Elevation gain - GPS, Barometric, Strava

@Tom Weymes

I saw a question on this from you and cant find it now - so here is my overview

GPS accuracy (simplified)
GPS measures the distance between you and a number of satellites.
If it can measure this for 3 satellites it can calculate where you are on the earth.
The technology and the geometric calculations can result in an error of +/- 15metres (95% of the time) for your horizontal position, (ie. latitude and longitude)
GPS calculates altitude relative to the center of the orbit of the satellite.
Then tries to relate that to the surface of the earth.
However the earth is not uniformly spherical
Its more like a squashed ball.
So GPS calculates where you are on a "model" of the earth
This can result in further inaccuracy
If the satellite reception is poor due to trees or sides of a valley this can increase the error

If your GPS can pickup 4 satellites this will increase the accuracy.

(Military GPS units do the same thing but use 2 frequencies for better results.
Since 2000 the US govt no longer messes with the GPS signal to degrade it for civilian use).

The errors are more pronounced for altitude measurement. These can be up to ±45 metres on the vertical axis.

Barometric altimeters measure pressure. Pressure changes with altitude (and temperature). Therefore if the pressure at a reference point is known, along with the altitude at that point - and the pressure at a location can be measured - then the altitude at that location can be calculated. Barometric pressure also changes with the weather. To get an accurate measurement at a specific location an altimeter should be re-calibrated regularly at a known altitude.
For cycling this is not as important as we are interested in the elevation change which can be calculated directly from pressure change.

Other factors
How often you measure the altitude
If you started at the coast and measured your altitude (0m)
did no more measurements as you cycled over Sally Gap to Laragh
and measured your altitude again at Glendalough Green (140m)
Then your elevation gain would be a mere 140m (nonsense)

If you took 3 measurements for the route (start-0m, SallyGap-500m, end-140m)
Then your elevation gain would be a 500m(close but no cigar)

Garmin edge units can record mesurements every second
but most devices default to "smart recording" and measure every time you change direction, elevation, speed.... (this is less acurrate but uses less storage space)
So the Garmin would probably record an elevation gain of 880m

However, Strava doesnt trust the elevation data from many devices.
Instead it uses its own digitized topographic map (but not always)
Then it lays your GPS track on top of this map
Then it reads your elevation at lots of points on our ride
and calculates the total elevation gained and lost
Unfortunately, many people say that Strava's map is not particularly accurate.

So if your GPS is out by 15m - then as you go over a bridge Strava might record your elevation on a different road

Net net. Pick one system and stick with it.
Does it really matter if a given ride has 437m gain or 447m gain.
If you stick with the same system all the time you should have similar errors on each ride.
But, the relative info between different rides should be reasonably OK
. eg Ride A has more gain than Ride B

On the other hand - I could be making all of this up as I go along