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Prompted by the excellent results obtained by Phil Holland in England, and by the work reported by several course measurers on this forum, especially Mark Neal in this and other posts, I recently acquired a GPS unit and have been carrying out some experiments to determine how it may be used as an accessory during course measurement with a calibrated bicycle and Jones counter.

I am writing up my experience on a series of web pages which I hope will be useful to course measurers. the first of these pages, The Repeatability of Marked Way Points, is now complete and I welcome any comments from GPS users with more experience than me.

My Conclusion is:

Without any averaging, Mike's ETREX H could be used to record and then refind reference points normally (ie 95% of the time) to better than about 8 m. Occasional errors might be greater. At this level of performance the Etrex H is not accurate enough the reliably record the exact start or finish of a race, which still needs to be measured by means of a Jones counter and a calibrated bicycle wheel, and referenced with an offset from a nearby piece of fixed street furniture. However a GPS can be very useful in recording the general location of the reference points and for example it could fairly reliably distinguish between trees of similar appearance separated by more than 20m along a road side.

However a measurer should be aware that not every race director or indeed other measurers may have suitable equipment to be able to refind the references or indeed be able to operate it, as I discovered when I used grid refs to record the reference furniture locations for the mile splits for a recent country half marathon. When the organiser went out with a borrowed GPS to paint the mile marks, he could not master use of his GPS to locate the points, and he had to rely on the fact that I had provided a sufficiently comprehensive description of the location references (trees, gates, openings in hedges etc). That said, my record of the locations will certainly be useful aid to me if I have ever to go back and refind a lost reference. For example if a tree is cut down, I might be able to locate the correct stump, with the aid of my GPS. Occasionally I end up with paint marks on the road and no close by reference for a mile split. Here again, a GPS reference would usually be good enough to get me within a few metres so I could search for faint paint remnants.

Even for this limited use I think recording GPS way points at reference points will now be a regular part of my measuring procedure. There are however, other possible applications in course measuring which will be considered in other articles.

Edited 1 April 2009: I have now added some new pages which can be reached from http://coursemeasurement.org.uk/GPS/index.htm. These include a simulation of measuring courses of polygonal shape using my GPS to measure the locations of the corners. I come to the conclusion that used in this way my GPS has about half the accuracy of a course measurer with his calibrated bike and Jones counter. ie The GPS would pretty much always be within 20 m on a 10k polygon course.
Edited 2 April 2009: I have now added a page showing the tracks from 11 rides around my 4.5k Loop course in Abingdon. These tracks are quite close to one another, generally within 5 m of the mean track position. There are occasional divergences up to 10 m which last for a few hundred meters. See Repeatbility of 11 GPS tracks of the Abingdon 4.5k loop
Edited 5 April 2009: I have completed my first draft of pages which give the results over 16 rides round the Abingdon 4.5 k loop. The ETREX H gives very good results only about a factor of two worse than is typically obtained by an experienced measurer with a Jones counter. The final page of the series is HERE

This message has been edited. Last edited by: Mike Sandford,


Mike Sandford -
Measurement Secretary South of England
UK Asssociation of Course Measurers
coursemeasurement.org.uk
email contact m.sandford at lineone dot net
 
Posts: 232 | Location: Abingdon, Oxfordshire | Registered: 24 October 2004Reply With QuoteReport This Post
Picture of Duane Russell
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Mike,

I have been using my Garmin Etrex HCx as a supplemental tool for about a year, and used another Etrex prior. I now post my GPS tracks with all of the maps of the courses I measure (see a sample at http://racemeasure.com/Races/C..._Colfax_Marathon.htm)

They are GPX files, so they should be importable into any GPS software.

An additional use for the GPS that I just started doing this year is to export the track from MapMyRun.com (I have race directors approve the course before I measure), and load it onto my GPS before I measure. I then can be sure I am on the correct route. This is especially helpful when measuring in an area that has lots of different trails, or where roads are close together. It saves me from taking a wrong turn. You can be writing a track file at the same time as you are following another track.

Mike, a word of caution. If you want to use the elevation profile from your track on the Garmin, don't "Save" your track on the GPS. If you do, you lose any intermediate point not needed. That is, if you have a straight path that goes over rolling hills for a mile, only the start and end point of that straight section will be saved all intermediate track points (and their elevations) will be discarded.

Let me know if you want more info on that, or if you want an Excel spreadsheet that parses the data so you can plot the elevation profile.

I completely agree with Mike that GPS units are a great tool for locating paint or washers. Makes sure that you are looking in the correct area.
 
Posts: 712 | Location: Denver, Colorado | Registered: 09 May 2006Reply With QuoteReport This Post
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Duane,

Thanks for the info. I had studied your website sometime ago before embarking on this study. In particular I had noted the large deviations in tracks which you reported on your website. I have now compared 11 rides with the GPS around my 4.5 k course. I find my tracks are more consistent than yours. Perhaps the GPS has improved. I am using an ETREX H with WAAS enabled and I am usually getting differential data from satellite 33.

I don't think I will try to do anything with elevation data just now. Course length interests me more.

When I started using a GPS just over a month ago under the guidance of Phil Holland, we had a very good demonstration of the value of GPS in alerting us to a short course. Runners complained the course was short by their GPSs and that too many people had unlikely PBs. A GPS on the arm of the average runner, more often than not gives a long course, so when there are many reports of short readings from runners' GPSs we need to sit up and take notice. Phil and I downloaded about 10 GPS tracks which had been posted on many websites including MotionBased which is an absolute goldmine of GPS downloads of runners' routes during races. After careful analysis of the tracks we concluded that the turn round appeared to be short of the correct position, so 3 of us went out and measured with our bikes. We confirmed the turn round was short. What is more we discovered faint paint marks at the correct turn round position. Phil has published a short write up


Mike Sandford -
Measurement Secretary South of England
UK Asssociation of Course Measurers
coursemeasurement.org.uk
email contact m.sandford at lineone dot net
 
Posts: 232 | Location: Abingdon, Oxfordshire | Registered: 24 October 2004Reply With QuoteReport This Post
Picture of Duane Russell
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The study of the short course was a good idea! I confirms that our descriptions need to be idiot-proof. And, that the markings should be updated annually (by each race director).

The tracks in the example on my Website were made while running across a bridge. The steel girders likely caused the GPS to become confused. The north/south disparity could have been caused by the football stadium across the street, also. Just confirms that nearby structures can cause improper results.
 
Posts: 712 | Location: Denver, Colorado | Registered: 09 May 2006Reply With QuoteReport This Post
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My ETREX H seems to work very well on the Abingdon loop which is subrban town/open country road. Mostly I am getting around 10 satellites and the accuracy indicated is 3 or 4 m. Sometimes but not always I get the EGNOS satellite and it switches to the DIFF mode, but I have not yet noticed any marked improvement.

I have now posted results from 16 rides. Perhaps I need to find a more challenging course to test it on.


Mike Sandford -
Measurement Secretary South of England
UK Asssociation of Course Measurers
coursemeasurement.org.uk
email contact m.sandford at lineone dot net
 
Posts: 232 | Location: Abingdon, Oxfordshire | Registered: 24 October 2004Reply With QuoteReport This Post
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