Tag Archives: health

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.

Colours Running Out: Analysis of 2016 running

Towards the end of 2015, I started distance running. I thought it’d be fun to look at the frequency of my runs over the course of 2016.

Most of my runs were recorded with a GPS watch. I log my cycling data using Rubitrack, so I just added my running data to this. This software is great but to do any serious number crunching, other software is needed. Yes, I know that if I used strava I can do lots of things with my data… but I don’t. I also know that there are tools for R to do this, but I wrote something in Igor instead. The GitHub repo is here. There’s a technical description below, as well as some random thoughts on running (and cycling).

The animation shows the tracks I recorded as 2016 rolled by. The routes won’t mean much to you, but I can recognise most of them. You can see how I built up the distance to run a marathon and then how the runs became less frequent through late summer to October. I logged 975 km with probably another 50 km or so not logged.

run2016

Technical description

To pull the data out of rubiTrack 4 Pro is actually quite difficult since there is no automated export. An applescript did the job of going through all the run activities and exporting them as gpx. There is an API provided by Garmin to take the data straight from the FIT files recorded by the watch, but everything is saved and tagged in rubiTrack, so gpx is a good starting point. GPX is an xml format which can be read into Igor using XMLutils XOP written by andyfaff. Previously, I’ve used nokogiri for reading XML, but this XOP keeps everything within Igor. This worked OK, but I had some trouble with namespaces which I didn’t resolve properly and what is in the code is a slight hack. I wrote some code which imported all the files and then processed the time frame I wanted to look at. It basically looks at a.m. and p.m. for each day in the timeframe. Igor deals with date/time nicely and so this was quite easy. Two lookups per day were needed because I often went for two runs per day (run commuting). I set the lat/lon at the start of each track as 0,0. I used the new alpha tools in IP7 to fade the tracks so that they decay away over time. They disappear with 1/8 reduction in opacity over a four day period. Igor writes out to mov which worked really nicely, but wordpress can’t host movies, so I added a line to write out TIFFs of each frame of the animation and assembled a nice gif using FIJI.

Getting started with running

Getting into running was almost accidental. I am a committed cyclist and had always been of the opinion: since running doesn’t improve aerobic cycling performance (only cycling does that), any activity other than cycling is a waste of time. However, I realised that finding time for cycling was getting more difficult and also my goal is to keep fit and not to actually be a pro-cyclist, so running had to be worth a try. Roughly speaking, running is about three times more time efficient compared to cycling. One hour of running approximates to three hours of cycling. I thought, I would just try it. Over the winter. No more than that. Of course, I soon got the running bug and ran through most of 2016. Taking part in a few running events (marathon, half marathons, 10K). A quick four notes on my experience.

  1. The key thing to keeping running is staying healthy and uninjured. That means building up distance and frequency of running very slowly. In fact, the limitation to running is the body’s ability to actually do the distance. In cycling this is different, as long as you fuel adequately and you’re reasonably fit, you could cycle all day if you wanted. This not true of running, and so, building up to doing longer distances is essential and the ramp up shouldn’t be rushed. Injuries will cost you lost weeks on a training schedule.
  2. There’s lots of things “people don’t tell you” about running. Blisters and things everyone knows about, but losing a toenail during a 20 km run? Encountering runner’s GI problems? There’s lots of surprises as you start out. Joining a club or reading running forums probably helps (I didn’t bother!). In case you are wondering, the respective answers are getting decent shoes fitted and well, there is no cure.
  3. Going from cycling to running meant going from very little upper body mass to gaining extra muscle. This means gaining weight. This is something of a shock to a cyclist and seems counterintuitive, since more activity should really equate to weight loss. I maintained cycling through the year, but was not expecting a gain of ~3 kilos.
  4. As with any sport, having something to aim for is essential. Training for training’s sake can become pointless, so line up something to shoot for. Sign up for an event or at least have an achievement (distance, average speed) in your mind that you want to achieve.

So there you have it. I’ll probably continue to mix running with cycling in 2017. I’ll probably extend the repo to do more with cycling data if I have the time.

The post title is taken from “Colours Running Out” by TOY from their eponymous LP.

Tips from the blog IX: running route

University of Warwick is a popular conference destination, with thousands of visitors per year. Next time you visit and stay on campus, why not bring your running shoes and try out these routes?

Route 1

track1

This is just over 10K and it takes you from main campus out towards Cryfield Pavilion. A path goes to the Greenway (a former railway), which is a nice flat gravel track. It goes up to Burton Green and back to campus via Westwood Heath Road. To exit the Greenway at Burton Green you need to take the “offramp” at the bridge otherwise you will end up heading to Berkswell. If you want to run totally off-road*, just turn back at this point (probably ~12K). The path out to the Greenway and the Greenway itself is unlit, so be careful early in the morning or late at night.

GPX of a trace put together on gpsies.

Track 2

track2

This is a variation on Track 1. Instead of heading up the Greenway to Burton Green, take a left and head towards Kenilworth Common. With a bit of navigation you can run on alongside a brook and pop out in Abbey Fields and see the ruins of Kenilworth Abbey. This is out-and-back, 12K. Obviously you can turn back sooner if you prefer. It’s all off-road apart from a few 100m on quiet residential streets as you navigate from the Common to Abbey Fields. GPX from Uni to around the lake at Abbey Fields.

Track 3

track3

 

This is a variation on Track 1 where you exit the Greenway and take a loop around Crackley Wood. The Wood is nice and has wild deer and other interesting wildlife. This route is totally off-road and is shorter at ~8K. GPX from Uni to around the Wood.

 

Other Routes

There is a footpath next to a bike lane down the A429 which is popular for runners heading to do a lap or two of Memorial Park in Coventry. This is OK, but means that you run alongside cars a lot.

If you don’t have time for these routes, the official Warwick page has three very short running routes of around 3 to 5 km (1, 2 and 3). I think that these routes are the ones that are on the signpost near the Sports Centre.

* Here, off-road means on paths but not alongside a road on a pavement. It doesn’t mean across fields.

This post is part of a series of tips.

Lemonade Secret Drinker: sober statistics

I read this article on the BBC recently about alcohol consumption in the UK. In passing it mentions how many people in the UK are teetotal. I found the number reported – 21% – unbelievable so I checked out the source for the numbers.

Sure enough, ~20% of the UK population are indeed teetotal (see plots). The breakdown by gender and age is perhaps to be expected. There are fewer teetotal men than women. Older women (65+) in particular are more likely to be teetotal. There has been a slight tendency in recent years for more abstinence across the board, although last year is an exception. The BBC article noted that young people are pushing up the numbers with high rates of sobriety.

There are more interesting stats in the survey which you can check out and download. For example, London has the highest rate of teetotallers in the UK (32%).

I thought this post would make a fun antidote in the run up to the holidays, which in the UK at least is strongly linked with alcohol consumption.

The post title is taken from “Lemonade Secret Drinker” by Mansun, which featured on their first EP (One). It’s a play on “Secret Lemonade Drinker” the theme from R Whites Lemonade TV commercial in the 70s/80s (which I believe was written and sung by Elvis Costello’s father).