Ascent calculation is inaccurateFORUMS HOME SEARCH FORUMS

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.

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

The ascent/descent-data for routes are often of with up to 50%. Happy to help out with raw-data if needed.

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

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. 

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

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.

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

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. 

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

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

I'm afraid we don't have the resources to keep doing detailed analysis of individual routes.  If Google's data isn't giving you the accuracy that you are looking for we have little chance of bettering that!  plotaroute is a tiny operation and is offered as a free service.  Yes, we do have an ad-free option but the nominal annual fee for this just offsets lost advertising income - our total revenues are not enough to pay the salary of one person let alone a whole team!  The project is very much a labour of love at present that I hope will grow into something bigger, so please bear with us, as we can only achieve what is feasible and within our means.

I understand the desire to try to get our Total Ascent figures to match those from other systems, but my main concern is to ensure that people can compare routes that we have on plotaroute on a like for like basis, so I would not want to apply different rules for different routes, for example by sometimes using GPS elevation data in place of the data we would normally use.  Also, we would need to make fundamental changes to the system to do this, as elevation data is not captured and stored on a route by route basis at present.

I think one way forward would be to calculate two figures for the Total Ascent - a 'raw' figue (the current one) and a 'smoothed' figure (sampling elevation readings less frequently).  This will retain the integrity and comparability of the existing routes while provding an alternative figure that reflects the degree of smoothing that is probably found in other applications. I can add this to our Feature Request list if you feel this would be helpful, where anyone interested in it can vote for it.  As you'll appreciate though, there are many other users of plotaroute who are asking for enhancements and new features, so I do need to take into account which ideas are most popular.

John

"I'm afraid we don't have the resources to keep doing detailed analysis of individual routes.  If Google's data isn't giving you the accuracy that you are looking for we have little chance of bettering that!"

I don't think anyone expects you to, but you could use what the other services do, heck you have already built it yourselves! :-) The Google data doesn't have the accuracy to be used for this application as raw data, it has to be smoothed a bit. This is what other services do to make the accumulated numbers more true to the real world. Without this, the totals can be off by 50%, as previously shown.

But good news, there is an easy way to solve this! What I (and possibly others) ask for is just that you use smoothing for the data. Plotaroute have the feature already in the "Route Profile Tool". If I use this for any of my done rides and set "Filter Bumps" to 10.0% and "Elevation Intervall" to ~200 meters, both the hills inclination and total data matches reality. If e.g. these numbers would be default things would be as I (and possibly others) would like it to be. 

Not only are the totals off as of today (annoying, but still just numbers), but the Route Timer-estimation gets so wrong it is useless. If the Route Timer and Download features used the smoothing from the "Route Profile Tool" instead of raw data, then those feature would be much more useful.

So, my suggestion is: make some smoothing default (if not for all users all the time, but could we please have it on a user base?). This will solve this problem to at least the use cases I have seen in this thread.  

I'm happy to look at smoothing the elevation data a bit to reduce the total ascent figures  if enough people feel this would be helpful.  I've added it to the Feature Requests list (number 52), so that anyone interested in this can vote for it.  At the moment only 3 people have asked for this on the posts below and we have over 50,000 registered users on the site, so I need to consider that some people may prefer that the data wasn't smoothed.  My personal opinion is that it would probably help for the Total Ascent figures to be more comparable with other applications and, so a bit of smoothing is probably a good idea, however  I think I would only want to increase the sampling interval from 30m to 90m.  This would also mean having to recalculate the Total Ascent figures for every route on the database, so it would be quite a big task.  And there would be additional development work if we also smoothed the actual elevation profiles, but if smoothed data is used to calculate the Total Ascent figutes it would probably be best to use smoothed data for the elevation profiles too. 

John

Reagrding ascent calculation I have to say estimation by Plotaroute is quite good during the editing. But once I saved the track, I see even a +40% increment without any reason.

If I go back to editing the same track and I just move the start-stop point by few meter, the system updates the total ascent and the number comes back to "real" values. So to me the problem seems to be related to the SAVE process... Even without saving, if I switch to the profile tool I see the increased value... It seems the right value is given only during the editing and only in the "HILLS" area (below the map).

Is this happening only to me?

Thanks,

Claudio

ps: plotaroute is a great tool! many compliments!

Glad you like the site Claudio - thanks.  

The Total Ascent figure shown on the elevation profile as you are plotting a route is just an estimate based on a small sample of elevation readings (a maximum of 500 at equidistant intervals across the route)  - a more detailed calculation is made when you save the route using elevation readings at 30m intervals, as it isn't feasible to recalculate this in such detail as you are plotting.  We do adjust the 'as-you-plot' figure by a factor to try to compensate for the smoothing effect of using elevation readings at bigger intervals, but the impact of using bigger intervals differs according to the amount of bumps and dips on the route, so it will sometimes vary quite a bit from the final figure.

John

Hello

I think this would be a nice idea, not to make it comparable with other applications but also to make it more accurate.

I leave in a mountain area (French Alps), so I ride frequently rides with a lot of elevation. The total elevation on plotaroute is often at least 50% too high. Example : I rode yesterday https://www.plotaroute.com/route/216481

Total ascent is 2671m on plotaroute. Yesterday a friend and I measured a total ascent of around 1820m (we had two different GPS)

I found similar results on pure uphill rides when comparing also with the altitudes written on a map.

The percentages as well are too high. I frequently get on plotaroute a max percentage of more than  35% (38.8% on the ride above) which is impossible to climb on a bike (at least for me!)

Keep on the excellent work you are doing. Your site is great!

Thanks Pierre for your feedback - glad you're enjoying using the site.  Adding some smoothing to our elevation data is on the Feature Requests list but please vote for it to move it up the list (number 52).  Please be aware though that GPS devices are not a reliable bechmark for ascent calculations - we've looked at numerous examples of raw GPS recordings and usually find big gaps in the data (probably due to poor signal reception) that result in ascent figures being understated.

John

Thanks. I voted or it.

There is just near my place an ascent which is pure uphill on 18km. It's about 1400m ascent. Next time I climb it I will see what plot a route gives and compare it both with a map and with what my GPS gives.

I will let you know.