How much did I climb?

[old_windbag]

Also where a garmin is used for course tracking and altitude( using barometric senssor ) it's best to use the wheel sensor option as GPS position isn't great over short distances, so grade measurement can suffer here without it. In terms of distance it will square corners and takes no account of non horizontal distance, so a 1 in 10 hill etc will be shorter than the hypotenuse you cycle up. I use a wheel sensor on two of my bikes but all my bikes have a standard cycle computer in parallel ( a £5 aldi one of late... not bad actually ).
As Audax67 says for some errors i.e different altitude at starting point on a circular ride manufacturers could put some correction in but I don't think they're bothered as most riders perhaps aren't( you could write an application to traverse the data and do this for fun? ). As I also stated earlier theres also the variation of pressure sensor output with temperature to factor in... whether garmin do this I don't know but I'm sure there'd be a correction table from the sensor manufacturer.... but it'll be a dirt cheap transducer, they're not scientific instruments but help for all of us who like stats. I sometimes think with some of the garmins, that it would be better for them to supply the basic hardware and data acquisition platform and let third parties produce software to download onto it to manipulate the acquired data. Then there'd be interfaces and functionality to suit everyones different needs, in a way that is the direction of smartphones but they aren't really designed primarily for cycling/outdoors activities and not something I'd want to have on my handlebars. But I think an open platform fitness device would be a better option than at present.

[Brucey]

I usually get home after a day's ride to find that my house has risen or sunk a few metres...

A few meters would not be realistic. Max over a semi-diurnal cycle would be around 30cm...

I think he means in respect of the barometric pressure being used as a proxy for true altitude. If there is a strong weather system coming through the altitude error can be quite large.

cheers

[Mick F]

This is all very well, but GPS altitude measuring can be ok. It isn't always out by a mile, it depends on your device.

My Garmin Montana tracks elevation gains and losses by a combination of BP and GPS.

Today, I calibrated my start altitude at 240ft as usual. You can do that with a Montana ....... ie you tell it at what altitude you are at any one place.

If I calibrated it at 2,400ft and left it alone, it would slowly correct itself to 240ft because that is the elevation of our house.
How does it correct itself?
By GPS.
How else?

By calibrating it, you make the start-up process quicker.

I always start my rides at the house and press START on the Montana, but I end my rides at the gate and press STOP there because I walk up the drive.

By picking out the elevation of the gate this morning on the way out - by looking at the data on Ascent.app - I see it's at 194ft.
Coming back home, the gate is at 202ft.
Gnat's whisker eh?

That is after a ride of nearly 30miles with a total ascent of nearly 3,500ft.

[Samuel D]

IIf I calibrated it at 2,400ft and left it alone, it would slowly correct itself to 240ft because that is the elevation of our house.
How does it correct itself?
By GPS.

There is an interesting test here that shows this calibration procedure takes a surprisingly long time. I suppose it must, given the noisiness of the data it’s working with.

[Psamathe]

I usually get home after a day's ride to find that my house has risen or sunk a few metres...

A few meters would not be realistic. Max over a semi-diurnal cycle would be around 30cm...

I think he means in respect of the barometric pressure being used as a proxy for true altitude. If there is a strong weather system coming through the altitude error can be quite large.

cheers

The movement of the Earth surface (tidal effect) could be up to 30 cm (depending on how long you are out and the Moon position) i.e Earth's surface moves up and down 30cm'ish per "tide". People often forget that the Earth's surface moves up and down and marine tides are not the only tidal phenomena.

Ian

[Mick F]

GPS altitude isn't Real Altitude.

GPS considers a different Earth to Real Earth. Just coz GPS devices say you are at a certain elevation, it doesn't mean this is geographically correct.

As far as I know, Mean Sea Level is measured differently all over the world. UK Ordnance Survey have different criteria to (say) US criteria.

The Pacific Ocean on the western side of Central America is much lower than the sea level in the Caribbean.

[Audax67]

The garmin altitude measurement based on barometric sensor is reasonable but I agree totally with the previous poster with regard to changing weather systems...

I usually get home after a day's ride to find that my house has risen or sunk a few metres. However, in most conditions, atmospheric pressure variation is approximately linear so it would be easy for manufacturers to offer a correction function across the curve. But AFAIK they don't.

A few meters would not be realistic. Max over a semi-diurnal cycle would be around 30cm (and the sinusoidal pattern means that the bulk of that movement would be happening over only part of that period - "rule of twelfths"). Maybe half a meter at the equator. And of course it depends on the Moon phase.

Ian

Earth tides? Don't think CicloSport thought of those. Wonder if Garmin did.

[beardy]

I would imagine that the same gravity that can deform the earth, can also attract the atmosphere and the satellites themselves and probably by a greater amount.

[old_windbag]

Well at least the earth tides aren't as bad as the gravitational forces subjected to some of Jupiter and Saturns moons. They create enough movement to generate internal heating to volcanic level. We've just got to put up with the moon drifting off away from us and the friction of the atmosphere slowly reducing the earths speed of rotation. Think there's also conservation of angular momentum interaction between the moon and earth related to rotation speed too.

[Brucey]

The movement of the Earth surface (tidal effect) could be up to 30 cm (depending on how long you are out and the Moon position) i.e Earth's surface moves up and down 30cm'ish per "tide". People often forget that the Earth's surface moves up and down and marine tides are not the only tidal phenomena....

Sure, but even if I were out in the fens I wouldn't worry about that overly much....

I would imagine that the same gravity that can deform the earth, can also attract the atmosphere and the satellites themselves and probably by a greater amount.

true enough, and that will have a significant effect on the barometric pressure over a tidal period.... but during that time all kinds of other things are going to change whether you like it not too. BTW the seas bulge tidally towards the moon, but they also bulge away from it...I do not know for sure that both the land and the atmosphere 'bulge'/vary in pressure in the same way. What is known is that there is also both a diurnal and a semidiurnal variation in atmospheric pressure that arises because of temperature variations, i.e. it is in phase with the 12/24hour clock regardless of the phase of the moon. These effects are small enough that they are often masked; they also vary with latitude.

The garmin altitude measurement based on barometric sensor is reasonable but I agree totally with the previous poster with regard to changing weather systems...

I usually get home after a day's ride to find that my house has risen or sunk a few metres. However, in most conditions, atmospheric pressure variation is approximately linear so it would be easy for manufacturers to offer a correction function across the curve. But AFAIK they don't.

Very roughly, atmospheric pressure varies by about 12mbar per 100m altitude near sea level. However it also varies with temperature and humidity. And when you change altitude there is a lapse rate (variation in temperature) that occurs anyway. So given that we have pressure/temperature / humidity variations arising through weather, too, I'd expect that a half-day loop bike ride (using barometric pressure alone as a proxy for altitude) would generate an error of a few metres to a few tens of metres on most days. When a strong frontal system moves through (clue; it'll be very windy!) it could be a hundred metres, easy.

Now if the GPS normalises the data (to correct for pressure variations that do not arise through altitude changes) as we go along, that is fine, but the GPS altitude data isn't just inaccurate, it can be skewed, too. So whilst it might improve the accuracy of the average static measured altitude at any one time, it might well introduce 'additional apparent climbing' depending on how the algorithm works that corrects the altitude as derived from barometric pressure.

In plain English this means that at the end of a loop ride using baro/GPS you will be 'back at the same altitude' but during the ride the altitude could be doing almost anything depending on how the data is used, exactly.

So it'd be interesting to see

a) how close the various measurements of 'climbing' actually are on various test routes (that could be chosen because they do or do not 'undulate' vs have steady gradients) and

b) how riding the same route in different/differently changing weather gives different altitude readings/total climbing, with/without GPS correction.

cheers

[old_windbag]

Bruceys answer above touches on another factor of cycling that I perceive to feel at this time of year, namely variation of air density with temperature. The figures below show that it is quite variable over the range of UK temperatures( as the south east are always boasting of breaking temperature records ).

T deg C density kg/m^3
35 1.145,5
30 1.164,4
25 1.183,9
20 1.204,1
15 1.225,0
10 1.246,6
5 1.269,0
0 1.292,2

So on the close to freezing days to mid summer heatwave theres roughly 11% difference, therefore 11% difference in drag... though viscosity increases slightly as far as I'm aware. So it may be be simply my perception wearing thinner clothes, better blood supply to extremities etc but I do feel I speed up in summer. I think it does have an effect and I feel that the wind in summer you seem to ride through rather than against as it feels in winter.
In the old days when there were seasons we normally had lots of lows in spring and autumn, march winds and autumn gales. Up north I see the wind vector around, as it does, as low pressure systems pass through. It'd be nice if you had time on your hands to ride a circular tour where each day you went with the wind or waited until it was going where you wanted it to then set off with it behind you..... patience might be very nicely rewarded, aside from erroneous altitude measurements.

[Psamathe]

...BTW the seas bulge tidally towards the moon, but they also bulge away from it......

Spaghettification. Astronomers sometimes chose great names for things.

Ian

[old_windbag]

from climb measurement errors to sitting on the event horizon of a black hole thats quite a bicycle trip. Strange to think that the atoms that make us have been inside one, and for the heavier atoms, two stars.

[Brucey]

...BTW the seas bulge tidally towards the moon, but they also bulge away from it......

Spaghettification. Astronomers sometimes chose great names for things.

indeed! Nice name or not I'm dashed if I can entirely understand why the seas bulge tidally on the far side from the moon though....

BTW one of my chums swear blind he goes faster when it is more humid, too; again lower density air....I guess provided you can still get enough oxygen into your blood you are going to faster whenever the air is less dense....

cheers

[Mick F]

............ plus you wear less and aren't restricted with thick clothing.
Not the weight of the clothing, but the restriction both in movement and in aerodynamics.