Ascent... by GPS (Edge 800)

[ericrobo]

Here's one for the nerds:

I know the Edge 800 uses barometric pressure to record ascent/descent.

Does anyone know if it's possible to take into account pressure changes to accurately (or more accurately) work out the ascent ?

My short ride is about 700 feet (sorry can't do metres, too old...ish )and there is a lot of variation on that depending on pressure changes.

I always set the Edge at 510 feet (my start point at home) and jot down what it is when I get back....

eg - 480 feet

So is there any arithmetical clever formula which can do this adjustment ?

With a start of 510ft and a finish of 480 feet (in an hour) pressure has been increasing, so I guess the Edge thinks that it has been going downhill a bit more than it has ?

[jam bo]

If you made the assumption that it was a linear change over the time and you had the raw elevation record then it would be fairly easy to apply a time based correction to each value and then recalculate the total climb.

Seems a lot of hassle though.

[Alpinum]

First, there are three ways relevant to us to measure altitude; with thermometers, barometers and GPS.

Temperature gradient tells you the temperature change in altitude - decreasing temp by increasing altitude. For my outdoor trips in the Alps (home base) I go by 6°C/1000 m. In the fells of the UK I think it's more like 7-8°C/1000 m. In the subarctic even more. As a generalised rule of thumb (var. literature) 0.0065°C/m is often stated. Lets go with that, a drop of 6.5 °C for 1000 m higher altitude.
Temp gradient = (temp at given altitude - temp at goal altitude) / altitude difference

Then there's the barometric altitude equation which tells you the air pressure at your goal altitude.
Air pressure at goal altitude = air pressure at given altitude × (1 - temp gradient × altitude difference / temp at given altitude in Kelvin) ^ (0,03416 / temp gradient)

I'm not going into GPS signals, that'd be boring.

So we're now stuck with two ways to calculate altitude; by temperature and barometer. As you mentioned the Edge calculating it by barometer only (which I'm not sure it does - thought it was a mix of GPS signal and barometer, at least for the 810 and 820 - but I may well be wrong) we have following situation with your example:

Start (given) alt. = 510 feet or, better (obviously) 155.448 m
End (goal) alt. = 480 feet or 146.304 m
Temp. = 15 °C or 288.15 Kelvin (just to fill in blank spots in the equation)
Air pressure at start = 1000 hPa (mBar) just as an example

This gives us quite precisely a difference of 1 hPa. So if air pressure changes by 1 hPa during your ride you'll get the alt. readings you used as an example.
If the air pressure hasn't changed, your home has changed altitude or you're lost and confused.
Or your Edge was a bit off.

I don't now how a Edge or a watch like Sunnto measure pressure - guess it's a simple load bearing membrane which probably doesn't need temp. compensation, but analogue barometer do. Something to consider if you compare the reading of the Edge with those of eg a mecury barometer.

Oh... nearly forgot to say that barometers tend to be a bit off. If I gain and loose 2000 m and have done no alt. correction, the alt. will be up to 70 m off on my Suunto (less on my Edge, but don't now the numbers, 'cause I don't need the Edge for navigating the way I do with my wrist barometer - mountaineering vs. biking)

Happy nerding

[In Reverse]

I think the correct method is to do the ride again in the opposite direction and everything should cancel itself out just fine. If it doesn't then just keep going until it does.

[ericrobo]

Thanks for your input... but has it left us any clearer ?
Or...
Does it not matter if the ascent recorded is out by 10% or even 20% ?

If you (think) have done 10,000 feet of ascent but it was actually 8000 feet ?

Well it’s only a measure.... but to my mind on mtb ascent is nearly everything... (doing a very steep 100 feet will push your heart rate a lot higher than a less steep 100 feet... and the more you’re in the red on a long ride, the more tired you will be.)

Is an accurate ascent beyond our technology ?

If the Edge 810 and 820 uses gps points as well as barometer can we assume it’s more accurate ?

Can anybody (who has these) do a test ?

Drawing a track in Memory Map uses contours to work out the ascent/descent, but this is probably only approximate.

Alpinum:
Impressive all that but does it help tell me what actual ascent I’ve done ?

Jamboree: (sorry, that was the outer... no, it was the puter...)
So what was the actual ascent ?
So if pressure has increased so your start point is 30 feet lower, what would that translate to ?

In reverse:
Going backwards always helps

[Dave Barter]

"Is an accurate ascent beyond our technology ?"

Simple answer = no but yes

GPS triangulation means that for near real time computation of height you are going to get errors. It's usually much less accurate that barometric. Reason being is that the vertical measure (from you to GPS) is MAHOOSIVE when compared to horizontal and hence more error induced. It can be +/- 20 metres on each measurement. Horizontally you get less than half of this

Barometric measurements are better for short durations when pressure changes do not occur. This needs stable weather and good calibration

There are many high end devices that can sort this out using ground stations to remove the errors but I can hear your northern accent now telling me that you ain't paying for that and in many cases you have to go and lay them out over your route prior to riding it.

So the final answer is to use a f**king map. Plot your route and work out the heights based upon distance from contours. No, I didn't think you'd want to do that. I've written some code to do it which I'm working on all the time, basically using a number of data sources to arrive at a definitive height for any point on the map. It ain't easy

[Scattamah]

Dave's call on GPS-derived altitude is spot on. I've watched my Etrex altitude value change +/- 50' whilst at a standstill. It's a good indicator, but it's by no means perfect.

Strava has an altitude adjust option, which I presume takes into account what the maps say vs what the GPS says.

Greetz

S.

[benp1]

"Is an accurate ascent beyond our technology ?"

So the final answer is to use a f**king map

[ScotRoutes]

Strava has an altitude adjust option, which I presume takes into account what the maps say vs what the GPS says.

.

Sort of. The sampling rate for altitude is poor. If you are going straight up and straight down a hill, it's not too bad. Any sort of contouring and you're fecked.

[ericrobo]

So... if you want a bit more accuracy, when you get to top of a hill, or somewhere where it's easy to see what the elevation is, if you then adjust the gps by setting it to that height, there's a good chance you'll end up somewhere near...
Never thought to do this before...

That will also tell you if pressure is rising or falling (from your last point of checking elevation...)

"I've written some code to do it which I'm working on all the time, basically using a number of data sources to arrive at a definitive height for any point on the map. It ain't easy"

That would be something though Dave and possibly worth doing... (but presumably would still need you to adjust your gps...?)

[Dave Barter]

That would be something though Dave and possibly worth doing... (but presumably would still need you to adjust your gps...?)

No, it uses your location (which is more accurate) to look up the height back at base when you upload to my secret evil genius server.

[Bearbonesnorm]

... and once again, I'm going to start by saying, "please bear with me"

I've being doing some thinking about peoples disconnection from the 'natural world' and I wondered how much our ever increasing reliance on technology might play a part, so my question is - does it matter? What difference does knowing make? Would you not go up if you knew how high it was? Will knowing the height you've climbed add to the experience or could it serve as a distraction and even deminish and detract? I don't know, hence I'm asking.

[Alpinum]

Impressive all that but does it help tell me what actual ascent I’ve done ?

It will at least help you get more precise readings once you know the barometric change.

So the final answer is to use a f**king map

At 10 m or in mountains 20 m equidistance you might wonder where many additional vert gains and losses have come from when going by a modern barometer/altimeter.
From the routes I draw on the Swiss maps (famous for their accuracy) I can add about 5-10 %. All systems have their flaws.

My GPS mostly is more like 2-5 m accurate, a resolution higher than the 10 or 20 m from a good map.

What difference does knowing make? Would you not go up if you knew how high it was?

It has not much to do with ericrobo's issue, but I've been in scary scenarios due to misreadings and understanding how exactly the systems I have at hand work, has helped to stay a bit safer. For my part that my answer to your question.
Thinking about it I've even been in such situations on my bike. You know, that moment when you go down a big scree slope in the mist, the hardly visible Alpine track hidden under fresh autumn snow and you have to ride towards a cliff to go round another one.

[ericrobo]

disconnection from natural world...

The sounds, the sound of the tyres, the air temperature, the quality of the track I’m on, the quality of the riding...
my lungs, my breathing, my legs, the mood I’m in...

I’m usually not thinking how my 15mm bottom bracket fits into a 20mm top sprocket, and loads of other technical mysteries (don’t mention dynamos...)

But I do find it very interesting to have an idea of ascent... and distance...

because these two things have a direct connection with the onset of fatigue...

It’s just very interesting knowing about these things and their effect on your body and how well your body is coping...

Sure if you ignored every aspect of this perhaps you wouldn’t be so motivated to get out and ride...

When I first started climbing I totally enjoyed doing v diffs, then it became somewhat boring, so had to move up to the next grade, until that was a bit boring, so it all became a bit of an interesting challenge...

And MTB (for me anyway) is about endurance, so nice to know whether it was a hard ride or a killer ride and so on...

Then to learn of Mr DB’s extremely clever programming is ever fascinating.. ( and to be honest the bike and its parts is fascinating)

And bikepacking is such an involved activity with all the different options re tents, tarp, sleeping bags, food, clothing, which oil you use... definitely a disconnection from the “natural world”

Ahem ! Have we nothing better to do... ?
(not at the moment...)

[Bearbonesnorm]

And bikepacking is such an involved activity with all the different options re tents, tarp, sleeping bags, food, clothing, which oil you use... definitely a disconnection from the “natural world”

This is true Eric and something I've also being thinking about.

[Dave Barter]

... and once again, I'm going to start by saying, "please bear with me"

I've being doing some thinking about peoples disconnection from the 'natural world' and I wondered how much our ever increasing reliance on technology might play a part, so my question is - does it matter? What difference does knowing make? Would you not go up if you knew how high it was? Will knowing the height you've climbed add to the experience or could it serve as a distraction and even deminish and detract? I don't know, hence I'm asking.

I also think a lot about this.

I am not sure it is a reliance on technology it is a use of it. Knowing my height gain is one more motivation to go out not the ONLY motivation. As is easing my navigation by using a GPS. It's not a reliance its an additional enabler. And in my case I never ever look at ascent or stats whilst riding and only occasionally when I return. I left Strava last year for many of the reasons you describe as it really was changing my riding patterns. But I ain't leaving my GPS nor the ever evolving technology as both of those take me to places that no paper map will. I don't care what the paper zealots say, for me it is backup. The technology allows me to go further and quicker into the wild. It does not disconnect me, exactly the opposite.

[ericrobo]

“Man spotted running through the woods, completely naked, and no shoes on... very near to BB Towers...”

When the police stopped him and asked him what he was doing running through the woods naked, he said he was “connecting with nature”...

They then asked him why he was carrying 3 small sheets of toilet paper... “I’m a poet and need to capture my thoughts ( the lying b##gger) on paper

[In Reverse]

Strava has an altitude adjust option, which I presume takes into account what the maps say vs what the GPS says.

.

Sort of. The sampling rate for altitude is poor. If you are going straight up and straight down a hill, it's not too bad. Any sort of contouring and you're fecked.

They've recently improved this feature - it's now calculated based on an average of the information provided by people using barometric devices. I think it's now pretty good. The old problems of one sample point per 200m or whatever are largely mitigated.