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.
