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The North side of Park Road Abingdon in normally clear of parked vehicles, since it is a private road with no parking allowed for the public. Two way riding is possible at quiet times of the day. When there are traffic movements at busier times it may be necessary to abort East to West rides and repeat them later when the traffic has cleared.
The road surface is smooth and the tarmac is in very good condition.
Between 0505 and 0520 on 23 June with a temperature of 11C I carried out the measurement using my Bosch GLM 250VF Professional laser range finder.
The specification of the range finder includes the statement:
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.
I consider the conditions were favourable, and with the use of a good retro-reflector I would expect a maximum error of 0.05* 286 mm = 14mm.
Since the length of the course was greater than the 250m maximum range, the measurement was carried out in two steps.
The reflector was placed at the west end and the bicycle carrying the laser was ridden just less than 250m along the course.
A temporary red tape was placed on the road to mark the position of the plumb bob suspended from the laser. The plumb bob position was 2mm East of the West edge of the tape, which was to be used as the reference therefore 2mm would need subtracting from this measurement. This close up of the road surface shows how smooth the finish is.
The 3 readings of the western section all gave 246.336 metres.
Next the reflector was moved to the red tape
For the 40m which remained the laser removed from the mount on the back of the bike and was placed directly on the road. This was done to reduce the correction for slope which would be quite large over 40m. Since the laser is about 96cm above the reflector spot this would have given a slope addition of 11mm. Over the short distance it was possible to aim the laser by means of small hand adjustments until the reflected spot was visible on the target.
Here are the 4 readings obtained at the west end. The first reading was disregarded since the laser was moved when the button was pressed. The average good reading of 40.392m was used.
The above picture also shows the point of attachment of the plumb bob behind on the extension behind the rear face of the laser. The rear face of the laser is the default reference point for the readings and this setting can be seen indicated in the small icons on the lower left corner of the laser LCD screen near the B of Bosch. This provides positive confirmation that the intended reference point was in fact being used.
Separate measurements with a ruler of the distance of the plumb bob cord from the rear face reference gave a value of 16mm which needs to be taken into account when the plumb bob is used.
| Laser Reading | slope correction* |
PLUMB TO LASER REF |
PLUMB TO ROAD MARKER |
REFLECTION SURFACE** |
CORRECTED DISTANCES |
| 246.336 | -0.0018 | +0.016 | -0.002 | -0.0015 | 246.347 |
| 40.392 | -0.0015 | 40.391 | |||
| TOTAL | 286.737 |
*Slope correction: Laser height = 1.04 m, Height of laser spot on retroreflector = 0.10 m.
Slope correction (by Pythagorous) = 246.336 - sqrt(246.336^2-0.94^2) = 0.0018
**The reflection surface correction is my estimate of half the optical path the laser beam undergoes during reflection by means of the 2mm hexagonal pyramids in the retro-reflector. Effectively the reflection surface is about 1.5mm behind the front face of the retro-reflector.
The calibration course length is 286.737m. The manufacturer's stated error is =/- 14mm, an allowance of 4mm needs to be added to cover my errors in measuring positions and corrections so the total error should not exceed +/- 18mm.