This topic has been archived, and won't accept reply postings.
I’ve recently bought a Suunto core and having troubles with the altimeter. I’ve been setting the reference altitude at the beginning of the day but by the time I’ve reached a summit a few hours later and climbed 7-900mtrs the altimeter reads about 30mtrs above my actual altitude.
Initially I noticed this during a period of unsettled weather over Christmas, so i put it down to changes in pressure through the day. But having used it at the weekend in very settled conditions i’ve had the same problem.
With no experience with altimeters am I expecting too much? Or should they only be relied upon for short climbs/periods of time? Or is there something i’m missing?
Initially I noticed this during a period of unsettled weather over Christmas, so i put it down to changes in pressure through the day. But having used it at the weekend in very settled conditions i’ve had the same problem.
With no experience with altimeters am I expecting too much? Or should they only be relied upon for short climbs/periods of time? Or is there something i’m missing?
In reply to asmith37:
30 metres amounts to approx. 3 bar which even on a "settled" day isn't unlikely over a few hours.
I find my altimeter more useful when using it as a datum for ascent or descent i.e. I need to climb 100m, set it to zero, climb up until it gets to 100m. For descent I'd set the amount as 100m then stop at zero.
30 metres amounts to approx. 3 bar which even on a "settled" day isn't unlikely over a few hours.
I find my altimeter more useful when using it as a datum for ascent or descent i.e. I need to climb 100m, set it to zero, climb up until it gets to 100m. For descent I'd set the amount as 100m then stop at zero.
In reply to asmith37:
The altimeter works through changes in atmospheric pressure,so any time the pressure changes, the apparent altitude will change. Over the course of a few hours, it's likely that there will be small changes in pressure (even when the weather seems to be settled). Depending on the model of Suunto watch you've got, you can check to see if the pressure changes are shown on the barometer settings.
The altimeter works through changes in atmospheric pressure,so any time the pressure changes, the apparent altitude will change. Over the course of a few hours, it's likely that there will be small changes in pressure (even when the weather seems to be settled). Depending on the model of Suunto watch you've got, you can check to see if the pressure changes are shown on the barometer settings.
In reply to asmith37:
Why do you say that? So long as you understand how they function and what can affect them and use them as part of a wider navigational aid, they're fantastic. Excellent for up and down as per my use; makes contouring a doddle.
I bet 30 metres accuracy is better than most people could walk on a compass bearing...
Why do you say that? So long as you understand how they function and what can affect them and use them as part of a wider navigational aid, they're fantastic. Excellent for up and down as per my use; makes contouring a doddle.
I bet 30 metres accuracy is better than most people could walk on a compass bearing...
In reply to asmith37:
Setting the reference altitude doesn't take long. Try resetting it at known points on your walk/run/climb. This will reduce the overall variation when you get to a summit.
> I’ve recently bought a Suunto core and having troubles with the altimeter. I’ve been setting the reference altitude at the beginning of the day
Setting the reference altitude doesn't take long. Try resetting it at known points on your walk/run/climb. This will reduce the overall variation when you get to a summit.
In reply to asmith37:
What does it read when you get back to base? Maybe the calibration is out?
Chris
What does it read when you get back to base? Maybe the calibration is out?
Chris
In reply to asmith37: I got to try a fancy altimeter from Suunto as I have a mate who is one of their design engineers. It was good fun, but on day out in Snowdonia, I found myself permanently checking the map for spot heights so that I could recalibrate it. So I did end up being much more aware of my altitude but mainly by checking the map!
30 mtrs out of 900 really doesn't sound that far off. If you want better, GPS is the answer I reckon.
30 mtrs out of 900 really doesn't sound that far off. If you want better, GPS is the answer I reckon.
In reply to Chris Craggs:
Strangely enough, I put a similar question into Suunto support yesterday about my X-lander (bought March 2010) and am awaiting their reply...
To paraphrase what I said, the altimeter has been consistently over-reading by about 40 to 50m in 1000m, I know about resetting it at known heights but am asking if they consider this acceptable and whether it could just have been wrongly calibrated from new and can be adjusted.
To answer Chris's question from my POV (giving just one example which I've also quoted to Suunto), I climbed a 1115m summit this Saturday starting from 95m, reset the altimeter from 1165m to 1115m at the top and it was reading 45m when I returned to my starting point. So think a consistent error (and mine's been consistent) should be capable of correction, but need to see what they say. Might just add that my GPS (despite what folk say about barometric altimeters giving more accurate readings) has been far more reliable (consistently to within a few metres) than this, so a bit disappointed with my +50m from the X-lander if that's the best they can do (will report back when I hear from them).
> What does it read when you get back to base? Maybe the calibration is out?
Strangely enough, I put a similar question into Suunto support yesterday about my X-lander (bought March 2010) and am awaiting their reply...
To paraphrase what I said, the altimeter has been consistently over-reading by about 40 to 50m in 1000m, I know about resetting it at known heights but am asking if they consider this acceptable and whether it could just have been wrongly calibrated from new and can be adjusted.
To answer Chris's question from my POV (giving just one example which I've also quoted to Suunto), I climbed a 1115m summit this Saturday starting from 95m, reset the altimeter from 1165m to 1115m at the top and it was reading 45m when I returned to my starting point. So think a consistent error (and mine's been consistent) should be capable of correction, but need to see what they say. Might just add that my GPS (despite what folk say about barometric altimeters giving more accurate readings) has been far more reliable (consistently to within a few metres) than this, so a bit disappointed with my +50m from the X-lander if that's the best they can do (will report back when I hear from them).
In reply to asmith37: As well as variations in atmospheric pressure over time, there is another source of inaccuracy inherent the Suunto barometric altimeters (and AFAIK almost all others out there). It is explained in the user manual for the Vector but not in that for the Core.
The inaccuracy arises because the altitude measurement is based on a reference model that assumes certain air temperatures for certain given altitudes. If the actual air temperature is lower than the model then the device will report altitude differences which are higher than actual, and vice versa. The model assumes that the air temperature at sea level is 15C. In the UK at this time of year the air temperature is very likely to be lower than that, hence it reports a higher than actual altitude as you ascend, even though you set the reference altitude correctly at your start point.
Although the device does have a thermometer, it can't use that to correct for the inaccuracy inherent in the model as it has no way of knowing whether the temperature it is reading is the actual air temperature, the temperature of your wrist exposed to the air, or the temperature of the inside of your glove.
(Note that this is nothing to do with the temperature compensation in the pressure sensor: that just makes sure that the sensor reports the same barometric pressure throughout the operational temperature range. It is the conversion of barometric pressure changes in to changes in altitude which is subject to the inherent inaccuracy in the model.)
The inaccuracy arises because the altitude measurement is based on a reference model that assumes certain air temperatures for certain given altitudes. If the actual air temperature is lower than the model then the device will report altitude differences which are higher than actual, and vice versa. The model assumes that the air temperature at sea level is 15C. In the UK at this time of year the air temperature is very likely to be lower than that, hence it reports a higher than actual altitude as you ascend, even though you set the reference altitude correctly at your start point.
Although the device does have a thermometer, it can't use that to correct for the inaccuracy inherent in the model as it has no way of knowing whether the temperature it is reading is the actual air temperature, the temperature of your wrist exposed to the air, or the temperature of the inside of your glove.
(Note that this is nothing to do with the temperature compensation in the pressure sensor: that just makes sure that the sensor reports the same barometric pressure throughout the operational temperature range. It is the conversion of barometric pressure changes in to changes in altitude which is subject to the inherent inaccuracy in the model.)
In reply to Martin W:
Been wondering whether it could be something like that (but missed it in the manual), so thanks for the lead!
> The inaccuracy arises because the altitude measurement is based on a reference model that assumes certain air temperatures for certain given altitudes. If the actual air temperature is lower than the model then the device will report altitude differences which are higher than actual, and vice versa. The model assumes that the air temperature at sea level is 15C. In the UK at this time of year the air temperature is very likely to be lower than that, hence it reports a higher than actual altitude as you ascend, even though you set the reference altitude correctly at your start point.
Been wondering whether it could be something like that (but missed it in the manual), so thanks for the lead!
In reply to brt:
In air, 30m ascent would approximate to roughly 4mb drop in pressure, within normal climbing altitudes. To give you an idea of what that means, the Core's storm alarm will sound if the atmospheric pressure drops by at least 4mb in a three hour period.
However, in the OP's case the vast majority of the error is probably due to the inherent inaccuracy in the model used to convert changes in barometric pressure in to altitude changes. Suunto state that the error in the height difference will be 0.2% for each degree C that the sum of the two temperature differences (ie variations from the model) at each altitude. The OP reported a ~30m error in ascents between 700m and 900m. If we take an average ascent of 800m and if we assume that the actual air temperature was, say, ten degrees lower than the model at the lowest and highest point (which would equate to 5C at sea level rather than 15C, which seems pretty reasonable for the UK at this time of year) then the expected error would be:
(10 + 10) x 0.2% x 800m = 32m
and because the temperature is lower than the model assumes, the altitude difference will be too high. Which accords very well with the OP's observations.
In other words, if the actual air temperatures are significantly different from those assumed by the model then the inaccuracy inherent in the model can easily be a more significant factor than actual changes in atmospheric pressure.
> (In reply to asmith37)
>
> 30 metres amounts to approx. 3 bar
>
> 30 metres amounts to approx. 3 bar
In air, 30m ascent would approximate to roughly 4mb drop in pressure, within normal climbing altitudes. To give you an idea of what that means, the Core's storm alarm will sound if the atmospheric pressure drops by at least 4mb in a three hour period.
However, in the OP's case the vast majority of the error is probably due to the inherent inaccuracy in the model used to convert changes in barometric pressure in to altitude changes. Suunto state that the error in the height difference will be 0.2% for each degree C that the sum of the two temperature differences (ie variations from the model) at each altitude. The OP reported a ~30m error in ascents between 700m and 900m. If we take an average ascent of 800m and if we assume that the actual air temperature was, say, ten degrees lower than the model at the lowest and highest point (which would equate to 5C at sea level rather than 15C, which seems pretty reasonable for the UK at this time of year) then the expected error would be:
(10 + 10) x 0.2% x 800m = 32m
and because the temperature is lower than the model assumes, the altitude difference will be too high. Which accords very well with the OP's observations.
In other words, if the actual air temperatures are significantly different from those assumed by the model then the inaccuracy inherent in the model can easily be a more significant factor than actual changes in atmospheric pressure.
In reply to ankyo: IIRC the margin for error in GPS altitude readings is inherently about twice that in GPS position readings. However, even handheld GPS devices are now capable of ~10m positional accuracy or better.
Traditonal barometric altitude readings did involve correcting for air temperature differences from the reference model, so they could be more accurate than GPS. Modern wrist-top altimeters can't make those corrections, though. Also, a lot of GPS devices will report their positional accuracy so you can factor that in to the accuracy of the altitude reading. You don't get the information with a wrist-top altimeter.
So the real problem is that it's harder to tell how far out the barometric altimeter might be just be consulting the device. The best way to get round that is to adjust the barometic altimeter at reference points of known altitude along your route. Which is exactly what the instruction manuals usually advise.
So, as is often the case, it's not that the device does its job poorly, it's knowing how to use it properly that matters.
Traditonal barometric altitude readings did involve correcting for air temperature differences from the reference model, so they could be more accurate than GPS. Modern wrist-top altimeters can't make those corrections, though. Also, a lot of GPS devices will report their positional accuracy so you can factor that in to the accuracy of the altitude reading. You don't get the information with a wrist-top altimeter.
So the real problem is that it's harder to tell how far out the barometric altimeter might be just be consulting the device. The best way to get round that is to adjust the barometic altimeter at reference points of known altitude along your route. Which is exactly what the instruction manuals usually advise.
So, as is often the case, it's not that the device does its job poorly, it's knowing how to use it properly that matters.
In reply to Martin W:
Thanks Martin, i'd be interested to try it out in warmer conditions having only used it where the temperature has been on or below zero. This weekend i was able to set it where we parked the car at a spot height of 101m by crummock water, on reaching Grasmoor (852m) it was reading 886m (about 2-1/2 hrs later) where i reset it, then walked round to whiteside where it was still reading correctly but by the time we had got back to the car it was reading 66m.
I'll just need to keep re-calibrating regularly as i ascend, but this is not always easy if there's no reference points. I guess it will still be more accurate when using for contouring, but i think i'll have more trust in my gps (which i thought would be less accurate) if i ever need to confirm my height.
Thanks Martin, i'd be interested to try it out in warmer conditions having only used it where the temperature has been on or below zero. This weekend i was able to set it where we parked the car at a spot height of 101m by crummock water, on reaching Grasmoor (852m) it was reading 886m (about 2-1/2 hrs later) where i reset it, then walked round to whiteside where it was still reading correctly but by the time we had got back to the car it was reading 66m.
I'll just need to keep re-calibrating regularly as i ascend, but this is not always easy if there's no reference points. I guess it will still be more accurate when using for contouring, but i think i'll have more trust in my gps (which i thought would be less accurate) if i ever need to confirm my height.
In reply to asmith37:
To see if temperature (on your wrist) is the factor, set the altimeter, note its reading, wear it for an hour and then note the reading.
To see if temperature (on your wrist) is the factor, set the altimeter, note its reading, wear it for an hour and then note the reading.
In reply to asmith37:
i wonder if theres something to be said for the pressure difference (+ humidity, temp etc) when a barmeter is worn under clothing.
especially goretex, event, conduit etc that claim to fundamentally work because of pressure differences.
me, i dont expect my altimeter (core) to be anything more than an extra dimension to go with a compass and map. i use it a lot, but as was mentioned above, to count altitude gain from a given point, ie 50m increments counted from zero, rather than a 'where am i' device.
theres too many factors involved on a single factor otherwise i feel.
i wonder if theres something to be said for the pressure difference (+ humidity, temp etc) when a barmeter is worn under clothing.
especially goretex, event, conduit etc that claim to fundamentally work because of pressure differences.
me, i dont expect my altimeter (core) to be anything more than an extra dimension to go with a compass and map. i use it a lot, but as was mentioned above, to count altitude gain from a given point, ie 50m increments counted from zero, rather than a 'where am i' device.
theres too many factors involved on a single factor otherwise i feel.
In reply to ice.solo:
Not the same kind of pressure, that'd be vapour pressure if I don't have my basic physics muddled too much.
Anyway, they aren't desperately accurate, 30 metres must be seen as ok really, you can't expect the world from them.
> especially goretex, event, conduit etc that claim to fundamentally work because of pressure differences.
Not the same kind of pressure, that'd be vapour pressure if I don't have my basic physics muddled too much.
Anyway, they aren't desperately accurate, 30 metres must be seen as ok really, you can't expect the world from them.
In reply to Flashy:
Blunder. Should have said mb - 1mb equalling approx. 8 metres.
I do think all the talk of dodgy calibration of the altimeter is a bit over played (could be wrong...). Altitude and air pressure is a complicated business. Using known datum and regular checks (it doesn't take long) keep me in the know.
That said, if you do suspect a fault I can vouch for Suunto after-care. I sent mine off after it stopped working and was promptly sent a new one; mine was three years old. Excellent. Their website has lots of info.
> (In reply to brt)
> [...]
>
> If you're under water.
> [...]
>
> If you're under water.
Blunder. Should have said mb - 1mb equalling approx. 8 metres.
I do think all the talk of dodgy calibration of the altimeter is a bit over played (could be wrong...). Altitude and air pressure is a complicated business. Using known datum and regular checks (it doesn't take long) keep me in the know.
That said, if you do suspect a fault I can vouch for Suunto after-care. I sent mine off after it stopped working and was promptly sent a new one; mine was three years old. Excellent. Their website has lots of info.
In reply to asmith37:
30m doesn't sound that bad in my experience.
I've seen a _much_ bigger inaccuracy at the top of Black Combe in a howling gale. Any physicists out there to explain that? Bernoulli Effect?
30m doesn't sound that bad in my experience.
I've seen a _much_ bigger inaccuracy at the top of Black Combe in a howling gale. Any physicists out there to explain that? Bernoulli Effect?
In reply to Tim Davies:
It shouldn't make any difference, the pressure sensors on Suunto altimeters are temperature compensated ie if the air pressure is 1000mb then the pressure sensor will report 1000mb throughout its operating temperature range (which for the Core is -20°C to +60°C, according to the manual).
> (In reply to asmith37)
>
> To see if temperature (on your wrist) is the factor, set the altimeter, note its reading, wear it for an hour and then note the reading.
>
> To see if temperature (on your wrist) is the factor, set the altimeter, note its reading, wear it for an hour and then note the reading.
It shouldn't make any difference, the pressure sensors on Suunto altimeters are temperature compensated ie if the air pressure is 1000mb then the pressure sensor will report 1000mb throughout its operating temperature range (which for the Core is -20°C to +60°C, according to the manual).
In reply to Martin W:
Would you think its possible to expect different inaccuraces then depending on whether you wearing the watch on your wrist or strapped to your bag, as the thermometer function seems to be affected by body heat?
Would you think its possible to expect different inaccuraces then depending on whether you wearing the watch on your wrist or strapped to your bag, as the thermometer function seems to be affected by body heat?
This topic has been archived, and won't accept reply postings.
Loading Notifications...