What Was The Actual Distance ?

runner100 is right on the ball in noting the wide range of distances reported by runners wearing GPS units when he says "surely if they are that reliable they would all read exactly the same?!"

The 26 distances reported by GPS have been graphed and anlaysed http://coursemeasurement.org.uk/GPS_run-to-the%20-beat-half.htm This shows that the GPS users on averaged measured the course to be 2.2% long, but they had a scatter about that average value with a standard deviation of 0.8%.

By contrast the the scatter in the results of course measurers, using a calibrated bicycle ridden along the Shortest Possible Route is typically 0.003% some 50 times more consistent than the GPS results reported here for the RTTB Half. That is why calibrated bicycles are used for official course measurements and not GPS units.

Actually it has been noticed that course measurers with a GPS fitted to their bike, usually get much more accurate results than runners do. This is probably due to a combination of factors eg

1. Course measurers are very careful about picking out the shortest possible line, 30 cm from the boundary on bends. Whereas runners in a crowded race on a twisty course have no chance of running such a good line.
2. The GPS unit on the bike may get better satellite view, resulting in fewer "jags" in the plotted track which tend to add to the distance recorded. Buildings and trees contribute to such loss of satellite view. However, even on a completely open airfield course GPS's do tend to read long so this by itself is not the whole story.
3. A few runners may be careless and not start and stop the GPS at the start and finish lines - however one would expect most runners to be aware of importance of this.
4. There is another possibility - that the course laid out by the race director is not exactly that measured by the official course measurer. The course measurer writes a report with a map , and these define the course which must be laid out by the race director. For courses in the South of England that have a UKA Certificate of Course Accuracy, the measurers course map can be found on the web at http://coursemeasurement.org.uk/south/index.htm. Unfortunately although it was properly measured the RTTB  Half did not get a UKA licence so no UKA Certficate of Course Accuracy was issued.

One thing that came to mind was the bits like at the Royal Arsenal Barracks where you ran in and around the water station then back out, living locallyt i know that usually this is all locked up so maybe areas like this were not measured when they went aorund on the bike?

Also the building work on the Royal Arsenal site further back is changing day to day with roads changing around.

Maybe these made a difference ?

This was my first run of any kind so 13 miles seemed like 26 to me anyway but just my thoughts

Mark Ward

At the Woolwich Baracks parade ground the course measurer went through the gates, marked the 5 mile point 41 metres from the line of bollards just outside the gates, and then marked the location of a turn round cone 110 metres from the line of bollards and then proceeded back to the gatesand turned right to continue along Grand depot road.

For the Woolwich arsenal Section the measurer approaced beside the Thames  and passed the statues in J.Clavell Square on his left hand side and then continued East until building 50where bhe made two right turns into Cadogan Rd. which then joined Duke of wellington Way where he turned right again 4 miles was then marked a little way along just 4 metres before lamp post 0p68 on his left he then passed through two gates and contuned westwardto reach woolwich high street again.

It is all given in great detail in the measurer's map and report.

Hi all,

Had to reply to this as I have a garmin myself, and have used it on many courses for about 3 years.

I originally had the Forerunner201 until about 3 months ago, this was always about 98% accurate, even on a 400m running track it was about the same, so I used to have it set to take mile splits at either 1.02 or 1.01 miles in races depending on how flat the course was, and this then gave me a much more accurate reading to agree when finishing.

I have now got a Forerunner205 which is much more accurate. Well, that is if you read the manual and change the default setting. As the 205 has an option to change a setting which is not on the 201 version, and so it plots your position every second of your run (battery only lasts 3.5 hours though then), this has proven to be very accurate. The Peterborough Half last month measured 13.12 miles, not bad eh!I have also run for 1 mile around a track, and it beeped for the mile just a few metres past the line on the 4th lap, which is pretty damned good.

If you are ever in doubt then check out the course measurement website first, look at the map that is often provided, and check the race has a valid accuracy number too, there are some races that dont!

SS