Garmin Forerunner - Elevation Inaccuracy

I've encountered the same problem using an Edge. If you look at the
elevation profile you'll find it's very noisy with occasional large
spikes in both directions thrown in; this all adds up to exaggerate the
total elevation.

Some mapping programs will smooth the elevation, taking a rolling
average over a certain period of time; I find this gives far better
results that match what the map says it should be. 'SportTracks'
(http://www.zonefivesoftware.com/SportTracks/) is one such program, and
is what I use to record my rides because it's a stunning piece of
software and is free to boot.

I don't think it's as simple as an "inaccuracy factor" though; on some
rides it's been dead on, others twice the value it should be. I presume
it all depends on whether your route has a few long climbs or many
short ones and how good a position the GPS can read along the way.

Phil

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phil

"Cattle Prods solve most of life's little problems."
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Tom,

My current solution is GPS/bluetooth module and PDA, this seems
reasonable whilst riding but goes bananas when stopped at the pub (big
height changes & very high speeds >50mph). However using SportsTracks,
as mentioned by Phil above, it is pretty quick to remove the quirks.

I think the only solution is to ride an identical route a few times and
get some idea of the total variation into a program like SportsTrack.
On the site they have done half of the exercise showing multiple repeat
trips of a single route, but haven't mentioned height variation.

I am looking at getting the Forerunner 205 so any feedback on altitude
variation with Garmin products will be interesting.

Keith

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I'm using the 301 and have had good success with it... as long as there
are not many tall trees or buildings. As a check of the accuracy in
your specific location, try to compare the elevation using google
earth. With google earth, try zooming in and point to a location. The
elevation will be shown at the bottom. I've also heard of people using
garmin data to retrace routes in google earth.

here's some info I got from a friend on email last month

---If you're into navigation by global positioning system
(GPS), British roadie Martyn Davis is your kinda guy. He's developed a
free web service that allows the plotting of routes anywhere in the
world using Google maps fed into a GPS device. Davis built his site
after buying the Garmin Edge 205 GPS-enabled bike computer and finding
its software "disappointing." First you'll want to read about his
development of the service at http://www.marengo-ltd.com/gps. To see
the service itself, click http://www.marengo-ltd.com/map. Even if this
technology is beyond you, it's fun to see your cycling roads (or those
you might be visiting) represented by Google maps and satellite photos.
When you get to Davis's site, zoom out, center the map over your
general area, then start zooming in. Once you've located your home
turf, select the "Tools" button and click "Save" to make it your
default start point.

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hudson
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Tom-

You are a human locomotive. If anything your GPS is underestimating
your altitude differentials. I've ridden with you. You can probably
ride up a vertical wall 1000 meters without breaking a sweat.

--
harper

-Greg Harper

B L U E S H I F T

*jc is the only main man. there can be no other.*

This is still my nth post....*AWESOME!!!*
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Semach.the.monkey said:

I have also created some static maps based on speed, altitude etc, and
I
would agree with you that the altitude seems to be one of the most
inaccurate aspects of a GPS system.

STM

Altitude readings require a lock on 4 or more satellites while 2D
(latitude and longitude) readings require a lock on only 3 or more
satellites. There will be times where you can get a 2D location but
the GPS receiver will not be able to calculate an altitude due to too
few satellites in view or poor satellite geometry.

The GPS receivers also geared towards giving you a 2D location and the
altitude is more secondary. The receiver will give you an accuracy
number of say 42 feet. That accuracy number is for 2D location. You
can assume that the 3D location accuracy is worse (may be much worse).
That accuracy number is also just a guess by the receiver based on
satellite geometry and other factors. It is only a guess. The 3D
accuracy is going to be worse than your 2D accuracy. The elevation
plots from the GPS are going to vary more than the 2D location plots.

The GPS calculations are done by triangulation with the satellites and
the time it takes the signals from each satellite to reach your
receiver. The accuracy of those calculations depends on the geometry
(location) of the satellites in the sky. The wider the triangles are,
and the more spread out the triangles are, the more accurate the
calculations can be. The satellites in the sky are constantly moving
so the geometry is also constantly changing over time.

Best bet for elevation plots from a GPS is to do some post processing
on the data to smooth out the elevation readings and get rid of the
jaggies. The jaggies in the elevation plot are probably erroneous
readings. I believe that SportTracks can automatically smooth out the
elevation data (I know you can manually edit the elevation data to
smooth it out one data point at a time). There is probably other
software that can edit or smooth out elevation data but I don't know of
any off-hand.

Another thing you can do is go to the high points and low points along
the ride and sit there with the GPS till it gets a good satellite lock
and good geometry and then take a waypoint. That waypoint should give
you a more accurate elevation reading than the reading you get while on
the move during a ride. Then use that more accurate waypoint elevation
to help smooth out the elevation data from during a ride.

A GPS receiver with a barometric altimeter built in will give a
smoother and more accurate elevation profile. But even then it can be
off based on weather factors (say a front moving through) or through a
poor calibration of the barometric sensor at the beginning of the ride.

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I have the Garmin e-trex Vista C with the built in altimeter. The
altimeter itself works good and appears fairly accurate, especially if
it is calibrated to the actual elevation first.

But I sometimes get different cumulative elevation gain/loss
information from the GPS display and the graph once I download it to my
computer. For example, after a recent ride the GPS said I only had a
cumulative elevation gain of 400' but when I add the total elevations
on the chart it is almost double that. The chart was more correct.

Haven't figured out why yet.

--
UniBrier

Steve

Hop Drop & Roll

“If something is to hard to do, then it's not worth doing. You just
stick that guitar in the closet next to your
shortwave radio, your karate outfit and your unicycle and we'll go
inside and watch TV.” – Homer
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UniBrier said:

when I add the total elevations on the chart it is almost double that.
The chart was more correct.

Not necessarily. I once spent a surprisingly easy day climbing Tryffan
in Snowdonia, only to realise afterwards it was muddy thumb print on
the map. I'd thought the view was a bit disappointing.

--
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If you meet someone superior to yourself, imagine yourself his equal; if
you meet someone inferior, look carefully at yourself.
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