GPS Elevation

[400bhp]

705 works off barometric pressure.

In short - it's bollox.

 

[Peteaud]

ive compared garmin (205) and bike hike and got roughly the same.

 

[Alien8]

The error in the fix is compounded by the dilution of precision. DOP is a function of the geometry of the tracked satellites around the receiver. For ground based receivers VDOP (vertical) is almost always greater than HDOP (horizontal) because you will typically have a good spread of satellites around the horizontal plane compared to the vertical plane where satellites will all have to be above the horizon, ie one side of the vertical plane. Hence vertical errors will tend to be worse than horizontal errors.

 

[psmiffy]

I think an illustration of how good electronic barometers/gps count altitude is that one of mine yesterday recorded an ascent of 60m while i was in the gym - never moved off of the second floor - started and finished at the same altitude

 

[Rob3rt]

I agree with smiffy. Elevation data coming from a braometer/gps is a fair bit off quite often. Also, if you stop, stand about a bit then restart cycling, you will often find some quite ridiculous shifts! I get this a lot after cycling to meet my club, standing around in the car park, then setting off again. Check my garmin when I get home and find some daft near binary shift sequence style elevation data. Unless my Gamin is on the fritz, I wouldn't rule that out!

 

[PK99]

b) Barometric (including some GPS units)
Barometric altimeters give much less short term variation so are pretty accurate over short periods, but are prone to long term drift and so need calibrating before every ride, and during the ride if possible. In good weather (static high pressure), they could vary by only a couple of metres during a full day's ride, but at other times you could find that the height of a fixed point (home) has varied by 100m or more during the ride. If it does vary, you've that much error included in the ascent figure. As well as that, most barometric altimeters are a bit slow to react and may miss the tops of hills or the bottoms of valleys if you go straight back down/up again.

).

See the screen captures below:

This was a canal ride out and back with lunch at about the mid point

https://www.dropbox.com/sh/v83z39osuntpipm/SXyyhkJKNA

Guess which one was Garmin 800 barometric and which one Correction enabled!!

 

[4F]

Guess which one was Garmin 800 barometric and which one Correction enabled!!

Did you fall in the canal ?

 

[andrew_s]

Guess which one was Garmin 800 barometric and which one Correction enabled!!

Just because something has a barometer doesn't mean it always makes a good job of using it
Perhaps I should have scattered a few "can be"s in there.

I'd also ask whether people who get these sudden shifts have calibrated them properly before the start of the ride?
If the barometric height is way out because it's a few days since you last used it, it may need a stationary spell unconfused by going up & down hills before the auto-calibration can do its thing and put the height right.

I have had a barometric altimeter stay within 2m of the true height all day, and give a total climbing figure within 5m of what I'd got from carefully going over large scale OS digital mapping (what used to be 1:2500 or 1:1250 before digital). That was a Ciclomaster CM414 bike computer.