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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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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 Tuesday 08 Mar 2016 16:35:04
From a quick bit a research it appears that other sources may indeed ignore some elevation readings within a certain threhsold, effectively applying smooting to the data collected. For example, Strava's site says
"A gain must exceed a threshold in order to be counted. The elevation data used for calculating gain is smoothed before elevation gain is calculated in order to reduce noise. If you are riding or running in a very flat area, and none of the individual climbs on your activity exceed that threshold, it is possible that Strava will list a total elevation gain of 0ft, even if the elevation profile is not totally flat. This is an unfortunate side effect of the thresholds necessary to reduce noise on the majority of Strava activities."
I suspect Garmin does the same - if anyone can confirm this I'd be interested to know. We don't ignore any readings or do any smoothing, so all lumps and bumps along the route are counted in our total ascent calculations, which would explain the higher figure.
John
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Andrea Ticci Tuesday 08 Mar 2016 13:27:38
Hi,
I believe plotaroute is processing correctly the data, the point maybe is in the accuracy of the elevation data and contour lines in the maps itself.
As an example if you are analazing data on a flat route in the mountains with steep gradients on each side of the road, a small error when plotting the road on the map (even few meters each side) would generate a large elevation change although the road is flat. When plotting in areas with contour elevation lines very close each other i.e. with considerable gradient, this error could become significant.
Regards
Andrea
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plotaroute admin Tuesday 08 Mar 2016 12:52:03
We'll certainly look into it a bit more but I think Garmin devices ignore readings with small elevation changes when calculating the total ascent due to assumptions about accuracy levels of the data, effectively applying smoothing to the data. So although it may be collecting data every second, this data isn't necessarly all used in the calculation of the total ascent figure. Also, by collecting data at time intervals, it will be at varying distance intervals, which creates another anonomly. And if the time interval is very short (e.g. one second), then changes in elevation may also be very small and may be discounted in calculations.
It might be quite hard to get to the bottom of it without having access to both the raw elevation data and formulae used by different systems. I'll try to do some research on it.
John
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George Slavov Tuesday 08 Mar 2016 12:02:54
Thanks for the quick response John
Unfortunately I don't think that's quite right. My Garmin is set to record a point every second, so on an avrage hill climb that would be 3-5m apart (I am not the fastest climber out there). This is a far higher sampling rate than what is present in any publicly available mapset and should in turn result in the highest elevation reading. The reality is that Plot a Route comes in roughly 50% above that (and this is consistent for all routes I've created or looked at in this area).
So I still think there is an issue with the underlying elevation data provider and it would be great if that was addressed at some point in the future.
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plotaroute admin Tuesday 08 Mar 2016 11:07:09
Hi George - glad you're enjoying using the site. Unfortunatley there is no "right answer" when it comes to total ascent, as it depends on the sampling interval (see my post below on 9 Nov 2015). Less frequent sampling of elevation readings will give a lower total ascent as it smoothes out lumps and bumps. I think we sample more frequently than most. Neither figure is wrong - they just need to be interpreted in context, so you can't really compare figures calculated using different sampling intervals.
John
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George Slavov Tuesday 08 Mar 2016 10:20:25
I was just looking at the same thing, I created a route here and then exported the GPX file. Plot a route estimates the altitude at 3300m, RWG estimates it at 2100m and actually riding it gives 2100m (Garmin 705 with barometric altimeter). I am guessing the underlying map data is a bit off.
Other than that this site is brilliant and has become my goto place for mapping out new routes.
George
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Deleted User Tuesday 10 Nov 2015 13:20:24
Hi John,
OK I see what you mean. I dont know what the others are doing but I can surely try to find out.
Can I ajust this somewhere when I'm creating a route?It though seems a bit strange to me that the total ascent in la Marmotte this year was 7310 mtrs (in reality)
AllanH
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plotaroute admin Monday 09 Nov 2015 13:42:58
Hi Alan,
Unfortunately there is no correct answer when it comes to calculating the Total Ascent. You can get a different figure each time by having different distances between the elevation readings, but that doesn't make each answer wrong. Longer distances between elevation readings will have the effect of smoothing out bumps and dips, so tend to result in lower Total Ascent. We take readings at regular 30m intervals by default (unless the route is very long), so probably more frequently than other sources you might be comparing to. GPS devices can be notoriously innacurate when it comes to calculating Total Ascent. Not only do they sometimes gather quite inaccurate readings, they also take readings at irregular intervals, so they are not good to compare with. Sometimes a barometric altimeter can help with accuracy but when we looked at the example quoted below, where a Garmin device with a baraometric altimeter was used, it was recording elevation readings of 63m at sea level, so quite a long way out.
You can see how the Total Ascent changes as elevation readings are sampled less frequently by having a look at your route with our route profile tool - drag the Elevation Interval slider on the right to increase the distance between each elevation reading and see how the Total Ascent changes in the data at the top.
John
