Laser Measurement

Hi Pete, I have a Leica and have measured a couple of calibration courses to check the accuracy.

I have white reflector boards that each mount to a short piece of 2x4 with a slot cut in it. They stand nice a straight and I can align them to the nail. I was not able to measure 300 m using this so I put a third board in the centre and measured 2 150 m sections. With my Leica mounted on a good tripod this worked well and I was able to get good agreement to my steel tape measurement.

There are a few issues. First, in the sunshine it is very hard to see the target in the Leica and is therefore almost impossible to use at 150 m. Second, with all the set up and going back and forth to get multiple measurements it takes about as long as measuring with a steel tape, as long as you have a helper. Third, it is fairly complex since you need to account for the height of the Leica compared to the target, ensure the proper alignment of the end points and the target, align the Leica reference correctly, and then add multiple sections. The steel tape method is simple and reliable.

After a few trials I decided it is better to rely on the steel tape.

The Leica is excellent for measuring offsets to fixed objects at the start, finish or turnarounds.

The Electronic Distance Measuring (EDM) rig I used was a total station rig. It had all the attributes of a surveyor's transit, plus the ability to shoot out a beam to target, and catch the reflection. It was not a consumer-grade product - those didn't exist at that time.

Both the EDM and the target were on tripods, and were set up over their points using plumb bobs.

Mike Sandford did a good bit of work on this and discovered that two things you need to make using a EDM fairly easy are a reflective target and some type of fine-adjustment mount.

Using a normal tripod as a mount doesn't work very well because it's very difficult to make the fine rotational adjustments necessary to hit a target 150 meters away. With normal target boards it can be difficult to tell if you have really hit them, especially on sunny days as Paul mentions. Mike used bicycle reflectors and I used a small board covered with reflective tape. With either one it is quite obvious when the laser hits the target.

The best method I found was to put one target at either end of the cal course, and take a measurement to each of them from the middle. There's really not much hope of getting a reading from a target 300 meters away.