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Measuring Ascent and Descent

Measuring Ascent and Descent


Postby kenny lochaber » Thu Jan 23, 2020 1:50 pm

Hi- Would anyone have a recommendation as to which bit of GPS kit- A Garmin watch or whatever ( Preferably analogue rather than smart watch / phone )would be best used to calculate Distance / speed ( I know they all do that)- but also can accurately calculate the ascent you have made and also the descent you have made on a mountain walk across pathless terrain( not the actual altitude you are at) . Would appreciate any opinions as to what more experienced mountaineers found good. Thanks a lot.
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Re: Measuring Ascent and Descent

Postby magicdin » Fri Jan 24, 2020 6:40 am

Or count the contours, kilometres on a paper map, that works too
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Re: Measuring Ascent and Descent

Postby Giant Stoneater » Fri Jan 24, 2020 8:16 am

This relates to part of you question regarding ascent/descent https://support.garmin.com/en-GB/?faq=fQOZqCDNUr9izx2Tz9g1k7
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Re: Measuring Ascent and Descent

Postby crfishwick » Fri Jan 24, 2020 10:51 am

kenny lochaber wrote:Hi- Would anyone have a recommendation as to which bit of GPS kit- A Garmin watch or whatever ( Preferably analogue rather than smart watch / phone )would be best used to calculate Distance / speed ( I know they all do that)- but also can accurately calculate the ascent you have made and also the descent you have made on a mountain walk across pathless terrain( not the actual altitude you are at) . Would appreciate any opinions as to what more experienced mountaineers found good. Thanks a lot.

I am not quite sure what you are asking? A normal gps will do that. Counting contours is rather hit and miss IMHO
If you want something like a watch and cheap I would recommend a Garmin Fortrex 401. Brilliant kit. Not there newest version. M.
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Re: Measuring Ascent and Descent

Postby malky_c » Fri Jan 24, 2020 11:11 am

I’d be wary of relying on any GPS unit to give accurate elevation info. I have a cheapish Garmin watch, Jaxter has a more expensive one. Mine always reads 5-10% less ascent than hers, and a count of the contours usually reveals the truth is somewhere in the middle. I think her watch works on air pressure while mine doesn’t calculate any ascent, and relies on the uploaded route being overlaid on a virtual 3D surface.

In short, counting contours is the only reliable way to know.
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Re: Measuring Ascent and Descent

Postby jmarkb » Fri Jan 24, 2020 11:22 am

malky_c wrote:mine doesn’t calculate any ascent, and relies on the uploaded route being overlaid on a virtual 3D surface.


.... and you will likely get different answers depending on the application you upload it to.

Contour counting is pretty good unless your route involves a lot of small ups and downs.
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Re: Measuring Ascent and Descent

Postby crfishwick » Fri Jan 24, 2020 11:31 am

malky_c wrote:I’d be wary of relying on any GPS unit to give accurate elevation info. I have a cheapish Garmin watch, Jaxter has a more expensive one. Mine always reads 5-10% less ascent than hers, and a count of the contours usually reveals the truth is somewhere in the middle. I think her watch works on air pressure while mine doesn’t calculate any ascent, and relies on the uploaded route being overlaid on a virtual 3D surface.

In short, counting contours is the only reliable way to know.

To be honest there has not been anything invented that gives you an accurate reading. :lol: That includes contour counting.
Why bother?
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Re: Measuring Ascent and Descent

Postby jmarkb » Fri Jan 24, 2020 2:10 pm

crfishwick wrote:To be honest there has not been anything invented that gives you an accurate reading.


It's hard because it's a mathematically ill-defined problem, akin to the "how long is the coastline" question. And the data is noisy.
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Re: Measuring Ascent and Descent

Postby crfishwick » Fri Jan 24, 2020 2:42 pm

jmarkb wrote:
crfishwick wrote:To be honest there has not been anything invented that gives you an accurate reading.


It's hard because it's a mathematically ill-defined problem, akin to the "how long is the coastline" question. And the data is noisy.

It's hard? Every measurement has an error even the Atomic clock :lol:
No point worrying about it. :wink:
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Re: Measuring Ascent and Descent

Postby davekeiller » Fri Jan 24, 2020 3:47 pm

Strictly speaking, you're confusing error and uncertainty in that last comment.

There are two ways to measure your altitude. One way is to use GPS to determine your location in 3D space, the other is to use air pressure.
The weakness of using air pressure is that it's strongly affected by the weather, so you have to re-calibrate regularly otherwise you'll get calibration drift resulting in erroneous calculated altitude and hence ascent and descent. The trouble with GPS is that it's only as accurate as the GPS calculation of your position - it is possible for it to be tens or even hundreds of metres out in areas where the signal is poor.
When using either method to calculate ascent and descent, you also have to consider sampling frequency. A device will generally calculate your altitude once every x minutes and calculate altitude by drawing straight lines between the points. Clearly if you go from the bottom of the hill to the top and then back down again, sampling frequency won't make a big difference beyond possibly missing the very top of the hill. If, however, a route goes up and down a lot (a ridge, say), then a low sampling frequency will smooth out the undulations and underestimate the ascent and descent.

Long story short, pretty much any GPS or GPS equipped device will do what you want. However, not all devices are created equal and there will always be a slight difference between what the device says and your true ascent and descent so take the readings with a pinch of salt.
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Re: Measuring Ascent and Descent

Postby crfishwick » Fri Jan 24, 2020 4:04 pm

davekeiller wrote:Strictly speaking, you're confusing error and uncertainty in that last comment.

There are two ways to measure your altitude. One way is to use GPS to determine your location in 3D space, the other is to use air pressure.
The weakness of using air pressure is that it's strongly affected by the weather, so you have to re-calibrate regularly otherwise you'll get calibration drift resulting in erroneous calculated altitude and hence ascent and descent. The trouble with GPS is that it's only as accurate as the GPS calculation of your position - it is possible for it to be tens or even hundreds of metres out in areas where the signal is poor.
When using either method to calculate ascent and descent, you also have to consider sampling frequency. A device will generally calculate your altitude once every x minutes and calculate altitude by drawing straight lines between the points. Clearly if you go from the bottom of the hill to the top and then back down again, sampling frequency won't make a big difference beyond possibly missing the very top of the hill. If, however, a route goes up and down a lot (a ridge, say), then a low sampling frequency will smooth out the undulations and underestimate the ascent and descent.

Long story short, pretty much any GPS or GPS equipped device will do what you want. However, not all devices are created equal and there will always be a slight difference between what the device says and your true ascent and descent so take the readings with a pinch of salt.

:wink: I think I will leave now! Got me hat and coat. :D
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Re: Measuring Ascent and Descent

Postby kenny lochaber » Fri Jan 24, 2020 4:12 pm

Many Thanks for all your responses- long and short- and those with the detail- very much appreciated
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Re: Measuring Ascent and Descent

Postby Glengavel » Sat Jan 25, 2020 8:27 pm

Just to add - GPS is less accurate in altitude because you can't see satellites below the horizon. In position, you can see satellites in every direction, which gives greater accuracy.
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Re: Measuring Ascent and Descent

Postby crfishwick » Sat Jan 25, 2020 8:29 pm

Glengavel wrote:Just to add - GPS is less accurate in altitude because you can't see satellites below the horizon. In position, you can see satellites in every direction, which gives greater accuracy.


??? An explanation may be required! As the first sentence goes against how all surveying organisation worlwide use gps to ascertain altitude height although in a static position.
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Re: Measuring Ascent and Descent

Postby Glengavel » Sat Jan 25, 2020 11:54 pm

crfishwick wrote:
Glengavel wrote:Just to add - GPS is less accurate in altitude because you can't see satellites below the horizon. In position, you can see satellites in every direction, which gives greater accuracy.


??? An explanation may be required! As the first sentence goes against how all surveying organisation worlwide use gps to ascertain altitude height although in a static position.


Can't remember the exact details but it was to do that, with position, you can see satellites on all directions, so you get a 'balanced' fix. With altitude, you can only see satellites above you, not below you (because the earth blocks them). So you get an 'unbalanced' fix. Something to do with errors cancelling out.
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