I have had my GPS altimeter stop working twice now, both times it was when doing long rides in drizzle.
I dont recall it failing in outright heavy rain.
I can see a small hole in the casing, above the battery case which I assume is the altimeter "breather" hole.
I guess it is getting blocked.
Anybody else come across this?
GPS altimeters in the wet.
GPS altimeters in the wet.
Yma o Hyd
Re: GPS altimeters in the wet.
on my cateye the hole is underneath and has never been cleaned in the 10 years since I've had it, but it does state in the manual to keep it clean
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mr bajokoses
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Re: GPS altimeters in the wet.
Yes, Garmin 705 seems to be affected in this way. Yesterday in the heavy drizzle in Lancashire the grade was reading between -2% and +2% no matter what the road looked like to me. The ascent display was incorrect too.
However, when I uploaded the ride into Garmin Training Centre and Garmin Connect, all was well with the ascent total. Strange.
However, when I uploaded the ride into Garmin Training Centre and Garmin Connect, all was well with the ascent total. Strange.
Re: GPS altimeters in the wet.
It's happened to me with both my Edge 305 and Edge 705.
Caused by moisture blocking the BP altimeter hole. It seems to happen during very wet drizzle rather than heavy rain. Must be something to do with the droplet size.
It's a well-known problem.
Caused by moisture blocking the BP altimeter hole. It seems to happen during very wet drizzle rather than heavy rain. Must be something to do with the droplet size.
It's a well-known problem.
Mick F. Cornwall
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insanityideas
- Posts: 161
- Joined: 18 Aug 2008, 6:18pm
- Location: Bristol
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Re: GPS altimeters in the wet.
In addition to water affecting the barometric sensor quick changes in atmospheric pressure durring the course of your ride can change the height reading. This only really becomes apparent in fast changing weather conditions when the pressure rises or falls quickly.
In the case of Garmin GPS the designers seem to have programmed it to behave in a particular way which can seem to give strange results on occasion. Every so often it will mis-estimate its starting height, gradually correcting it up durring the ride. Via some experimentation (taking the GPS on an aircraft) I can however explain how their particular algorithm works at least in part:
1) Durring a ride the height measurements are made only by the barometric altimeter they are not corrected by the GPS. (in a plane regardless of actual height above ground, the displayed height is always based on cabin pressure).
2) Because of the way the GPS system works it is more accurate in determining longditude and latitude than it is in determining height which is why the barometric sensor is required rather than relying on the height data obtained by the GPS receiver.
3) The GPS display does report getting a 2D or 3D GPS lock but it doesn't appear to use the GPS height data. I don't think it uses GPS to calibrate the barometric sensor when the unit first switches on, but I havn't gone to the trouble of confirming this.
4) There are various smoothing algorithms in play, although some are more successful than others, for example gradient going up and down gradually.
I was in the process of working out exactly how the GPS records height and calculates distance as well as how it takes height into account when calculating calories but I have been busy of late. I was prompted to do this because Training Centre (or the GPS) occasionally records some crazy high speeds and I was wondering where the errors came from and how to correct them.
In the case of Garmin GPS the designers seem to have programmed it to behave in a particular way which can seem to give strange results on occasion. Every so often it will mis-estimate its starting height, gradually correcting it up durring the ride. Via some experimentation (taking the GPS on an aircraft) I can however explain how their particular algorithm works at least in part:
1) Durring a ride the height measurements are made only by the barometric altimeter they are not corrected by the GPS. (in a plane regardless of actual height above ground, the displayed height is always based on cabin pressure).
2) Because of the way the GPS system works it is more accurate in determining longditude and latitude than it is in determining height which is why the barometric sensor is required rather than relying on the height data obtained by the GPS receiver.
3) The GPS display does report getting a 2D or 3D GPS lock but it doesn't appear to use the GPS height data. I don't think it uses GPS to calibrate the barometric sensor when the unit first switches on, but I havn't gone to the trouble of confirming this.
4) There are various smoothing algorithms in play, although some are more successful than others, for example gradient going up and down gradually.
I was in the process of working out exactly how the GPS records height and calculates distance as well as how it takes height into account when calculating calories but I have been busy of late. I was prompted to do this because Training Centre (or the GPS) occasionally records some crazy high speeds and I was wondering where the errors came from and how to correct them.
Re: GPS altimeters in the wet.
Just a few points .....
Yes, I agree about the cabin pressure in an aircraft. I took my 305 on a flight to Malta and we stayed at 7,000ft, despite actually being at 30,000ft.
The Garmin system uses all its sensors all the time to give the co-ordinates, speed and distance etc, but ignores any that are patently at odds with something.
For instance - long story - I was experimenting with Auto Wheelsize vs Custom Wheelsize to see if the speed and distance were affected. I made the custom wheelsize down to 90mm and up to 9000mm, but the Garmin ignored them because the distance travelled and speed were obviously wrong, so it used the default 2100mm. In Auto Wheelsize, it continually corrects the wheel diameter to keep it accurate, therefore it MUST use GPS as well as the Speed Cadence unit.
Also, the BP altitude is regularly checked against GPS. The pressure can change over the day, but the altitude will be correct - or at least it tries to be! I contacted Garmin to ask about this, and they assured me that the 305 and the 705 use BOTH GPS and BP to calculate the altitude.
The problem with the aircraft cabin, is that the two readings would be well out with each other, so the Garmin reverts back to BP only, not bothering with the GPS after it checks it, as it's way out. Just the same with a stupidly low or high wheelsize.
What I don't know, and perhaps someone could experiment, is how a 205 or a 605 operate without having a BP altimeter. How would they behave in an aircraft cabin? Would they show the real altitude? Also, how accurate/inaccurate are their altitude readings?
Yes, I agree about the cabin pressure in an aircraft. I took my 305 on a flight to Malta and we stayed at 7,000ft, despite actually being at 30,000ft.
The Garmin system uses all its sensors all the time to give the co-ordinates, speed and distance etc, but ignores any that are patently at odds with something.
For instance - long story - I was experimenting with Auto Wheelsize vs Custom Wheelsize to see if the speed and distance were affected. I made the custom wheelsize down to 90mm and up to 9000mm, but the Garmin ignored them because the distance travelled and speed were obviously wrong, so it used the default 2100mm. In Auto Wheelsize, it continually corrects the wheel diameter to keep it accurate, therefore it MUST use GPS as well as the Speed Cadence unit.
Also, the BP altitude is regularly checked against GPS. The pressure can change over the day, but the altitude will be correct - or at least it tries to be! I contacted Garmin to ask about this, and they assured me that the 305 and the 705 use BOTH GPS and BP to calculate the altitude.
The problem with the aircraft cabin, is that the two readings would be well out with each other, so the Garmin reverts back to BP only, not bothering with the GPS after it checks it, as it's way out. Just the same with a stupidly low or high wheelsize.
What I don't know, and perhaps someone could experiment, is how a 205 or a 605 operate without having a BP altimeter. How would they behave in an aircraft cabin? Would they show the real altitude? Also, how accurate/inaccurate are their altitude readings?
Mick F. Cornwall
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insanityideas
- Posts: 161
- Joined: 18 Aug 2008, 6:18pm
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Re: GPS altimeters in the wet.
Mick Thanks for clarifying that, I did wonder if it compared against all its sensors which would make more sense. I can add a bit more information to what you have already said.
Based on personal observation I have noted that the wheel sensor is only used in the absence of a GPS signal and then only used to display current speed. Distance is still calculated by GPS location. You can verify this in two ways: stand stationary and spin the wheel, speed will be displayed based on the wheel sensor but you will never travel any distance. I am fortunate enough to live in Bristol which has the Bristol-Bath bike path, this has a 0.8 of a mile tunnel, it makes the GPS behave very strangely: It looses lock and relies on the wheel sensor to display speed, but when it regains lock at the other end it will sometimes calculate a crazy speed (~3000mph) as a maximum speed, which is probably equivilent to travelling between two ends of the tunnel instantly. Doesn't always do this but usually records very long speeds durring an extended loss of GPS. The recorded track of the ride will show a straight line through the tunnel and distance is calculated on that basis. This makes buying the speed and cadance sensor a bit of a swizz if you thought it improved accuracy, although having cadance is its main purpose!
I know quite a lot about the inner workings of the GPS system, the limitations of which apply to all GPS receivers including the EDGE, manufacturers can employ extra technology to overcome some of these, in the case of the EDGE this is the compare and averaging logic and its barometric sensor to remove obvious errors. The GPS system was designed to give a very accurate 2 dimensional position lock, however the height position is less accurate typically stated as a factor of 1.5 or greater.
This is compounded by the fact we are using the device to travel forwards more than it travels up and down making such height errors more obvious, after all deviation side to side of 1 meter as you travel down a road doesn't really show up, but moving up and down by the same amount would be like cycling a roller coaster and would make some mad numbers for the height profile. Because the barometric sensor doesn't suffer these per reading ocilations it makes for smoother readings, but it does need to be calibrated to the current height using the GPS, and will suffer from compounded errors over time as air pressure changes due to the weather.
The reason I suspected barometric sensor calibration was only when the unit was turned on is because every so often I will see a 100ft discrepency over the height of my house that will persist for the entire ride to work and then fix itself on the journey home, however I think this probably shows that calibration is subject to its own rules on eronious data (i.e. not changing by 100ft in one measurement), so if it gets garbage on first fix it won't correct it later, or it only calibrates once since turn on.
Because altitude measurement is less accurate even with barometric sensor correction most of the online ride mapping tools correct your track according to DEM data they have for your route. There is an accuracy limitation for DEM data though, it is compiled using satilites which measure height using radar as they pass accross the surface of the planet. These measurements will have an accuracy error for the height measurement and they also have a resolution for the samples, so typically they will measure the height of a square of land 10 meters x 10 meters, the resolution of these measurements is improving as technology advances.
As far as measuring distance traveled GPS will always be more accurate than wheel based cycle computers however well they are calibrated, however GPS will not be perfectly accurate because they calculate using straight lines between points, each point being created every 1-10 seconds depending on the reporting rate of the receiver. most accurate would be painstakingly drawing the route on google earth. BTW I still havn't got to the bottom of if the Garmin edge measures distance and speed in 2d or 3d.
In short nothing's perfect.
Based on personal observation I have noted that the wheel sensor is only used in the absence of a GPS signal and then only used to display current speed. Distance is still calculated by GPS location. You can verify this in two ways: stand stationary and spin the wheel, speed will be displayed based on the wheel sensor but you will never travel any distance. I am fortunate enough to live in Bristol which has the Bristol-Bath bike path, this has a 0.8 of a mile tunnel, it makes the GPS behave very strangely: It looses lock and relies on the wheel sensor to display speed, but when it regains lock at the other end it will sometimes calculate a crazy speed (~3000mph) as a maximum speed, which is probably equivilent to travelling between two ends of the tunnel instantly. Doesn't always do this but usually records very long speeds durring an extended loss of GPS. The recorded track of the ride will show a straight line through the tunnel and distance is calculated on that basis. This makes buying the speed and cadance sensor a bit of a swizz if you thought it improved accuracy, although having cadance is its main purpose!
I know quite a lot about the inner workings of the GPS system, the limitations of which apply to all GPS receivers including the EDGE, manufacturers can employ extra technology to overcome some of these, in the case of the EDGE this is the compare and averaging logic and its barometric sensor to remove obvious errors. The GPS system was designed to give a very accurate 2 dimensional position lock, however the height position is less accurate typically stated as a factor of 1.5 or greater.
This is compounded by the fact we are using the device to travel forwards more than it travels up and down making such height errors more obvious, after all deviation side to side of 1 meter as you travel down a road doesn't really show up, but moving up and down by the same amount would be like cycling a roller coaster and would make some mad numbers for the height profile. Because the barometric sensor doesn't suffer these per reading ocilations it makes for smoother readings, but it does need to be calibrated to the current height using the GPS, and will suffer from compounded errors over time as air pressure changes due to the weather.
The reason I suspected barometric sensor calibration was only when the unit was turned on is because every so often I will see a 100ft discrepency over the height of my house that will persist for the entire ride to work and then fix itself on the journey home, however I think this probably shows that calibration is subject to its own rules on eronious data (i.e. not changing by 100ft in one measurement), so if it gets garbage on first fix it won't correct it later, or it only calibrates once since turn on.
Because altitude measurement is less accurate even with barometric sensor correction most of the online ride mapping tools correct your track according to DEM data they have for your route. There is an accuracy limitation for DEM data though, it is compiled using satilites which measure height using radar as they pass accross the surface of the planet. These measurements will have an accuracy error for the height measurement and they also have a resolution for the samples, so typically they will measure the height of a square of land 10 meters x 10 meters, the resolution of these measurements is improving as technology advances.
As far as measuring distance traveled GPS will always be more accurate than wheel based cycle computers however well they are calibrated, however GPS will not be perfectly accurate because they calculate using straight lines between points, each point being created every 1-10 seconds depending on the reporting rate of the receiver. most accurate would be painstakingly drawing the route on google earth. BTW I still havn't got to the bottom of if the Garmin edge measures distance and speed in 2d or 3d.
In short nothing's perfect.
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insanityideas
- Posts: 161
- Joined: 18 Aug 2008, 6:18pm
- Location: Bristol
- Contact:
Re: GPS altimeters in the wet.
http://freegeographytools.com/2007/can-you-use-gps-data-to-make-a-dem to see what I mean about height accuracy.
Re: GPS altimeters in the wet.
Garmin GPS that have a barometric altimeter will report the height from the barometer. As I understand it, they calibrate the barometer from the GPS height when turned on, but not thereafter (you can always force a recalibration by a quick off/on). As a result, you'll get better heights if you leave the GPS somewhere with a clear view of the sky for 10 minutes or so before starting your ride, so that you have a good GPS height to work from.
The barometric height is used in preference because the GPS heights are subject to random fluctuations. They are accurate if you stay in one place and average them out for 10 or 15 minutes, but the height reported at any one instant could be a fair bit out. I remember trying to calibrate my cyclecomputer off someone's Foretrex whilst sitting at a cafe table, and seeing the height drift up and down over a 40m range several times before I gave up (very few spot heights on Spanish maps).
In contrast, barometric altimeters are stable and accurate over a short period, but drift off with atmospheric changes. The only systematic error is that they take a few seconds to react, and so may miss the bottom of a steep down and immediately straight back up dip, or the top of a col. This is deliberate - if they reacted too quickly, they would react to dynamic pressure changes due to wind buffet. This damping is done by the very small air vent.
As far as the accuracy of amount of climbing is concerned, if you use the GPS height (i.e. a model with no barometer), you end up including random fluctuations and spikes in the climbing, and end up with a figure considerably too high. With a barometric altimeter, you get a figure slightly low due to missed peaks/troughs, plus an error of maybe 60% of the height drift during the ride (only drift whilst ascending matters).
The barometric height is used in preference because the GPS heights are subject to random fluctuations. They are accurate if you stay in one place and average them out for 10 or 15 minutes, but the height reported at any one instant could be a fair bit out. I remember trying to calibrate my cyclecomputer off someone's Foretrex whilst sitting at a cafe table, and seeing the height drift up and down over a 40m range several times before I gave up (very few spot heights on Spanish maps).
In contrast, barometric altimeters are stable and accurate over a short period, but drift off with atmospheric changes. The only systematic error is that they take a few seconds to react, and so may miss the bottom of a steep down and immediately straight back up dip, or the top of a col. This is deliberate - if they reacted too quickly, they would react to dynamic pressure changes due to wind buffet. This damping is done by the very small air vent.
As far as the accuracy of amount of climbing is concerned, if you use the GPS height (i.e. a model with no barometer), you end up including random fluctuations and spikes in the climbing, and end up with a figure considerably too high. With a barometric altimeter, you get a figure slightly low due to missed peaks/troughs, plus an error of maybe 60% of the height drift during the ride (only drift whilst ascending matters).
Re: GPS altimeters in the wet.
Have a read at this:
https://forums.garmin.com/showthread.php?t=10268
I'm SMICKF on the Garmin Forum.
(The forum software didn't want the space between K and F, and it didn't want so few characters, so I added an S, as I'm actually Stephen Michael.)
The GPS does NOT give speed or distance when you have a Speed/Cadence unit fitted and paired to your Edge, unless you have it set for Auto Wheelsize. With Custom Wheelsize set, the magnet does it all! Also, if you spin your wheel off the ground and press START, the speed and distance is measured and recorded even though you are stationary.
The problem I have with all this, is that the GPS trace that you can display on a computer map, will be at odds with the recorded distance if your Custom Wheelsize is incorrect from actual fact or if you run on rollers with a good GPS signal.
Any road up, I maintain that if you have paid a deal of money for a GPS bike computer, you have to believe what it says, and leave it in Auto to do its stuff. Anything less, you may as well have bought a basic £11.99 computer and use an OS map in your back pocket.
Nothing's perfect.
https://forums.garmin.com/showthread.php?t=10268
I'm SMICKF on the Garmin Forum.
(The forum software didn't want the space between K and F, and it didn't want so few characters, so I added an S, as I'm actually Stephen Michael.)
The GPS does NOT give speed or distance when you have a Speed/Cadence unit fitted and paired to your Edge, unless you have it set for Auto Wheelsize. With Custom Wheelsize set, the magnet does it all! Also, if you spin your wheel off the ground and press START, the speed and distance is measured and recorded even though you are stationary.
The problem I have with all this, is that the GPS trace that you can display on a computer map, will be at odds with the recorded distance if your Custom Wheelsize is incorrect from actual fact or if you run on rollers with a good GPS signal.
Any road up, I maintain that if you have paid a deal of money for a GPS bike computer, you have to believe what it says, and leave it in Auto to do its stuff. Anything less, you may as well have bought a basic £11.99 computer and use an OS map in your back pocket.
Nothing's perfect.
Mick F. Cornwall