Garmin 520 under calculating distances

sy_flembeck

Wondering if anyone has any ideas on this.

My 2 year old Garmin worked fine until recently. I was going to sell it so I factory reset it. Had a change of heart so set it up again. I noticed that my average speeds were quite low but didn't pay any real heed to it till yesterday when I did an easy, flat loop that I do a lot so know well. It should read 53km but I finished with a recording of 44km. Today I hit a PB on a climb yet when I was on it I was constantly recording 7.6-7.8kmh and on the descent when I know I was doing mid 50skmh the highest it showed was 33kmh.

I stopped the clock and restarted and it appeared to do the trick but then I measured a particular segment near home at 9.2km but drove it later and it's 10.3km.

Any ideas on what to do?

TIA

TheBlaaMan

Sounds like it has defaulted back to a different wheel size than is on your bike. Go into the settings and see what way it is assessing your bike/wheels set-up

Wishbone Ash

sy_flembeck wrote: »

n I measured a particular segment near home at 9.2km but drove it later and it's 10.3km....

The odometers in motor vehicles are not very accurate as they use the rolling circumference of the wheel/tyre to determine distance/speed. A new tyre has a greater circumference than a worn tyre. They also overestimate speed to prevent legal hassle from drivers. A Sat Nav would be much more accurate.

crosstownk

If you're using a wheel speed sensor then try resetting it and repairing it. I've had that happen before with a speed sensor.

Weepsie

Is it recording in imperial rather than metric?

smacl

sy_flembeck wrote: »

I stopped the clock and restarted and it appeared to do the trick but then I measured a particular segment near home at 9.2km but drove it later and it's 10.3km.

If you use an odometer you measure slope distance whereas on a map you measure plan distance, so the odometer will typically show more distance. When you measure a distance in plan you also assume that you're holding a perfectly straight line from one corner to the next, whereas in fact your path contains many more small curves that when straightened out add quite a bit to the distance.

Not as extreme as your case, but a hilly map route I'd planned out as 132k on bikemap.net a couple of weeks ago came out a 141k on the Garmin. At a guess about 2% of that (2.6k) was due to gradient and the rest was due to change of position within the lane travelled.

barrier86

I'm getting odd readings from my year old 520. Anytime I turn it on the elevation is really off. Just checked it there and its saying elevation is -92 metres. Even with sattelite connection... very annoyong.

Cee-Jay-Cee

Change the battery in your speed/cadence sensor and re-pair it with the unit.

TheBlaaMan

barrier86 wrote: »

I'm getting odd readings from my year old 520. Anytime I turn it on the elevation is really off. Just checked it there and its saying elevation is -92 metres. Even with sattelite connection... very annoyong.

The altimeter is barometric, ie it relies on atmospheric pressure for its reading - it is not a GPS-set altitude. Give it a minute or two after switch on and it often improves, but you may need to leave it for longer than that.
If it can get a satellite signal and therefore knows where it is, you can force it to recalibrate. Find out the altitude of your house/location Then, in the setup pages choose > Set Elevation. Enter the known elevation, and select enter/save. Now, each time it knows it is at that location (using GPS), it will recalibrate itself to the altitude you have set into the unit.

crosstownk

barrier86 wrote: »

I'm getting odd readings from my year old 520. Anytime I turn it on the elevation is really off. Just checked it there and its saying elevation is -92 metres. Even with sattelite connection... very annoyong.

I had this a few months back. Elevation was fine if I started from home but was all over the place if I started elsewhere. The metres gained was always OK - it was the elevation that was wrong.

I made many attempts to set the elevation at my work location but the 520 had a mind of it's own.

Eventually, it all got sorted after a software update from Garmin. Connect to your PC/laptop and make sure you have the latest software update via Garmin Express.

sy_flembeck

TheBlaaMan wrote: »

Sounds like it has defaulted back to a different wheel size than is on your bike. Go into the settings and see what way it is assessing your bike/wheels set-up

Thanks for the above, folks.

Didn't realise the wheels needed to be set so I did as suggested and went further into the unit. The wheel size had been gauged automatically and was incorrect so I changed that. I also changed the battery as suggested and re-paired them. Hopefully I'll get out tomorrow and see if the changes work but it appears that may have been the issue.

Thanks again, much appreciated

sy_flembeck

Quick update.

That did the trick.

Many thanks again

smacl

TheBlaaMan wrote: »

The altimeter is barometric, ie it relies on atmospheric pressure for its reading - it is not a GPS-set altitude. Give it a minute or two after switch on and it often improves, but you may need to leave it for longer than that.
If it can get a satellite signal and therefore knows where it is, you can force it to recalibrate. Find out the altitude of your house/location Then, in the setup pages choose > Set Elevation. Enter the known elevation, and select enter/save. Now, each time it knows it is at that location (using GPS), it will recalibrate itself to the altitude you have set into the unit.

Finding big differences between the Garmin 810 elevation gain/loss versus map corrected factors here too. Last weekend, uncorrected (i.e. barometric) elevation gain was 2,908 (2946 loss) corrected was 2475. Was trying to figure this out as I calibrated the Garmin when I got it, and noticed the following from their site

The elevation profile has climbs, but the elevation gain is very large. This often happens with activities recorded in mountainous areas outside of the US. The resolution of the data sets can be coarse and the profile may be bumpier than it should be even with interpolation and smoothing. Sometimes they are so coarse it's like trying to find a smooth elevation profile over a lego block model of your area. These bumps may pass the threshold in gain calculations and the total gain from the bumps may add up to more than the gain that was perceived on the ride.

I'm guessing that Garmin use 30m SRTM as their base elevation map at this stage which means that Europe should be as accurate as the states, about 30m grid with 8m RMSE vertically. Still surprised to see ~20% variation between the two methods and given the barometric values closed to just over 1% I'd suspect it is more accurate. Must see if I can grab a high accuracy LIDAR DEM from the ordnance survey and do a comparison.

Unknown Soldier

smacl wrote: »

Finding big differences between the Garmin 810 elevation gain/loss versus map corrected factors here too. Last weekend, uncorrected (i.e. barometric) elevation gain was 2,908 (2946 loss) corrected was 2475. Was trying to figure this out as I calibrated the Garmin when I got it, and noticed the following from their site



I'm guessing that Garmin use 30m SRTM as their base elevation map at this stage which means that Europe should be as accurate as the states, about 30m grid with 8m RMSE vertically. Still surprised to see ~20% variation between the two methods and given the barometric values closed to just over 1% I'd suspect it is more accurate. Must see if I can grab a high accuracy LIDAR DEM from the ordnance survey and do a comparison.

Unless there are extremely averse weather conditions, or your unit is borked, the Garmin Barometric Altimeter is usually the correct reading. The Strava App and thier recalculation Elevation usually robs you of about 10-15% of climbing.

For Garmins, one handy tip is to set an Elevation point at your gaff, assuming that's where you mostly start cycling from. Another would be to turn your device on and leave it on an outside table for a 5-10 mins or so before heading off. I've seen mine go from 180m down to 100 and from -69 up to 90 when left outside for 10 mins after being turned on.

If you have a set elevation point (my gaff's about 95m), once you hit start it defaults to that (more or less) regardless of its current reading. The Garmin then records every rise/fall, especially if you have it set to record data every second, rather then "smart" mode.

If you were to not cycle, and put a ladder at the front of your house, and did some repeats up it, that will all register as "climbing/descending" on your garmin. On map based Elevation, you'd get Zero climbing, as that works off set elevation points, as far as I can gather.

A handy way to check the more or less accuracy of you unit is to look at a Sportive or group ride you did on your Phone/Strava app, and click on the bit where it shoes where you road with other. In there is give a summary of distance & elevation of the other people, and you can spot those using their phone/app and those on garmin type yoke.

Another handy check is to compare elevation gained/lost if you start and finish at the same point. Regardless of your starting elevation, if they are 10-15m difference, its not bad. Mine is usually 2-4m difference.

smacl

Unknown Soldier wrote: »

Another handy check is to compare elevation gained/lost if you start and finish at the same point. Regardless of your starting elevation, if they are 10-15m difference, its not bad. Mine is usually 2-4m difference.

I was seeing 38m of difference over 2.9k of climbing equating to 1.3% error which I'd consider very good for this class of device, particularly over a long spin that started on a cool morning and went through a sunny afternoon. The base elevation map only changes elevation every 30m and I'm guessing its being heavily smoothed even then with 8m rmse in height. OSi data has a height every 2m accurate to 25cm rmse so must do a check on the same route at some point. (I have access to the OSi data through work).

Only thing that strikes me is that any route map your liable to get for a sportive which includes amount of climbing will be based on the same 30m base elevation map that strava uses, so probably worth keeping that in mind from a training point of view. e.g. the 2.7k of up on the WW200 is likely to come out at 3k+ on the 810.

Unknown Soldier

ah here...

I just upload three of my last rides to a test Strava account I have and you wouldn't believe what happened!*

Sorry, I keep seeing that online

regardless...


and the following happened when I was testing elevation.

Garmin to Strava:
312 > 311
280 > 284
708 > 778

My bleedin garmin is robbing me of climbing!