Since I've Googled and not found what I want to know, I thought there might be people here who know how surveyors' instruments work. The reason I am asking is that three different surveyors have surveyed the same boundary peg (and others) and put the peg in three different places, by up to 500mm away. They have all used the same Council survey mark in the footpath outside.
While I would have thought that a GPS unit would do the job very easily, the surveyors did not use a GPS unit, except maybe to get the position of North (an assumption by me). My cheap GPS unit locates the pegs to within a metre.
They all used a device (probably with a laser) on a tripod, and a rod with a mirror on top. I'm actually not interested in discussing any other kind of equipment. I want to know how three surveyors could get different results in the last few years using equipment that they say is accurate to within a millimeter. The Council survey mark has not moved and there have been no landslips and nobody has moved the pegs. The pegs are next to trees and structures that have not moved.
The process for a survey appears to be:
- find a spot across the road that is visible from the section being surveyed.
- put a peg there, put the laser above it and the mirror above the survey mark and measure the angle and distance and declination. Somehow the equipment knows where North is - I hope it's not using a compass.
- find a spot inside the section being surveyed that is visible from where the boundary pegs are guessed to be. Put the laser there and measure to the mirror on the spot in 2 above.
- Put the mirror above each peg position. At the laser, key in the distance and angle where the peg should be, and move the mirror up and around until the peg position is deemed to be correct by the equipment. This process takes seconds only and requires no calculations by the operator. The raising and lowering of the mirror is allowed for by the equipment by some magic unknown process (obviously that changes the distance, if the laser is a different altitude from the mirror). The mirror needs to be raised above vegetation in the vicinity. The mirror has a small bubble level on it so can be made vertical.
All around me people are putting up buildings and fences, and we need to know where the boundaries are. My neighbour's fence is reinforced concrete blocks and will never be moved again I'd say.
I can't measure most of the peg positions accurately with a tape measure because the land has a slope of up to 60 degrees and is covered in trees.
Now surveyors are highly paid professionals who in theory know what they are doing. How come they don't agree where boundary pegs should be put?