Tag Archives: cycling

Elevation: accuracy of a Garmin Edge 800 GPS device

I use a Garmin 800 GPS device to log my cycling activity. including my commutes. Since I have now built up nearly 4 years of cycling the same route, I had a good dataset to look at how accurate the device is.

I wrote some code to import all of the rides tagged with commute in rubiTrack 4 Pro (technical details are below). These tracks needed categorising so that they could be compared. Then I plotted them out as a gizmo in Igor Pro and compared them to a reference data set which I obtained via GPS Visualiser.

commute3d

The reference dataset is black. Showing the “true” elevation at those particular latitude and longitude coordinates. Plotted on to that are the commute tracks coloured red-white-blue according to longitude. You can see that there are a range of elevations recorded by the device, apart from a few outliers they are mostly accurate but offset. This is strange because I have the elevation of the start and end points saved in the device and I thought it changed the altitude it was measuring to these elevation positions when recording the track, obviously not.

abcTo look at the error in the device I plotted out the difference in the measured altitude at a given location versus the true elevation. For each route (to and from work) a histogram of elevation differences is shown to the right. The average difference is 8 m for the commute in and 4 m for the commute back. This is quite a lot considering that all of this is only ~100 m above sea level. The standard deviation is 43 m for the commute in and 26 m for the way back.

cda

This post at VeloViewer comparing GPS data on Strava from pro-cyclists riding the St15 of 2015 Giro d’Italia sprang to mind. Some GPS devices performed OK, whereas others (including Garmin) did less well. The idea in that post is that rain affects the recording of some units. This could be true and although I live in a rainy country, I doubt it can account for the inaccuracies recorded here. Bear in mind that that stage was over some big changes in altitude and my recordings, very little. On the other hand, there are very few tracks in that post whereas there is lots of data here.

startmidIt’s interesting that the data is worse going in to work than coming back. I do set off quite early in the morning and it is colder etc first thing which might mean the unit doesn’t behave as well for the commute to work. Both to and from work tracks vary most in lat/lon recordings at the start of the track which suggests that the unit is slow to get an exact location – something every Garmin user can attest to. Although I always wait until it has a fix before setting off. The final two plots show what the beginning of the return from work looks like for location accuracy (travelling east to west) compared to a midway section of the same commute (right). This might mean the the inaccuracy at the start determines how inaccurate the track is. As I mentioned, the elevation is set for start and end points. Perhaps if the lat/lon is too far from the endpoint it fails to collect the correct elevation.

Conclusion

I’m disappointed with the accuracy of the device. However, I have no idea whether other GPS units (including phones) would outperform the Garmin Edge 800 or even if later Garmin models are better. This is a good but limited dataset. A similar analysis would be possible on a huge dataset (e.g. all strava data) which would reveal the best and worst GPS devices and/or the best conditions for recording the most accurate data.

Technical details

I described how to get GPX tracks from rubiTrack 4 Pro into Igor and how to crunch them in a previous post. I modified the code to get elevation data out from the cycling tracks and generally made the code slightly more robust. This left me with 1,200 tracks. My commutes are varied. I frequently go from A to C via B and from C to A via D which is a loop (this is what is shown here). But I also go A to C via D, C to A via B and then I also often extend the commute to include 30 km of Warwickshire countryside. The tracks could be categorized by testing whether they began at A or C (this rejected some partial routes) and then testing whether they passed through B or D. These could then be plotted and checked visually for any routes which went off course, there were none. The key here is to pick the right B and D points. To calculate the differences in elevation, the simplest thing was to get GPS Visualiser to tell me what the elevation should be for all the points I had. I was surprised that the API could do half a million points without complaining. This was sufficient to do the rest. Note that the comparisons needed to be done as lat/lon versus elevation because due to differences in speed, time or trackpoint number lead to inherent differences in lat/lon (and elevation). Note also due to the small scale I didn’t bother converting lat/lon into flat earth kilometres.

The post title comes from “Elevation” by Television, which can be found on the classic “Marquee Moon” LP.

A Day In The Life III

This year #paperOTD (or paper of the day for any readers not on Twitter) did not go well for me. I’ve been busy with lots of things and I’m now reviewing more grants than last year because I am doing more committee work. This means I am finding less time to read one paper per day. Nonetheless I will round up the stats for this year. I only managed to read a paper on 59.2% of available days…

The top ten journals that published the papers that I read:

  • 1 Nat Commun
  • 2 J Cell Biol
  • 3 Nature
  • 4= Cell
  • 4= eLife
  • 4= Traffic
  • 7 Science
  • 8= Dev Cell
  • 8= Mol Biol Cell
  • 8= Nat Cell Biol

Nature Communications has published some really nice cell biology this year and I’m not surprised it’s number one. Also, I read more papers in Cell this year compared to last. The papers I read are mainly recent. Around 83% of the papers were published in 2015. Again, a significant fraction (42%) of the papers have statistical errors. Funnily enough there were no preprints in my top ten. I realised that I tend to read these when approving them as an affiliate (thoroughly enough for #paperOTD if they interest me) but I don’t mark them in the database.

I think my favourite paper was this one on methods to move organelles around cells using light, see also this paper for a related method. I think I’ll try again next year to read one paper per day. I’m a bit worried that if I don’t attempt this, I simply won’t read any papers in detail.

lgs

I also resolved to read one book per month in 2015. I managed this in 2014, but fell short in 2015 just like with #paperOTD. The best book from a limited selection was Matthew Cobb’s Life’s Greatest Secret. A tale of the early days of molecular biology, as it happened. I was a bit sceptical that Matthew could bring anything new to this area of scientific history. Having read Eighth Day of Creation, and then some pale imitations, I thought that this had pretty much been covered completely. This book however takes a fresh perspective and it’s worth reading. Matthew has a nice writing style, animating the dusty old main characters with a insightful detail as he goes along. Check it out.

This blog is going well, with readership growing all the time. I have written about this progress previously (here and here). The most popular posts are those on publishing: preprints, impact factors and publication lag times, rather than my science, but that’s OK. There is more to come on lag times in the New Year, stay tuned.

ladidadiI am a fan of year-end lists as you may be able to tell. My album of the year is Battles – La Di Da Di which came out on Warp in September. An honourable mention goes to Air Formation – Were We Ever Here EP which I bought on iTunes since the 250 copies had long gone by the time I discovered it on AC30.

Since I don’t watch TV or go to the cinema, I don’t have a pick of the year for that. When it comes to pro-cycling, of course I have an opinion. My favourite stage race was Critérium du Dauphiné Libere which was won by Chris Froome in a close contest with Tejay van Garderen. The best one-day race was a tough pick between E3 Harelbeke won by Geraint Thomas and Omloop Het Nieuwsblad won by Ian Stannard. Although E3 was a hard man’s race in tough conditions, I have to go for Stannard outfoxing three(!) Etixx Quick Step riders to take the win in Nieuwsblad. I’m a bit annoyed that those three picks all involve Team Sky and British riders…. I won’t bore everyone with my own cycling (and running) exploits in 2015. Just to say, that I’ve been more active this year in any year since 2009.

I shouldn’t need to tell you where the post title comes from. If you haven’t heard Sgt. Pepper’s Lonely Hearts Club Band by The Beatles, you need to rectify this urgently. The greatest album recorded on 4-track equipment, no question. 🙂