Elevation and GPS

On Sat, 15 Aug 2020 14:34:29 -0700 (PDT),
Andre Jute wrote:
On Saturday, August 15, 2020 at 2:52:44 PM UTC+1, Ted Heise wrote:
On Sat, 15 Aug 2020 13:49:53 +0000 (UTC),
Ted Heise wrote:

Oh, just to add a point that some cutoff is probably needed to
reduce noise from minor variations in barometric readings.
It's not a highly reliable measurment, though moreso than
elevation from GPS.

Ted's explanation. I've never had a bike computer with
elevation that didn't require some kind of baseline rise
minimum as a stabilising measure -- what the designers probably
described as "noise elimination".

All the same, I would be very surprised if even a cheap modern
system, working off GPS, consistently gets it wrong. I'd take
that 7 feet in 1540 any day as accurate enough. As an old sail
circumnavigator, I know only too well how unreliable the
natural ambient barometer can be -- it is an instrument
(courtesy title) on which you interpret trends rather than
particular pinpoint readings.

Well, the accuracy of GPS systems is markedly less for elevation
than it is for horizontal position. This site suggests it's more
than an order of magnitude difference (i.e., +/- 400 feet as
compaed to the 7 feet you mention).

https://support.garmin.com/en-US/?fa...Uv1QyoxITW2vZ6

And here's a site that says, "[the commercial grade] of GPS
handheld unit provides elevation data with poor accuracy."

https://water.usgs.gov/osw/gps/

I can't find anything now to substantiate it, but my understanding
is that the barometric sensors of today are still more accurate
for elevation than is the GPS system itself. If that were not so,
it seems pretty unlikely the barometer would be included.

My Garmin (Edge 820), as many of the units do, allows one to set
various locations with their known elevation. In particular, my
driveway is set as such, and each time I start a ride the unit
calibrates the barometer to give that elevation. I've definitely
seen artifactual changes in elevation when big fronts have come
through while riding, but such changes have generally been
relatively small (i.e., on the order of 20 feet or so).

It also seems possible (at least in theory) that elevation could
come from map data in the GPS unit. I know my hiking/backpacking
unit (Garmin etrex Vista HCx) has topographical maps available,
and they seem pretty accurate. On the other hand, these map files
are huge in size compared to the street maps, so it would surprise
me if they can cram them into the small GPS units. For example,
my Vista will only hold about a U.S. state's worth of topos.
Still, the Edge can display the elevation profile of the upcoming
section of a route, so it must get the elevation data from
somewhere. Maybe it's baked in when the route is created (i.e.,
drawn from the web).

--
Ted Heise West Lafayette, IN, USA

On Sat, 15 Aug 2020 16:04:32 -0700 (PDT),
Tom Kunich wrote:
On Saturday, August 15, 2020 at 6:37:16 AM UTC-7, wrote:
I have a garmin 910xt that on Garmin Connect says I rode 56
miles with 1040 feet of elevation. I did all the hills one can
do in the flatlands. I go to Ride with the GPS and it shows
+1533 and then -1540.I am going to assume that it records the
elevation change at 1533. So what gives the two are off by a
pretty good margin? Strava seems to just take what Garmin
Connect says?

The elevation measurement on a Garmin is a barometric pressure
unit. Between the time you left and the time you returned the
pressure changed so that in fact is a TINY change of only 6'.
Around here we can have a difference between morning and
afternoon of up to 20'. While it is possible for a satellite
link to measure altitude changes it is inaccurate and depends
entirely on the SATNAV satellites being on or very near the
horizon.

Tom, this is entirely consistent with my understanding and
experience.

--
Ted Heise West Lafayette, IN, USA

On Sunday, August 16, 2020 at 5:19:58 AM UTC-7, Ted Heise wrote:
On Sat, 15 Aug 2020 16:04:32 -0700 (PDT),
Tom Kunich wrote:
On Saturday, August 15, 2020 at 6:37:16 AM UTC-7, wrote:
I have a garmin 910xt that on Garmin Connect says I rode 56
miles with 1040 feet of elevation. I did all the hills one can
do in the flatlands. I go to Ride with the GPS and it shows
+1533 and then -1540.I am going to assume that it records the
elevation change at 1533. So what gives the two are off by a
pretty good margin? Strava seems to just take what Garmin
Connect says?
The elevation measurement on a Garmin is a barometric pressure
unit. Between the time you left and the time you returned the
pressure changed so that in fact is a TINY change of only 6'.
Around here we can have a difference between morning and
afternoon of up to 20'. While it is possible for a satellite
link to measure altitude changes it is inaccurate and depends
entirely on the SATNAV satellites being on or very near the
horizon.
Tom, this is entirely consistent with my understanding and
experience.
--
Ted Heise West Lafayette, IN, USA
Since I do a lot of climbing I watch the elevation depending upon unit go up in 3' or 6' segments. This isn't very heart warming when you have 1000 feet left to go.

On Sun, 16 Aug 2020 10:13:44 -0700 (PDT),
Tom Kunich wrote:
On Sunday, August 16, 2020 at 5:19:58 AM UTC-7, Ted Heise wrote:
On Sat, 15 Aug 2020 16:04:32 -0700 (PDT),
Tom Kunich wrote:
On Saturday, August 15, 2020 at 6:37:16 AM UTC-7, wrote:
I have a garmin 910xt that on Garmin Connect says I rode
56 miles with 1040 feet of elevation. I did all the hills
one can do in the flatlands. I go to Ride with the GPS and
it shows
+1533 and then -1540.I am going to assume that it records
the elevation change at 1533. So what gives the two are
off by a pretty good margin? Strava seems to just take
what Garmin Connect says?
The elevation measurement on a Garmin is a barometric
pressure unit. Between the time you left and the time you
returned the pressure changed so that in fact is a TINY
change of only 6'. Around here we can have a difference
between morning and afternoon of up to 20'. While it is
possible for a satellite link to measure altitude changes it
is inaccurate and depends entirely on the SATNAV satellites
being on or very near the horizon.
Tom, this is entirely consistent with my understanding and
experience.

Since I do a lot of climbing I watch the elevation depending
upon unit go up in 3' or 6' segments. This isn't very heart
warming when you have 1000 feet left to go.

Yeah, I fully agree with that as well!

--
Ted Heise West Lafayette, IN, USA