Neal,
Yes that is the type of thing I am looking for but that Orion min-EQ sells for £65 including PP in UK. Also I would have to put it on a taller tripod, since one of the objectives is not to have kneel on the dirty ground.
At present I am looking out for second hand systems on ebay: tall tripod plus equatorial mount plus a telescope. I suppose I would throw away the telescope or try to sell it on ebay.
After 4 months I managed to buy an EQ1 Equatorial Telescope Mount with a sturdy tripod on ebay for £24. After a few minutes filing the sloping shoulders of the rectangular slot of telescope mounting point, the Bosch 250m laser range finder fitted snugly secured with rubber bands:
The tripod is much sturdier than the puny one shown in the background of the picture above and which I initially bought for the range finder. The EQ1 telescope mount has proper slow motion drives with hand knobs and is far better than the adjustments on any cheap alt-azimuth mount with friction bearings and a tilt/pan handle to adjust. One adjusts the EQ1 mount for the equator so that the right ascension axis points horizontally in the following configuration:
The right ascension drive is extremely precise. The declination drive does slightly perturb the RA drive setting due to a tiny amount of slop in a bearing, but this does not prevent one doing hand adjustments using the slow motion knobs at the minute of arc level.
The laser beam diverges with an angle of 2 minutes of arc, and alignment with a small reflector target is easy with this mounting arrangement.
My first trial was with a target consisting of a pair of vehicle retro reflectors taped to a post 248 metres away over rough ground. The retro-reflecting surface measured 10 cm by 8cm which was smaller than the laser spot size at this distance - about 15 cm diameter. Alignment is done using a small sighting viewer on the rangefinder, and when the laser spot hits the retro-reflector a bright red spot is visible in the viewer and also with the unaided eye.
The readings I got in this initial trial fluctuated by about 1 or 2 cm - completely adequate reading precision for laying out a calibration course. Not that anyone would have ever laid out a 248 m calibration course with a steel tape over this rough, bumpy ground.
When I tried increasing the range to about 300m, I was not able to get a reading, so maybe the instrument is programmed to reject readings above 250m which is the stated maximum range, or maybe I had alignment problems since I could not see the reflected laser light at the increased range. More tests at ranges greater than 250 m are needed. However, even if range is limited to 250m it will still be very suitable for measuring short calibration courses in a single hop, or longer ones with two measurements.
I have started to evaluate the reproducibility using an 83m distance between a nail, and a small retorreflector of 4 sqcm area on a fence. I can position the laser ranger exactly, within 1 mm over the nail using a plumb bob:
Here is an magnified photo of the laser spot on the retroreflector. I think it appears white rather than red due saturation of the digital camera CCD. The scale is such that the small white patch is 5 cm above the edge of the retroreflector:
At 18 C today the reading came out consistently within 1 mm of 83.259 metres. I shall repeat this set up at other temperatures during the next few days. If reproducibility is confirmed, I will then steel tape a 249m cal course and then measure it with the laser.
The tripod is much sturdier than the puny one shown in the background of the picture above and which I initially bought for the range finder. The EQ1 telescope mount has proper slow motion drives with hand knobs and is far better than the adjustments on any cheap alt-azimuth mount with friction bearings and a tilt/pan handle to adjust. One adjusts the EQ1 mount for the equator so that the right ascension axis points horizontally in the following configuration:
The right ascension drive is extremely precise. The declination drive does slightly perturb the RA drive setting due to a tiny amount of slop in a bearing, but this does not prevent one doing hand adjustments using the slow motion knobs at the minute of arc level.
The laser beam diverges with an angle of 2 minutes of arc, and alignment with a small reflector target is easy with this mounting arrangement.
My first trial was with a target consisting of a pair of vehicle retro reflectors taped to a post 248 metres away over rough ground. The retro-reflecting surface measured 10 cm by 8cm which was smaller than the laser spot size at this distance - about 15 cm diameter. Alignment is done using a small sighting viewer on the rangefinder, and when the laser spot hits the retro-reflector a bright red spot is visible in the viewer and also with the unaided eye.
The readings I got in this initial trial fluctuated by about 1 or 2 cm - completely adequate reading precision for laying out a calibration course. Not that anyone would have ever laid out a 248 m calibration course with a steel tape over this rough, bumpy ground.
When I tried increasing the range to about 300m, I was not able to get a reading, so maybe the instrument is programmed to reject readings above 250m which is the stated maximum range, or maybe I had alignment problems since I could not see the reflected laser light at the increased range. More tests at ranges greater than 250 m are needed. However, even if range is limited to 250m it will still be very suitable for measuring short calibration courses in a single hop, or longer ones with two measurements.
I have started to evaluate the reproducibility using an 83m distance between a nail, and a small retorreflector of 4 sqcm area on a fence. I can position the laser ranger exactly, within 1 mm over the nail using a plumb bob:
Here is an magnified photo of the laser spot on the retroreflector. I think it appears white rather than red due saturation of the digital camera CCD. The scale is such that the small white patch is 5 cm above the edge of the retroreflector:
At 18 C today the reading came out consistently within 1 mm of 83.259 metres. I shall repeat this set up at other temperatures during the next few days. If reproducibility is confirmed, I will then steel tape a 249m cal course and then measure it with the laser.
Very impressive Mike. It gives me confidence that when I am able to test out my telescope mount I'll be able to hit my targets as well. But here in Michigan it is currently much closer to 18 degrees F than it is to C.
I have taken the following series of readings to check for the reproducibility of the Bosch distance meter:
metres temp C date time
83.259 18 24/03/11 03:30 PM
83.256 17 24/03/11 05:00 PM
83.256 8 25/03/11 08:30 AM
83.259 8 28/03/11 08:40 AM
83.256 8 28/03/11 08:55 AM
83.258 10 28/03/11 09:35 AM
83.256 11.5 28/03/11 10:30 AM
83.259 11.5 28/03/11 11:30 AM
83.256 16 28/03/11 04:00 PM
The variation includes both the variation of the instrument, stated to be the sum of +/- 1 mm plus +/- 0.005% of the length, and also the accuracy of positioning the plumb bob over the nail which may be of the order +/- 1 mm.
There is no indication of a significant thermal coefficient in these readings.
These results are rather similar to the reproducibility of a 50m steel tape used in two lengths over 100m, but without the hassle of temp corrections, end corrections and tension measurements as well as scrabbling around on the ground, all of which are needed for steel taping.
The next test will be comparison of measurement of a 250m cal course with the Bosch laser distance meter and with a steel tape.
Here is how I attached the plumb bob to the back of the laser meter:
metres temp C date time
83.259 18 24/03/11 03:30 PM
83.256 17 24/03/11 05:00 PM
83.256 8 25/03/11 08:30 AM
83.259 8 28/03/11 08:40 AM
83.256 8 28/03/11 08:55 AM
83.258 10 28/03/11 09:35 AM
83.256 11.5 28/03/11 10:30 AM
83.259 11.5 28/03/11 11:30 AM
83.256 16 28/03/11 04:00 PM
The variation includes both the variation of the instrument, stated to be the sum of +/- 1 mm plus +/- 0.005% of the length, and also the accuracy of positioning the plumb bob over the nail which may be of the order +/- 1 mm.
There is no indication of a significant thermal coefficient in these readings.
These results are rather similar to the reproducibility of a 50m steel tape used in two lengths over 100m, but without the hassle of temp corrections, end corrections and tension measurements as well as scrabbling around on the ground, all of which are needed for steel taping.
The next test will be comparison of measurement of a 250m cal course with the Bosch laser distance meter and with a steel tape.
Here is how I attached the plumb bob to the back of the laser meter:
I'm impressed at the experimental technique. Well done, Mike.
I hope a stranger does not send me an application for certification using unsupported measurement data employing a laser gun. I am not sure I'd know how to find it credible. I'd expect to be asking lots and lots of questions.
I'd certainly have no problem with an application from Mike, but few would document their methodology so well.
I hope a stranger does not send me an application for certification using unsupported measurement data employing a laser gun. I am not sure I'd know how to find it credible. I'd expect to be asking lots and lots of questions.
I'd certainly have no problem with an application from Mike, but few would document their methodology so well.
I've received a couple cal course applications where the measurement was by Total Station. The only thing in the documentation that really convinced me that the measurement was accurate was the "professional surveyor" by the measurer's name.
I think the only way we could accept this type of measurement is if the measurer has proven ahead of time that he is capable of accurately measuring a course with this technique. That's why I suggested in another thread that we allow EDMs to be used for the second measurement of a cal course. Once a measurer has shown 2 or 3 times that his EDM measurement of cal courses is very similar to his taped measurement of cal courses, he could use only his EDM in the future.
Actually, a better practice would be to allow EDMs for the first measurement, with taping required for the second. Best for them to test the accuracy of their method without knowing what the right answer is.
I think the only way we could accept this type of measurement is if the measurer has proven ahead of time that he is capable of accurately measuring a course with this technique. That's why I suggested in another thread that we allow EDMs to be used for the second measurement of a cal course. Once a measurer has shown 2 or 3 times that his EDM measurement of cal courses is very similar to his taped measurement of cal courses, he could use only his EDM in the future.
Actually, a better practice would be to allow EDMs for the first measurement, with taping required for the second. Best for them to test the accuracy of their method without knowing what the right answer is.
Pete,
Thanks for your confidence in my work.
I know you are cautious about un-proven techniques, and especially complicated techniques which have plenty of scope for operator mistakes or misunderstandings. Please treat this as work in progress. I am not going to over advocate its use in comparison with the basic tape measuring method. First of all, I have to demonstrate beyond doubt that the device can give reliable results. Then if it proves reliable, I expect to use it myself to avoid scrabbling around on the ground and to layout calibration courses much more quickly than I can with solo taping. If I can layout a calibration course quickly, I will be more likely to do so on the site of a distant course measurement, rather than relying on using my home calibration course and sometimes suffering large temperature changes when the journey is lengthy.
I have more progress to report today:
I located a quiet piece of road about 330 m long suitable for a calibration course. I will call it Allotment Road, Abingdon. The road has a locked vehicle gate which is occasionally opened by people visiting their allotments. There are occasional pedestrians. It is very suitable for firing a class I laser along the road without dazzling road users.
I steel taped a 249.90575 metre calibration course using the standard solo method. This took about 1hr and 40 minutes. I am afraid I am not as fast as you Pete when you quote 30 mins to measure a cal course. That time might be just possible if I had a team of 2 helpers to hook/unhook the end and to tension the tape, and if I was more rapid when getting on the ground to take the reading - but I think anyone would be hard pressed do two solo measurements in 30 mins.
Next I set up the retro reflector at the far end and the laser range finder at the end nearest my car. Took a series of measurements which varied by less than +/- 2 mm, and then packed up the equipment this took 25 minutes - about 4 times faster than my solo steel taping.
Here is a view down the course with the laser spot visible to the eye (bright red) and the camera (small white spot).
The laser gave 249.942 m. Two corrections are required.
+ 0.016 (plumb bob offset)
- 0.003 (slope correction. Laser 1.25m higher than reflector - I have assumed road is horizontal - this needs testing by means of a similar measurement from the far end)
Corrected laser measurement for distance between course end nails: 249.955 m
So the laser gives a result 49 mm longer than the steel tape. This is about half a Jones count, or 0.02%.
My tape is a class II tape specified accurate to 0.02%.
The Bosch GLM250VF is stated to be accurate to 0.005% under favourable conditions.
My preliminary conclusion is that since the difference of the two measurements lies within the sum of the accuracy specs ie 0.025%, I have confirmation that the method works within the advertised calibration accuracy of the instruments.
Much more work to do, but on returning home I found that a job lot of 60 retroreflectors measuring 12 cm x 8 cm had arrived:
I can now set up many targets!
I decided to make my home laser test range better, extending it from 83m to 93m. With the target mounted higher in the tree the beam clears any parked cars, so it should now be usable at any time. The plan would be to test the laser on this short range before and after going to make a measurement elsewhere. It will be a lot quicker than deploying the equipment on the Allotment Road calibration course every time. So the first aim will be to show consistent measurements at Allotment road, and for each occasion to take checks on the 93m laser test range.
Thanks for your confidence in my work.
I know you are cautious about un-proven techniques, and especially complicated techniques which have plenty of scope for operator mistakes or misunderstandings. Please treat this as work in progress. I am not going to over advocate its use in comparison with the basic tape measuring method. First of all, I have to demonstrate beyond doubt that the device can give reliable results. Then if it proves reliable, I expect to use it myself to avoid scrabbling around on the ground and to layout calibration courses much more quickly than I can with solo taping. If I can layout a calibration course quickly, I will be more likely to do so on the site of a distant course measurement, rather than relying on using my home calibration course and sometimes suffering large temperature changes when the journey is lengthy.
I have more progress to report today:
I located a quiet piece of road about 330 m long suitable for a calibration course. I will call it Allotment Road, Abingdon. The road has a locked vehicle gate which is occasionally opened by people visiting their allotments. There are occasional pedestrians. It is very suitable for firing a class I laser along the road without dazzling road users.
I steel taped a 249.90575 metre calibration course using the standard solo method. This took about 1hr and 40 minutes. I am afraid I am not as fast as you Pete when you quote 30 mins to measure a cal course. That time might be just possible if I had a team of 2 helpers to hook/unhook the end and to tension the tape, and if I was more rapid when getting on the ground to take the reading - but I think anyone would be hard pressed do two solo measurements in 30 mins.
Next I set up the retro reflector at the far end and the laser range finder at the end nearest my car. Took a series of measurements which varied by less than +/- 2 mm, and then packed up the equipment this took 25 minutes - about 4 times faster than my solo steel taping.
Here is a view down the course with the laser spot visible to the eye (bright red) and the camera (small white spot).
The laser gave 249.942 m. Two corrections are required.
+ 0.016 (plumb bob offset)
- 0.003 (slope correction. Laser 1.25m higher than reflector - I have assumed road is horizontal - this needs testing by means of a similar measurement from the far end)
Corrected laser measurement for distance between course end nails: 249.955 m
So the laser gives a result 49 mm longer than the steel tape. This is about half a Jones count, or 0.02%.
My tape is a class II tape specified accurate to 0.02%.
The Bosch GLM250VF is stated to be accurate to 0.005% under favourable conditions.
My preliminary conclusion is that since the difference of the two measurements lies within the sum of the accuracy specs ie 0.025%, I have confirmation that the method works within the advertised calibration accuracy of the instruments.
Much more work to do, but on returning home I found that a job lot of 60 retroreflectors measuring 12 cm x 8 cm had arrived:
I can now set up many targets!
I decided to make my home laser test range better, extending it from 83m to 93m. With the target mounted higher in the tree the beam clears any parked cars, so it should now be usable at any time. The plan would be to test the laser on this short range before and after going to make a measurement elsewhere. It will be a lot quicker than deploying the equipment on the Allotment Road calibration course every time. So the first aim will be to show consistent measurements at Allotment road, and for each occasion to take checks on the 93m laser test range.
I have used the Leica Disto D5 only to double check the calibration course measured by steel tape, expecially in raining condition. I found it quite difficult to hold it steady even mounted on a tripod when trying to press the button.
FUNG Wang-tak
FUNG Wang-tak
Mike wondered how I can lay out 300 m in a half hour, single-handed.
The trick is to traverse the cal course only once, not twice. I use a tape that is marked with metric scale on one edge and decimal feet on the other edge. By reading each, I get two measurements in one pull, with the readings not expected to agree. The same procedure could employ two tapes hooked over the same nail. By having the tapes use different scales, this reduces the chance of making the same reading mistake on both measurements.
If I should make a reading error I will not find it until I work out the measured lengths upon completion of the taping. This is the reason I don't pull out the nails until I have checked the work.
When one measures in both directions, the memory of the previous reading is gone, and it is unlikely that the same reading error will occur.
The trick is to traverse the cal course only once, not twice. I use a tape that is marked with metric scale on one edge and decimal feet on the other edge. By reading each, I get two measurements in one pull, with the readings not expected to agree. The same procedure could employ two tapes hooked over the same nail. By having the tapes use different scales, this reduces the chance of making the same reading mistake on both measurements.
If I should make a reading error I will not find it until I work out the measured lengths upon completion of the taping. This is the reason I don't pull out the nails until I have checked the work.
When one measures in both directions, the memory of the previous reading is gone, and it is unlikely that the same reading error will occur.
quote:Originally posted by Pete Riegel:
The trick is to traverse the cal course only once, not twice. I use a tape that is marked with metric scale on one edge and decimal feet on the other edge. By reading each, I get two measurements in one pull
Now that is a really good idea. My tape does have both scales, but not everyone will have chosen that type of tape marking.
So is this accepted as two proper measurements for IAAF/AIMS international measurement purposes?
If so, would a modification of the principle by taking a camera shot of the reading also be a suitable way recording results without the possibility of error? (One hand holding the camera, the other hand tensioning the tape with a spring scale.)
Mike,
I don't know whether it is specifically accepted or not. As far as I can see, pulling the tape twice and getting two readings would constitute two measurements. But, because of a possible tendency to read the same thing wrong, when doing it a few seconds apart, I felt that using the metric/imperial scales would be a good check. The mind does not instantly convert the distance, so there is no lingering knowledge of a "correct" reading - thus the two measurements are independent.
At least that's the way I see it.
Your camera idea is a good check. I don't use a spring scale.
I don't know whether it is specifically accepted or not. As far as I can see, pulling the tape twice and getting two readings would constitute two measurements. But, because of a possible tendency to read the same thing wrong, when doing it a few seconds apart, I felt that using the metric/imperial scales would be a good check. The mind does not instantly convert the distance, so there is no lingering knowledge of a "correct" reading - thus the two measurements are independent.
At least that's the way I see it.
Your camera idea is a good check. I don't use a spring scale.
My response would be that, taking two readings with one pull does not ascertain that the proper amount of tension has been placed on the tape. Two separate measurement actions would go further to assure that the proper tension was applied. Similarly, if, by chance, the beginning end of the tape (opposite the end being read) happened to slip a quarter-inch (or 1 cm) due to pavement giving-way, you wouldn't have a second measurement to cross-check against.
Simply reading two indicators with one pull only verifies one aspect of the measurement. If we put two counters on a bike, and read them both at each split (and overall length), we would only verify counts for the path ridden; it does not verify that the ridden path was the SPR. I think pulling once and reading two different scales (metric and SAE) is not a good indicator that all the variables have been double-checked.
Simply reading two indicators with one pull only verifies one aspect of the measurement. If we put two counters on a bike, and read them both at each split (and overall length), we would only verify counts for the path ridden; it does not verify that the ridden path was the SPR. I think pulling once and reading two different scales (metric and SAE) is not a good indicator that all the variables have been double-checked.
Two pulls are used. The tape is pulled and read. I put down the end and write down the reading. The I do a new pull, and a new reading, using a different scale.
If a nail should pull loose (which is unlikely) it will show up when the two readings are compared. I believe I'd feel it give when I pulled the tape. In any case, is it really likely that the nail would move just a little? It it was that loose it would simply pull out of the pavement. This I'd be sure to notice.
As for tension, pulling by feel is good enough.
Use of the two different scales makes this method robust. When using only one scale there is a tendency to repeat wrong readings.
I believe the solo method, as I've done it, fulfills the requirement for two independent measurements.
See the example below for actual data.
Solo Example
If a nail should pull loose (which is unlikely) it will show up when the two readings are compared. I believe I'd feel it give when I pulled the tape. In any case, is it really likely that the nail would move just a little? It it was that loose it would simply pull out of the pavement. This I'd be sure to notice.
As for tension, pulling by feel is good enough.
Use of the two different scales makes this method robust. When using only one scale there is a tendency to repeat wrong readings.
I believe the solo method, as I've done it, fulfills the requirement for two independent measurements.
See the example below for actual data.
Solo Example
I have come up with a new mounting arrangement for my Bosch GLM250 VF. I removed the tripod legs from my equatorial mount, and attached it directly on a piece of wooden floor board which has been shaped to wedge securely on my bicycle rack so that the bike, propped on its kick stand, provides a stable mounting, with the eyepiece at a convenient 105 cm above the ground.
With this arrangement it is easy to jump on the bike and ride it along a calibration course longer than 250 metres in order to very quickly obtain readings on different sections. Even small adjustments of position become very much quicker than moving a tripod. I just wheel the bike along, put down the kick stand aim and take a reading.
For transport to and from a calibration course, the mount packs away in my pannier bag, and my retro reflector target is clipped onto the rack:
I have not timed myself yet, but I should think that on arrival at an already nailed calibration course I can now set up, measure, and pack up in between 5 and 10 minutes. I certainly beats my measured speed of solo taping: 1 hour 40 minutes reported earlier in this thread. Also at no point do I have to kneel on the ground; I just bend slightly to place the box carrying the retro-reflectors over one nail and to check the alignment of the plumb bob on the nail at the other end.
With this arrangement it is easy to jump on the bike and ride it along a calibration course longer than 250 metres in order to very quickly obtain readings on different sections. Even small adjustments of position become very much quicker than moving a tripod. I just wheel the bike along, put down the kick stand aim and take a reading.
For transport to and from a calibration course, the mount packs away in my pannier bag, and my retro reflector target is clipped onto the rack:
I have not timed myself yet, but I should think that on arrival at an already nailed calibration course I can now set up, measure, and pack up in between 5 and 10 minutes. I certainly beats my measured speed of solo taping: 1 hour 40 minutes reported earlier in this thread. Also at no point do I have to kneel on the ground; I just bend slightly to place the box carrying the retro-reflectors over one nail and to check the alignment of the plumb bob on the nail at the other end.
Just a year ago in the previous post I showed my set up for using my Bosch Laser Rangefinder on my bike. It worked well. But could I rely on the results? An opportunity to test it against a calibrated tape came up last June.
When David Katz was preparing to come to London to oversee for the IAAF the official measurement of the London Olympic marathon he purchased a new steel tape and had its calibration checked at the NIST in Maryland (see his post here)
Realising this would be an excellent opportunity to check my laser rangefinder method against a calibrated tape, I took my Bosch Laser and the setup for my bike with me when I went to join David and Hugh Jones for the marathon measurement on 13 June 2012. David and Hugh had measured the calibration course in The Mall before I arrived. After we had finished our measurement ride, I checked the calibration course length with my laser. At first the results were in fair agreement.
While at The Mall for the measurement I was told by David and Hugh that the result from their steel tape measurement was 328.57 m
I went home and worked applied Pythagoras to my raw laser readings to correct for the sloping beam and obtained 328.621 m.
LASER - TAPE = 51 mm 0.015%
This difference was slightly more than the 0.005% to 0.013% error that was claimed by Bosch for the laser, but not by so much to be of concern or cause too much worry about the calibration course length.
When I sent my numbers to Hugh he reported that they had first used the NIST correction for the whole 100m tape rather than the 60m tape lengths which they had used. The corrected NIST calibration steel tape result should be 328.56m.
My initial laser ranger result was 328.621 m.
The difference was getting slightly larger: now LASER - TAPE = 61 mm 0.019%
I asked questions about the steel tape tension and temperature and the correct NIST calibration to use for the Steel Tape.
I was told the Steel Tape was pulled at an estimated 10 lbs. However it was clear from what David told me on the phone after he returned to New York, that the NIST calibration figures which had been so far been applied were for 10 kg tension.
As a result David changed The Mall Calibration Course steel tape result to 328.517 m.
My initial laser ranger result remained at 328.621 m.
The difference was now much worse. LASER - TAPE = 104 mm 0.032%
At this point I became worried about my laser measurement methodology, so I performed a new calibration course layout at home, reproducing the method I had used on The Mall and I photographed every stage. It is written up here. I worked out some further very small corrections additional to the simple correction for beam slope which I had initially used. I then applied these corrections to the laser result from The Mall. The overall effect was to increase my Mall result by 10 mm. This was principally due to my previous omission of a 12 mm misalignment of the plumb bob with the marker on the road when I was measuring the East end of The Mall cal course. My son Geoffrey, assisting me, had clearly recorded this in my notebook. When we lined up the bike there had been a small offset which we measured and recorded rather than fiddle again with the bike position. Unfortunately the overall effect was to add 10 mm to the laser result and so increase the the difference between tape and laser.
The Mall laser ranger result was initially 328.621 m, now was changed to 326.631 m. Latest steel tape calculation is 328.517 m.
LASER - TAPE = 114 mm 0.034%
At 0.034% the discrepancy was much more then could be explained from the laser manufacturer's figure for error. The effect on the marathon if we were to use the laser result for The Mall calibration would be to reduce the length we had calculated from our measurement rides by 14 m, which was still quite a bit less than the 42 m SCPF, so it would probably not render the marathon short.
However an unexplained discrepancy of this magnitude would be a worry for me that would prevent me from trusting the laser method in future.
Fortunately the reason became clear early in July when David sent his calculation of the steel tape corrections together with his draft measurement results to Pete Riegel for checking. Pete saw that the tension correction had been applied in the wrong direction.
Mall Calibration Course steel tape result was changed to 328.597 m. Final laser ranger result 328.631 m.
LASER - TAPE = 34 mm 0.010%
The specification for the laser, a Bosch Professional GLM 250VF, is as follows:
In unfavourable conditions (e.g. at intense sunlight or an insufficiently reflecting surface), the maximum deviation is ±20 mm per 150 m. In favourable conditions, a deviation influence of ±0.05 mm/m must be taken into account.
This spec is 0.013% in unfavourable conditions and 0.005% in favourable conditions. (There was low sun shining almost perpendicularly on to the target reflector during when I used the laser on The Mall, so it is reasonable to assume that the conditions were somewhat unfavourable.)
This was now a very satisfactory agreement with and NIST calibrated tape. Taken in conjunction with previous comparisons between the laser and my own class 2 steel tape which itself had in the past been compared with several other tapes, I now conclude that the soundness of my laser rangefinder measurement method has been demonstrated beyond all reasonable doubt - at least for my my particular instrument, the Bosch Professional GLM 250VF.
As far as the The Mall Calibration Course we all agreed to take the final steel tape value of 328.597 m as derived here for the calculation of the marathon course.
When David Katz was preparing to come to London to oversee for the IAAF the official measurement of the London Olympic marathon he purchased a new steel tape and had its calibration checked at the NIST in Maryland (see his post here)
Realising this would be an excellent opportunity to check my laser rangefinder method against a calibrated tape, I took my Bosch Laser and the setup for my bike with me when I went to join David and Hugh Jones for the marathon measurement on 13 June 2012. David and Hugh had measured the calibration course in The Mall before I arrived. After we had finished our measurement ride, I checked the calibration course length with my laser. At first the results were in fair agreement.
While at The Mall for the measurement I was told by David and Hugh that the result from their steel tape measurement was 328.57 m
I went home and worked applied Pythagoras to my raw laser readings to correct for the sloping beam and obtained 328.621 m.
LASER - TAPE = 51 mm 0.015%
This difference was slightly more than the 0.005% to 0.013% error that was claimed by Bosch for the laser, but not by so much to be of concern or cause too much worry about the calibration course length.
When I sent my numbers to Hugh he reported that they had first used the NIST correction for the whole 100m tape rather than the 60m tape lengths which they had used. The corrected NIST calibration steel tape result should be 328.56m.
My initial laser ranger result was 328.621 m.
The difference was getting slightly larger: now LASER - TAPE = 61 mm 0.019%
I asked questions about the steel tape tension and temperature and the correct NIST calibration to use for the Steel Tape.
I was told the Steel Tape was pulled at an estimated 10 lbs. However it was clear from what David told me on the phone after he returned to New York, that the NIST calibration figures which had been so far been applied were for 10 kg tension.
As a result David changed The Mall Calibration Course steel tape result to 328.517 m.
My initial laser ranger result remained at 328.621 m.
The difference was now much worse. LASER - TAPE = 104 mm 0.032%
At this point I became worried about my laser measurement methodology, so I performed a new calibration course layout at home, reproducing the method I had used on The Mall and I photographed every stage. It is written up here. I worked out some further very small corrections additional to the simple correction for beam slope which I had initially used. I then applied these corrections to the laser result from The Mall. The overall effect was to increase my Mall result by 10 mm. This was principally due to my previous omission of a 12 mm misalignment of the plumb bob with the marker on the road when I was measuring the East end of The Mall cal course. My son Geoffrey, assisting me, had clearly recorded this in my notebook. When we lined up the bike there had been a small offset which we measured and recorded rather than fiddle again with the bike position. Unfortunately the overall effect was to add 10 mm to the laser result and so increase the the difference between tape and laser.
The Mall laser ranger result was initially 328.621 m, now was changed to 326.631 m. Latest steel tape calculation is 328.517 m.
LASER - TAPE = 114 mm 0.034%
At 0.034% the discrepancy was much more then could be explained from the laser manufacturer's figure for error. The effect on the marathon if we were to use the laser result for The Mall calibration would be to reduce the length we had calculated from our measurement rides by 14 m, which was still quite a bit less than the 42 m SCPF, so it would probably not render the marathon short.
However an unexplained discrepancy of this magnitude would be a worry for me that would prevent me from trusting the laser method in future.
Fortunately the reason became clear early in July when David sent his calculation of the steel tape corrections together with his draft measurement results to Pete Riegel for checking. Pete saw that the tension correction had been applied in the wrong direction.
Mall Calibration Course steel tape result was changed to 328.597 m. Final laser ranger result 328.631 m.
LASER - TAPE = 34 mm 0.010%
The specification for the laser, a Bosch Professional GLM 250VF, is as follows:
In unfavourable conditions (e.g. at intense sunlight or an insufficiently reflecting surface), the maximum deviation is ±20 mm per 150 m. In favourable conditions, a deviation influence of ±0.05 mm/m must be taken into account.
This spec is 0.013% in unfavourable conditions and 0.005% in favourable conditions. (There was low sun shining almost perpendicularly on to the target reflector during when I used the laser on The Mall, so it is reasonable to assume that the conditions were somewhat unfavourable.)
This was now a very satisfactory agreement with and NIST calibrated tape. Taken in conjunction with previous comparisons between the laser and my own class 2 steel tape which itself had in the past been compared with several other tapes, I now conclude that the soundness of my laser rangefinder measurement method has been demonstrated beyond all reasonable doubt - at least for my my particular instrument, the Bosch Professional GLM 250VF.
As far as the The Mall Calibration Course we all agreed to take the final steel tape value of 328.597 m as derived here for the calculation of the marathon course.
One might want to consider paying a land surveyor in the local area to install a straight baseline. Typical cost for a single party chief for less than an hr is probably less than $125. You just want to make sure he shoots the line at least 8 times and has broken his setup at least twice. They can stop on the way to another job and take care of your needs. Money well spent on experienced personnel.
About 15 years ago I laid out a 1/2 mile cert course on the road outside my house.
The road is dead straight, level and just over 1/2 mile long. I decided to do a 1/2 mile cert course.
The idea being that any error in a cert course would be multiplied as one scaled up to the race distance. If a 1/2 mile cert course was as much as 6" out that would only end up with a three feet error at 5K.
Additionally riding that distance on the cert course would be very similar to riding on the race course. One rides differently when looking down than up into the distance.
And any fractional error from the discreet digits on a Jones counter would be negligible on that distance.
For years I have wanted to validate my 1/2 mile cert course. However redoing the two steel tape measurements by the same method would only confirm any error in method or any error in the length of my tape.
Over the years asked various survivors working on my street if they would help.
I have had various answers from my equipment cant shoot that distance, to hire us and book a job, to sod off.
However today two very nice gents were surveying for a house sale a few houses away. I can spot a shiny new high end survey rig, and knew this was good crew to ask. They were happy to help.
After confirming their equipment would have no problem with 1/2 a mile, I went out by bicycle and placed cones over the marks.
Then lent the bike to his man who biked out to hold the reflector pole. (Accurate pole with bubble level). This was shot from their high end TDR system. Not a GPS based system, so no GPS errors.
I was flabbergasted when they told me the result.
With no prompting or suggestion from me on what I expected, they reported 2,640.17 feet.
It took three people the best part of a day to do the orignal measurements. Took a good tape loaded with a spring gauge, sturdy nails at each point, and a table of corrections for the error or correction at each nail. 2 inches off in 1/2 mile.
Will spend the rest of the day feeling rather validated.
The road is dead straight, level and just over 1/2 mile long. I decided to do a 1/2 mile cert course.
The idea being that any error in a cert course would be multiplied as one scaled up to the race distance. If a 1/2 mile cert course was as much as 6" out that would only end up with a three feet error at 5K.
Additionally riding that distance on the cert course would be very similar to riding on the race course. One rides differently when looking down than up into the distance.
And any fractional error from the discreet digits on a Jones counter would be negligible on that distance.
For years I have wanted to validate my 1/2 mile cert course. However redoing the two steel tape measurements by the same method would only confirm any error in method or any error in the length of my tape.
Over the years asked various survivors working on my street if they would help.
I have had various answers from my equipment cant shoot that distance, to hire us and book a job, to sod off.
However today two very nice gents were surveying for a house sale a few houses away. I can spot a shiny new high end survey rig, and knew this was good crew to ask. They were happy to help.
After confirming their equipment would have no problem with 1/2 a mile, I went out by bicycle and placed cones over the marks.
Then lent the bike to his man who biked out to hold the reflector pole. (Accurate pole with bubble level). This was shot from their high end TDR system. Not a GPS based system, so no GPS errors.
I was flabbergasted when they told me the result.
With no prompting or suggestion from me on what I expected, they reported 2,640.17 feet.
It took three people the best part of a day to do the orignal measurements. Took a good tape loaded with a spring gauge, sturdy nails at each point, and a table of corrections for the error or correction at each nail. 2 inches off in 1/2 mile.
Will spend the rest of the day feeling rather validated.
Great story, James. Thanks for sharing. All us technocrats here will be quick to point out that the 2640.17 value is in FEET, hence, the measured error is not 1/4 inch but 2.04 inches. Still, that's not bad for the long course! There's nothing like a good, certified, steel tape for engendering confident accuracy. Nice work.
JJ
JJ
You are right, 2 inches off. My mistake. In a hurry and not thinking straight. Will edit.