Ascent calculation is inaccurate

Posts for this topic...

• Post your comments.... Sign In to Post
• Nisse Son   Monday 28 Mar 2016 19:20:09

  John, I first of all must give credit to how you and the Plotaroute team takes on customer complaints, issues and feedback. You dive into things fast and with engagement, this is an area where you stand out positively from a lot of competitors!

   

  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.

   

  Trust me, there are none. I would not use hypothetical examples when I post my findings, I have ridden all of them. If in doubt, post a question to any of the bigger road cycling forums and ask how much downhill it is on the first 9.9km of Sa Calobra. :-) The total from Plotaroute is ~200 meter of.

   

  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.

   

  This is where the problem arises, you assume Google's data to be accurate. It is not when it comes to small roads climbing up and down mountains. The simplified model they use doesn't give correct data for this use case when used without smoothing of it. Lets take the part between 6-7km of the Sa Calobra as an example: https://www.plotaroute.com/route/187724 What according to Google is an undulating road is not, this is an error due to their simplified model of terrain. Again, I have ridden this road and know where there are and are not downhill. Do download the .kml and look at the 1 kilometer stretch in Google Earth street view, and you will see for your self that there are definitely no downhills to speak of. (The slope is quite the same all way through, which makes sense as the road is built to accommodate for the tourist buses and therefore is about 6-7%). 

   

  This is not a problem with these specific roads, but a general issue. For 95% of roads the error is so small you won't notice, but please remember that it is for those last 5% where a service like this is used a lot.

   

  Pleas bear in mind that this is a free service and there is a limit to what we can realistically do.

   

  Plotaroute is not a free service, you work in Freemium-model. (I'm happily paying for the service.) I for one think it is an important one for how I use the service. But I fully agree on that you need to make decisions on what bugs to fix vs. what features to build and this might not be the most important one.

   

  What I and others propose is not that you try to find correct data for all roads in the world, but look into how smoothing of the data would give more accurate information for total ascent and for the time estimations. 
• Andrea Ticci Monday 28 Mar 2016 18:34:44

  Thank you John,

  You have done a fantastic job with this site and I understand there is a limit to what can be realistically done calculating elevation from map.

  Just to clarify what I was meaning,I am providing what seems to be a good example:

  This route I have generated with plotaroute https://www.plotaroute.com/route/187663 is quite famous in the area. The route is a continuos climb with not a single meter going downhill over the entire length (13km). The starting point is at 154m above sea level, the end is at 814m above sea level.

  The real elevation gain for this route is 814 - 154 = 660 m (source: google earth, in good agreement with plotaroute values and altimeter readings)

  Plotaroute elevation profile for this route https://www.plotaroute.com/routeprofile/187663 shows a fragmented pattern with continuous up and down that do not exist.

  The unfiltered elevation gain from plotaroute provides 1305 m  that is about twice the real elevation gain.

  I believe the limit is provided by the maps used to generate the elevation profile.

  You are right in saying that barometric GPS absolute elevation readings can be inaccurate especially if you do not calibrate the altitude with a known value at the beginning of the ride to eliminate drifts due to barometric pressure variations (sometimes I read negative altitudes at sea level). In any case the differential elevation gain remains quite accurate (e.g.  My GPS with barometric altimeter is quite in good agreement with the  calculated 660 m elevation gain of the above route, the deviation from  the 660 m is always less than 4% ).

  I would suggest when uploading rides to plotaroute, to provide the option of recalculating the elevation data or to leave the data in the uploaded route.

  Regards

  Andrea

   

• 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

• 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. 

   

• 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

• 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.

• 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

• 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. 

   

• 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:

  1. Inaccuracies in the elevation readings being measured by the GPS device
     
     
  2. Long gaps with no readings
     
     
  3. 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

• 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.