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George Slavov Wednesday 09 Mar 2016 00:01:31
Garmin records raw data from the barometric sensor, there is no smoothing of the data when it is recorded. After uploading to Strava, the evelavtion differs (I presume that is when the smoothing happens) but the change is not significant, usually within 3-5% of the original reading. In saying that Strava is known to be wildly inaccurate when using their mobile app as that relies on the map elevation data rather than sensor readings.
I agree with Andrea that it is most likely the accuracy of the underlying data as the best data set available for the area where I ride has 10m resolution and the public version of it has 30m. I had a quick look at some ride profiles and it seems that on long steady climbs where the road is mostly cutting across elevation lines the data from Plot a Route is quite accuarate. On twisty undulating roads it is quite far out as per my original post.
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plotaroute admin Wednesday 09 Mar 2016 09:18:30
Thanks George. I'll delve a bit deeper and see if I can work out where the differences come from. Do you have a sample file directly from your GPS device that you could send me (ideally a fairly short route)? It would be helpful to see the raw data recorded in the file and then compare this with elevation data generated for the same route on plotaroute. I'll email you directly with an address to send it to.
John
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Nisse Son Sunday 20 Mar 2016 15:07:26
The ascent/descent-data for routes are often of with up to 50%. Happy to help out with raw-data if needed.
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plotaroute admin Sunday 20 Mar 2016 22:20:07
Hi Olof - We've had a few people contact us about this and have done quite a bit of analysis of raw data from GPS devices. I'm afraid every time it turns out that the data from the GPS device was a long way out. This has been true when the GPS device used a barametric altimeter too.
I've just recently looked into a sample route provided by George below. I compared our own elevation data with the elevation data recorded by his GPS (with barametric altimeter) and also with data from Google. The Google and plotaroute readings are almost identical but the GPS readings are a long way out (and not by the same amount either, so it is not just a calibration issue).
When it comes to calculating the Total Ascent figure, the GPS data was also flawed, mainly becasue of large gaps between the data, presumably where the signal had been lost. On plotaroute the distance between readings is a fixed interval of 30m. The GPS file that we looked into had readings on average 41m apart but up to 2920m apart! Scanning through the data I could see there were several instances where there are gaps of over 1km between readings.
So, the main reasons GPS devices give lower total ascent figures than we calculate on plotaroute is due to a combination of:
- Inaccuracies in the elevation readings being measured by the GPS device
- Long gaps with no readings
- Irregular intervals between readings
Differences between the total ascent figures on our site and other sites are most likely due to different sampling intervals. You can see the effect of this by changing the sampling interval in our route profile tool - increasing the distance between readings makes a big difference to the total ascent, as it effectively smoothes out lumps and bumps. We sample readings quite frequently - every 30m for most routes but less frequently for longer routes.
Unfortunately no method is going to be 100% reliable but I think the main conclusions are that you can’t really compare ascent calculations from different sources as they are calculated differently and you can't rely on elevation readings measured by GPS devices.
John
- Inaccuracies in the elevation readings being measured by the GPS device
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Nisse Son Thursday 24 Mar 2016 19:20:49
Hi John, happy you are look into this so thoroughly!
I think the problem is elsewhere, look at this route: https://www.plotaroute.com/route/186065 According to the overview the total ascent is 5031m (see also this screen shot: https://drive.google.com/file/d/0B9MeEBv9khGkZF9aN21xMFBIVGM ) I have riden it and it is about 2900-3000 total ascent, see Strava track: https://www.strava.com/activities/417517234 but also make an estimate by yourself using tools.
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plotaroute admin Friday 25 Mar 2016 10:29:08
I'm afraid it's not possible to compare Total Ascent figures from different sources, as they are calculated differently and also elevation data from GPS devices is not reliable (see my post below 20 March).
I believe that Strava uses the elevation data from your GPS device if it has a barametric altimeter, so I would expect Strava and your GPS device to give very similar results as Strava is just reporting what your GPS device says, possibly wirth some minor smoothing. Unfortunately GPS devices are not infallible but there is a tendancy to place unwaivering trust in their results, even though closer scrutiny of the raw data often shows big measurement errors and gaps.
John
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Nisse Son Friday 25 Mar 2016 18:40:54
Yes, Strava probably uses my barometric data from my GPS (Garmin Efdge 500) and yes, barometric data can be way of. What I try make a point of is that Plotaroute have some major bug in calculating elevation data, either based on data or algorithms. Take a look again at the route (https://www.plotaroute.com/route/186065 ). It is basically going straight up from sea level to 1936MASL and then back down again. There are a few small hills during the way, but nothing that adds 3000 meter of climb more (up to at thousand, yes maybe). The end result can never be 5000+ meter of ascent in total. Look closely at this screen shot of the profile Plotaroute shows for it (https://drive.google.com/file/d/0B9MeEBv9khGkRGQ2dTB0UElxVjQ ) and we get a hint where this comes from, the road seems to go up/down constantly when it is just a steady hill. (Again, it is obvious that there isn't 3000 meter extra of ascent to find on this route.) The problem is probably that Plotaroute doesn't do any smoothing whatsoever of the data. This results in the error that for hillier parts of the world makes the total ascent hugely incorrect. I have ridden these roads a lot of times and can assure you they do not go up/down/up/down as the profile says (it is easy to take a look at Google Street view for yourself to confirm if you wish).
Another, even clearer example: https://www.plotaroute.com/route/186461 This is the famous Sa Calobra-climb on Mallorca. It starts at five meters above sea level and tops out at ~715-720 m.a.s.l. and I would say it has 0 meters of downhill. Hence it has 720-5=715 of ascent, remember that regardless of where during the climb you climb those meters, there can never be more than (hight at end - hight at start). Lets say I'm wrong about the 0 meter of downhill and there actually is 10, this would lead to a total ascent of 725 meter (again, have a look at Google Street view if you wish to validate (or even better, go there yourself, I can recommend it! :) )) Plotaroute states 914 meters of ascent, ie more than 25 % too much.
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plotaroute admin Saturday 26 Mar 2016 11:09:33
Hi Olof - I've had a quick look at your Sa Calobra route, which has a total ascent of 914m on our calculations but 715m if you assume there are no downhills. However, it does appear that there are some downhill stretches, at least according to our data but also according to Google's data.
It's not really possible to see this clearly in Google Street View, as it's a 2D image, but you can see it in Google Earth. If you download the route as a KML file you can open it in the Google Earth software and move your mouse along the line of the route to see the elevation readings at the bottom of the screen. If you look at some of the parts of the route where our data shows a downhill stretch, say 2.7km or 6.3km, you should see that Google also reports this as a downhill stretch. It is all these short downhill stretches that make the total ascent higher than the difference between the altitudes of the start and the summit. These can be smoothed out in our route profile tool by using elevation readings that are further apart - so if elevations readings were taken every 120m instead of every 30m the total ascent would be 765m instead of 914m. I'm not saying that our data is perfect, as no solution is perfect and there is a limit to what we can provide as part of a free service, but it does seem like our data is fairly consistent with Google's.
I'll will do some further investigation though, but it does take quite a lot of time to look into individual routes on a case by case basis and I'm conscious that time spent on this is diverting us away from working on other feature requests that people have asked for.
John
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Andrea Ticci Sunday 27 Mar 2016 23:18:56
Hi John,
In my view the issue is plotaroute source data: the elevation profile is given by the overlay of the route coordinates an the elevation data in the map. Errors in the road's coordinates and in the elevation data contained in the maps (google earth or any other map source) will generate a deviation from the real altimetric profile. In other terms the issue is in the input data that plotaroute processes. The accuracy of barometric readings from GPS devices seems to be much better than the accuracy of data taken from road plotted on charts. This is confirmed by the relatively good agreement of different instruments from different manufacturers when riding the same route.
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plotaroute admin Monday 28 Mar 2016 11:20:14
I'm afraid elevation data from GPS devices is not a reliable benchmark against which to judge whether ths source data is accurate, even if the device does have a barometric alitmeter. When we analysed the route you provided at the start of this thread, your barometric GPS device was measuring 63m above sea level at points where the route was next to the sea. I've seen other examples of this too.
Also, it doesn't matter how accurate the altitude measruement is if the device loses the GPS signal - it doesn't just stop recording coordinates when the GPS signal is lost, it also stops recording elevation data, so this creates gaps in the elevation data, which leads to some climbs not being counted.
Unfortunately no solution is perfect, but our source data does seem to tally quite closely with Google's data, so I'm fairly sure that differences in Total Ascent calculations between different applications and devices are largely due to size of the gaps between the data, with larger gaps giving a lower result.
Pleas bear in mind that this is a free service and there is a limit to what we can realistically do.
John
