tell you that the hill
is 200 +/- 200 feet high.
And if we change the hill to 2000 feet and still use the same +/-
figures, suddenly the +/-20 feet of the GPS starts to look VERY
accurate.

My experience from a number of years of use of both types of
instruments is that I'll take the overall consistency/accuracy of the
GPS over the barometric altimeter any day, in fact the GPS is about
the only way to conveniently reset a baromeric altimeter after unknown
changes in elevation/barometric pressure.

Heh, heh, my barometric altimeter watch is a really cheap one. I'm almost certain I see abrupt changes in the reported altitude due to the pressure wave of passing trucks. It's still a fun toy, considering it was cheap, and it's usually not off by too much over the course of a day.

Mark J.
.

Relevant Pages

  • Re: Roads on Topo Canada
    ... The best you can expect from a barometric altimeter is +/- ... to smooth the noisy altitude data from the GPS. ... The GPS is constantly calibrating the barometer, with a time constant of about 3 hours. ...
    (sci.geo.satellite-nav)
  • Re: Barometric vs GPS altitude
    ... > better than most (weather depending!) barometric units. ... advantage of a combination of GPS and a barometric altimeter. ... that error in altitude read out of my Vista is within 2 meters. ...
    (sci.geo.satellite-nav)
  • Re: Bicycle Computer Altimeter Accuracy
    ... Take a GPS altimeter reading. ... almost always within 20 feet or so of the correct altitude and NEVER ... So while the barometric altimeter under ideal ...
    (rec.bicycles.tech)
  • Re: Calculate sea-level air pressure from GPS altitude?
    ... However, since the actual sea-level atmospheric pressure at any point on the journey is unlikely to be exactly 29.92 inches Hg, then the ACTUAL altitude will NOT be 36,000 feet and it will vary continuously. ... your in-cabin hand-held GPS will show the true altitude. ...
    (sci.geo.satellite-nav)
  • Re: Digital Photography On Aircraft Not Permitted on Take Off or Landing
    ... because the displayed altitude is nowhere near accurate. ... Garmin GPS units with barometric altimeters are capable of providing ... and i can even see where i made lane changes on maps. ... I'm aware that many users of barometric altimeter GPS units report ...
    (rec.photo.digital)