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Discussion Forum - Gear ! - GPS vs Course measuring


Author: Ian Koszalinski
Posted: Fri 29th Jun 2007, 1:49
Joined: 2004
Local Group: High Peak
dont want to be in a plane that flies under the trees tho
Author: John King
Posted: Sun 24th Jun 2007, 22:11
Joined: 2002
Planes fly using GPS so i guess there is not much room for error.

John
Posted: Thu 21st Jun 2007, 14:18
Joined: 2006
Read it again! I am saying that losing signal beneath trees could only reduce the distance measured - so why do most peoples GPS's read higher milage than the apparent event distance?
Author: Nick Ham
Posted: Thu 31st May 2007, 12:46
Joined: 1998
Local Group: South Manchester
Hi Gary,
Don't you mean the GPS recorded distance is always less than or equal to the apparent event distance? If not, it contradicts the argument that loss of signal results in assumed straight lines.
Posted: Thu 31st May 2007, 9:22
Joined: 2006
I have previously found that the distance recorded by my GPS has only ever been more than the apparent event distance. When people say about the accuracy of a GPS being affected by losing signal beneath trees it is only possible to lose distance on a GPS when out of signal and not gain distance. ie. if you were to lose signal beneath tree cover the GPS would then measure this as a straight line from where it lost signal to where it was restored despite all the twists and turns that you may have done.
Author: Philip Powell
Posted: Sun 20th May 2007, 8:47
Joined: 1981
Local Group: Northumbria
I'm sure Lambton and Everest were wrong.

We'd better do our own - more accurate - survey, Peter!
Posted: Sat 19th May 2007, 23:00
Joined: 1994
With all this talk of accuracy of GPS thingies, maybe you should all read John Keay's book 'The Great Arc'. It tells the amazing story of the longest measurement of the earth's surface ever attempted. It was 1600 miles from the southernmost point of India to the Himalayas. Begun in 1800, it took 50 years, cost an enormous number of lives, mainly to malaria, involved equations more complex than any in the pre computer age, and had measuring instruments weighing half a ton. Started by William Lambton, it was finished by Col. George Everest - who is well known for other reasons. Despite all the trials and tribulations, the accuracy they achieved is amazing to the extent that they were able to measure the height of Everest (until then unknown to westerners) to within a few feet of its true height - after 1600 miles from sea level. Whilst they were at it, they were able, with that great distance surveyed, able to accurately calculate the earth's diameter.
And some argue over 0.1 of a mile!
Author: Garfield & Helen Southall
Posted: Fri 18th May 2007, 22:17
Joined: 1991
Local Group: Merseystride
Foretrex 101 duly walked-in (will be run-in on Sunday !). Very nice piece of kit and remarkably easy to use and manipulate whilst on the hoof. Top marks. Nokia LD-3W was a bit shy and didn't communicate with phone at all. Ah well, back to Google. Garfield
Author: Garfield & Helen Southall
Posted: Tue 15th May 2007, 22:29
Joined: 1991
Local Group: Merseystride
Then again, the Parisians and NPL people only do very, very, very precise approximations.....

Rebecca's checkpoint chap is doomed to a life of disappointment, I'm afraid....!

(Helen)
Author: Garfield & Helen Southall
Posted: Tue 15th May 2007, 22:26
Joined: 1991
Local Group: Merseystride
"Is there such things as accurate measurements anyway???!!!!"

'Fraid so! They're based in Paris (the spiritual home of the metre....)
( http://www.bipm.org/en/home/ )

There's also a team at the NPL who do nothing but measure things - very carefully - for a living. ( http://www.npl.co.uk/measurement_solutions/mini_case_studies/environment.html ) They even run "metrology clubs" for the very keen (but don't tell Rebecca's checkpoint chap!)

(Helen)
Author: Garfield & Helen Southall
Posted: Tue 15th May 2007, 22:12
Joined: 1991
Local Group: Merseystride
Elton, the 101 does "elevation", which is a trigonometric calculation. I received my new Foretrex 101 this afternoon and splendid it looks too. I was disappointed to see no PC cable included. When I tracked one down I noted it finished in a serial plug! Which means buying a serial to USB cable to partner it. Very few new computers have serial sockets and certainly none of my laptops. Never mind, really looking forward to putting it through it's paces and comparing it to my new GPS bluetooth module which links to my Nokia Phone !!
Garfield
Author: Tony Willey
Posted: Tue 15th May 2007, 21:08
Joined: 1989
Local Group: Lakeland
My experience is that there is a difference of up to 2% between any two GPSs when measuring a walk distance. Cumulative height gain on my Etrex Summit is even more doubtful, I usually end a circular fell walk with at least 100 metres difference between recorded ascent and descent. It's pretty accurate at telling me where I am, how high I am and which direction I am going, provided I have calibrated it, and that's what is important.
I used to be a SCAAA road race course measurer. I used a road bike calibrated against a carefully measured straight of half a mile or so, and would find that course distances were consistent to within a foot or two. Once did a couple of laps of the Silverstone GP circuit on my bike to measure a 10k course. Not too practical for the Hundred, I suppose!
Author: Elton Ellis
Posted: Tue 15th May 2007, 18:38
Joined: 2006
Local Group: Surrey
Rebecca
<i>3/4m discrepency (not in our favour) at the first checkpoint</i> I noticed that one too. I've measured distances on the Thames Path using my GPSr and they compared very well with distsnces measured on my electronic Anquet maps (1:50K)

Short article on 101 accuracy here. Garmin checked their distance with a wheel on a track, hence no ruts.

http://db.tidbits.com/article/8190

Garfield <i>Barometric v GPS altitude being the new debate.</i> Didn't think the 101 did altitude? Unless it is by trig. I understand that any barometric altitude measurements need calibration and accuracy is variable. Certainly lots of questions/complaints/misunderstandings on GPS forums on altitude readings. Likewise GPSr compass readings. I always use my old aluminium Silva - powered by the Earth's magnetic field, so it doesn't run out of battery power (Doesn't use fractal theory either! ;) )

Ian <i>
personally i count my strides i find this a very accurate measure</i>
Ah well, I have an attention span of a two year old, so would lose count after 10 paces, distracted by a passing butterfly. Garfield would tell us if that is a function of Chaos Theory.

I measure distance by time: at 3 mph, a walker covers 100m per minute (roughly), so if the course description say turn left after 1100m, I check my watch at the start, then start looking for the turn after about ten minutes.
Author: Ian Koszalinski
Posted: Tue 15th May 2007, 12:23
Joined: 2004
Local Group: High Peak
personally i count my strides i find this a very accurate measure

A bloody long way

it's a method which is seldom wrong
Author: Garfield & Helen Southall
Posted: Mon 14th May 2007, 22:38
Joined: 1991
Local Group: Merseystride
Glad you mentioned the 101, Elton. As I write this my new Foretrex 101 is sitting in a Parcel Line van trundling up the motorway to Chester. It's what I do with my Suunto that now concerns me. Barometric v GPS altitude being the new debate. Garfield
Author: Rebecca Lawrence
Posted: Mon 14th May 2007, 21:53
Joined: 2003
Local Group: Marches
Too true! Interesting comments. I think some people do take it too seriously - at the end of the day I use my GPS mainly for speed so i can get depressed at how slow i am going after 30m or so. I am quite a robust, stiff upper lip type of woman but I have to admit to be quite upset by what happened at the weekend.

I was actually amazed at how good and accurate the GPS was on the Ridgeway and it agreed very much with the checkpoint mileage given on the walk - but I suppose there is minimal tree cover. It was also surprisingly very good on the oxon 40 considering the ammount of tree cover. The only time so far it didn't agree was the Dorset Giant where at the point I had to pull out at 41m, there was about 1m discrepency, and even 3/4m discrepency (not in our favour) at the first checkpoint but I'm not sure how that course was measured. I stand to be corrected but I believe the Oxon was measured using tracklog which seems to agree more readily with GPS?

At the end of the day it doesn't really matter, but is very interesting!

I work as an audiologist and we have free field speakers calibrated by sound level meters which are calibrated using a sound generator, but what exactly calibrates that and is that accurate? Is there such things as accurate measurements anyway???!!!!
Author: Elton Ellis
Posted: Mon 14th May 2007, 19:33
Joined: 2006
Local Group: Surrey
Author: Rebecca Lawrence
Posted: Sun 13th May 2007, 22:42
Joined: 2003
Local Group: Marches
I would be very impressed if he walked a 40m course with a wheel - I could barely do it without, but perhaps he did? And if he did walk it with a wheel, how did he get it over the ruts and could it have bounced off?!!!
Author: Ian Koszalinski
Posted: Sun 13th May 2007, 22:14
Joined: 2004
Local Group: High Peak
over 40mls if you walked the inside of the path and your partner walked the outside how much father would your partner have travelled?
compare it with the olympic 400m race and the size of the stagger between lanes and thats only on a 400m oval track, so if you see someone hogging the inside on the 100, it'll be me
i don't think there would be much difference taking a measure on a map with a wheel or using a gps, but perhaps he walked the course with a wheel.
Author: Garfield & Helen Southall
Posted: Sun 13th May 2007, 22:12
Joined: 1991
Local Group: Merseystride
According to fractal theory, the only true way would be to measure the ground with a tape rule! I use Tracklogs mapping software, which is based upon 1:25,000 maps and you can zoom the map and get very very close to the route. Together with very accurate contour counting, I've found this to the most accurate method and regularly coincides with "on the ground" measurements.

GPS readings lie in-between and illustrate the fractal theory perfectly. The measurement is taken of a straight line between two recorded locations. The further apart the locations, the more likely the measurement is innaccurate as it misses bends taken between the two readings - also, the faster you run the further the two points are apart !
I have my old GPS12 record the location every three minutes, so I'll do a bit of wiggling in that time, so the measurement won't be as accurate as Tracklogs. Then again, how many paths are marked perfectly on OS maps?
I tend to use my GPS when a) I'm lost and b) when I have a really tricky piece of navigation to do (i.e. Yockenthwaite Moor on the Fellsman). With the latter, I've noticed that the distance remaining to the target fluctuates.
Get yer tape measure out .... (but watch out for expansion due to heat)
Garfield
Author: Rebecca Lawrence
Posted: Sun 13th May 2007, 16:29
Joined: 2003
Local Group: Marches
Had a rather unpleasant experience at the weekend - I was wearing my Garmin and was asked at a checkpoint by a fellow runner what distance we were at. the checkpoint was meant to be at 19.5, and my GPS showed 19.4 which wasn't bad at all. This guy who was at the checkpoint overheard the conversation and just said, GPS are NOT ACCURATE in a very dictatorial way. Trying to be pleasant I said that obviously if ther is tree cover then yer he is right, although we have done measured 10k races where it has proven to be extremely accurate.

With this he said 'ITS NOT ACCURATE, I'M RIGHT AND YOUR WRONG' getting more and more irate. It turns out that he actually measured the course we were doing and seemed to resent people going about with their GPS saying it was wrong.

We tried to make the point that it would be very difficult to accurately measure a cross country course and how does he know his measurement is correct and the GPS is wrong?

I just wondered what people thought about this - is it possible to accurately measure a cross country course, and if so, how do you do it, and would it be any more accurate than the measurement from a GPS?

How accurate is a GPS compared to measuring? At least armed with this information I could give as good back!

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