GPS Discrepancies

General cycling advice ( NOT technical ! )
Jdsk
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Re: GPS Discrepancies

Post by Jdsk »

mattsccm wrote: 9 Apr 2021, 8:14pm I plonked 4 Garmin GPS on the bonnet of my Land Rover once whilst green laning. They all said different things. I knew where I was by the OS map (road junction scarily close to a cross of grid lines) . None were correct with nearly half a mile between the extremes.
Laterally... nothing to do with height?

When was it: that's a much greater discrepancy than is now typical.

Jonathan
PT1029
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Re: GPS Discrepancies

Post by PT1029 »

Does charge left in the battery have any effect.
Different technology, but barometric based. I had a Cateye computer (1990's I think) with altitude. The altitude got higher and higher, even when not going anywhere. The battery ran out, new battery and problem solved, I had this happen twice. I remember looking out on a beach in Brittany, the Cateye told be I was at 39,000 feet!
KTHSullivan
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Re: GPS Discrepancies

Post by KTHSullivan »

PT1029 wrote: 9 Apr 2021, 9:09pm Does charge left in the battery have any effect.
Different technology, but barometric based. I had a Cateye computer (1990's I think) with altitude. The altitude got higher and higher, even when not going anywhere. The battery ran out, new battery and problem solved, I had this happen twice. I remember looking out on a beach in Brittany, the Cateye told be I was at 39,000 feet!
:lol: :lol: :lol:
Just remember, when you’re over the hill, you begin to pick up speed. :lol:
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RickH
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Re: GPS Discrepancies

Post by RickH »

My oldest GPS (edge 605) relied on GPS for altitude & tended to give much higher ascent figures, presumably due to the inaccuracies causing the recorded height to "wander".

My newer units (edge 1000 &, more recently, 1030 Plus) have a barometer & are much more consistent. I also have waypoints set on the map at places I regularly start rides from (home, various train stations that I use, etc) that automatically sets the altitude when I start recording there.

With regards to displayed location, if you have a unit with a "lock to roads" setting enabled and are travelling off road you can get "interesting" results. - such as when following an urban canal (you can guess how I know).
Former member of the Cult of the Polystyrene Head Carbuncle.
KTHSullivan
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Re: GPS Discrepancies

Post by KTHSullivan »

Thanks for all of the comments and the input. Obviously the general consensus is that GPS units are roughly in the parish as far a distance and where am I is concerned; but with regard to altitude they are effectively pants.
Just remember, when you’re over the hill, you begin to pick up speed. :lol:
Jdsk
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Re: GPS Discrepancies

Post by Jdsk »

KTHSullivan wrote: 9 Apr 2021, 9:16pmObviously the general consensus is that GPS units are roughly in the parish as far a distance and where am I is concerned; but with regard to altitude they are effectively pants.
I think that latter bit should say something about calculation of height lost and gained. The software that does that might be causing more error than the GPS device's estimation of altitude.

Jonathan
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Mick F
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Re: GPS Discrepancies

Post by Mick F »

KTHSullivan wrote: 9 Apr 2021, 9:16pm Thanks for all of the comments and the input. Obviously the general consensus is that GPS units are roughly in the parish as far a distance and where am I is concerned; but with regard to altitude they are effectively pants.
Not all "effectively" pants.

My Montana is quite good at elevation if you let it settle.
When I first got it, I went to a few OS spot heights and placed it on the ground. It was correct within a few feet.

What you can do with many devices, is tell it at what elevation it is at, and then it'll track the ascent accurately ........... plus or minus wind across the breather hole.

I set mine at the elevation of our front drive as I know that from the OS map. It's a menu item in the Settings page.

When I had the Edge705, you couldn't set the elevation, but you could create a waypoint at your front gate, and the waypoint menu allowed an elevation input. As you ride away, you go through the waypoint, and the 705 then flipped to the correct elevation.
Mick F. Cornwall
mattsccm
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Re: GPS Discrepancies

Post by mattsccm »

No idea abouitv height on the 4 I tried. Could be a decade ago so, as you said, things change.
pete75
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Re: GPS Discrepancies

Post by pete75 »

KTHSullivan wrote: 9 Apr 2021, 3:07pm Out yesterday for my daily dozen, wandered into the Lincolnshire Alps . Cat Eye GPS gave me 28.38 miles and 1840 feet ascending. Mio GPS provided 28.4 miles and 921 feet ascending. Obviously the distance provided by both units were more or less the same. However which one do I believe for height gained. Somewhat slightly spooky that Mio gave me more or less half that of the Cat Eye. Is the Cat Eye measuring the up and down or is the Mio only measuring half of the up? I have looked at the settings on both units and it appears that they are set up correctly.

Any suggestions?
Plot the same route on something like this https://ridewithgps.com/. and compare. It takes ascending/descending from map data. Draws a nice graph too.
'Give me my bike, a bit of sunshine - and a stop-off for a lunchtime pint - and I'm a happy man.' - Reg Baker
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NATURAL ANKLING
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Re: GPS Discrepancies

Post by NATURAL ANKLING »

Hi,
My bicycle GPS device is useless when compared to my mobile phone when walking.
I think possibly mobile phones uses app s so the software probably comes into it a bit.
My bicycle satnav device (lezyne mega xl) Is pretty useless under trees et cetera, I know that cloud cover also affects satnav too.
I measured the course on a map online and it came out at just over 5 miles, my mobile phone app doesn't read less than 4.9 miles normally, but the bicycle satnav says just 3.6?
I think all satnav uses software to interpolate.
It's a bit like using an app for calories and a watts Without any input from the body.
When I used to do some longer distances I used to adjust the wheel input on my Speedo to match a online map and always make sure my Speedo read less than any map could.
Over the years I've listened to many people tell me how many miles they ran last night.
I would quiz them on their actual course, 4.5 miles worked out at just over 3 Plotted on a map! The days before mobile phones et cetera of course.
When I was running I always plot my courses on an accurate large scale map.
No point in kidding yourself is there.
There is a setting on my bicycle satnav that's my lezyne, normal or adaptive.
Adaptive Assumed you were going in a straight line until knows you're not-

"The default setting for our GPS devices when recording is Adaptive. What this means is that, if you are traveling in a straight line without changing speed, it may not record unnecessary GPS data points where nothing changed. This minimizes the file size and maximizes battery life. The exception is if you are running sensors. Heart rate or power data tends to fluctuate enough that the device is going to sample more frequently.

If you want the device to sample more frequently because you're mountain biking, riding under tree cover, or riding a curvy road, you can manually change the device to always sample once per second. Here's how you do that..."


Maybe once a second is not enough when you are walking although I'm inclined to think that tree cover cloud cover and darkness? Has something to do with it.
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Mick F
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Re: GPS Discrepancies

Post by Mick F »

I've set my Montana to record every 4yds.
It can go down to 0.001yds and up to 9,999.99yds.
I could swap to metric so it would be 0.001mtr up to 9,999.99mtrs.

I could set it to record at time intervals, from 1sec all the way up to 99hrs 59mins and 59secs.
Mick F. Cornwall
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NATURAL ANKLING
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Re: GPS Discrepancies

Post by NATURAL ANKLING »

Hi,
https://support.lezyne.com/hc/en-us/art ... Correction

"As mentioned in a previous article, once you upload your ride files to the Lezyne Root site, an algorithm runs to correct any discrepancies in the recorded elevation. This algorithm is based on topographical survey data and is similar to how Strava treats files on their site.

We do offer the ability to disable the correction from the Lezyne Root website interface. When disabled, the data on the ride page will show the raw barometer data."
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Psamathe
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Re: GPS Discrepancies

Post by Psamathe »

NATURAL ANKLING wrote: 10 Apr 2021, 11:33am Hi,
https://support.lezyne.com/hc/en-us/art ... Correction

"As mentioned in a previous article, once you upload your ride files to the Lezyne Root site, an algorithm runs to correct any discrepancies in the recorded elevation. This algorithm is based on topographical survey data and is similar to how Strava treats files on their site.

We do offer the ability to disable the correction from the Lezyne Root website interface. When disabled, the data on the ride page will show the raw barometer data."
I think Garmin Connect does something similar. On the web interface when viewing a recorded track there is an option "Elev corrections" (Elevation Corrections are calculated with data from professional surveys instead of the data from your device. If your device has a barometric altimeter, Elevation Corrections are disabled by default.

Ian
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NATURAL ANKLING
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Re: GPS Discrepancies

Post by NATURAL ANKLING »

Hi,
https://support.lezyne.com/hc/en-us/art ... n-Numbers-

"Strava's correction and our correction are two different algorithms, so they aren't always going to come up with the same number. Unfortunately, elevation gained is a bit more intangible than most other ride metrics. If you'd like to have your mind blown a little, but learn a lot more about elevation, this is a great video:"

Watch the video :)
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
KTHSullivan
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Re: GPS Discrepancies

Post by KTHSullivan »

Went out his morning for another re-run on the MIO/CATEYE GPS anomaly mystery. Got more or less the same outcome as previously. Both gave me roughly the same distance, but MIO logged 717 ft of gain whereas the CATEYE logged 1430 ft. Both devices were in the back pocket of my cycle top so no difference was due to the vagaries of wind speed etc. Both devices were allowed to settle and checked for altitude before setting off. Plotting the route on B Router gives me 780ft of gain. The plot thickens.
Just remember, when you’re over the hill, you begin to pick up speed. :lol:
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