Heighting IV Flashcards
How can theoretical precision be deduced?
By the theory of Propagation of Errors.
How do you estimate the error in a single setup and propagation over a distance?
Error = +-sqr(Dl)
where:
D= sight length of fore and back sight
l= the distance levelled
How to do calculate the error over flat ground?
+-12xsqr(k)mm
How to do calculate the error over hilly ground?
+-24xsqr(k)mm
How to do calculate the error for precise levelling e.g. deformation monitoring?
+-6sqr(k)mm
Nam 4 instrument errors when levelling?
1) Level out of adjustment (collimation error).
2) Inaccurate staff divisions
3) Parallax- must be eliminated before reading taken
4) Tripod legs loose
Nam 3 natural errors when levelling?
1) Strong winds
2) Heat haze/shimmer on sunny days
3) Earth Curvature
What is atmospheric Refraction?
Greatest near to ground and particularly over water bodies and generally only important for long sights beyond normal spirit level.
How do you minimise the effects of the earth curvature and refraction?
If BS and FS are equal.
What does combined correction curvature + refraction equal?
-0.067 x s^2m, where s is the horizontal distance in km.
Give 4 observation errors?
1) Level bubble not centred
2) Level and staff not firmly in the ground
3) Moving level and staff together
4) Faulty staff holding
Give 3 booking errors?
1) Transposing figures
2) Booking in wrong column
3) Arithmetical errors in the reduction process
How do we check collimation error?
Two peg test
When does collimation error arise?
When the level is out of adjustment, i.e. the line of sight is not horizontal when the bubble is central.
How do I carry out two peg test?
1) Set up level between two pegs and read staff at each peg.
2) Move the level beyond one of the pegs by distance D and re-read both staff readings a2 and b2
3) Not the original
4) Peg positions A and B have not moved
