CIVL1810 Chap.10 Levelling and Booking Reduced Levels
Levelling and Booking Reduced Levels
The surveying half of the unit begins here. A level run starts on a point of known reduced level, works forward through backsights, intermediate sights and foresights, and closes back onto a known datum or benchmark so that it can be checked.
Every clause of the field procedure exists to make an error detectable: readings go in the field book, backsight and foresight distances are equalised so the collimation error cancels in the subtraction, and sights are kept under 100 metres. Two booking methods reduce identical readings, and they differ in how many quantities must agree at the end.
What this chapter covers
- 01
The field procedure: all staff readings recorded in the field book
- 02
Equal backsight and foresight distances to eliminate errors from the line of sight or collimation
- 03
Keeping the length of sight under 100 metres
- 04
Commencing and finishing on a known datum or benchmark, and closing the level traverse
- 05
Backsight, intermediate sight and foresight, and which of them are structurally guaranteed
- 06
The change point read twice, as a foresight and then as a backsight
- 07
The rise and fall method: each consecutive pair of readings gives a rise or a fall
- 08
The height of collimation method: one line of sight level per instrument setup
- 09
The three way check of backsights less foresights, rises less falls, and last level less first
- 10
Why the collimation method offers only two agreeing quantities
- 11
Misclosure when a run is closed back onto its starting benchmark
- 12
Error sources: collimation, long sights, an unstable change point, a staff off vertical, and booking slips
- 13
Why an arithmetic check that closes does not prove the observation
- 14
Distributing a misclosure within the allowance specified for the job
Why the run closed on paper but not on the ground
- +1Separate the two kinds of correctness. The three way check tests the reduction only. Its agreement proves that backsights less foresights, rises less falls, and last level less first are arithmetically consistent, so no booking or subtraction error exists.
- +1Locate the fault. Because the arithmetic is sound, the 42 mm misclosure must arise in the observation. That is precisely why the field procedure requires a run to commence and finish on a known datum or benchmark and to be closed.
- +1Give two causes with mechanisms. A collimation error combined with unequal backsight and foresight distances leaves a residual that does not cancel in the subtraction. An unstable change point that settled between its foresight and its backsight corrupts the level carried forward while leaving every arithmetic check intact. Long sights beyond 100 metres or a staff held off vertical are further candidates.
Key terms
- Reduced level
- The height of a point relative to the datum a run is based on, obtained by working forward from a benchmark of known level.
- Backsight
- The first reading taken from an instrument setup, onto a point whose level is already known. Every setup has one.
- Foresight
- The last reading taken from an instrument setup, onto the point the instrument is about to leave behind. Every setup has one.
- Intermediate sight
- Any reading between the backsight and the foresight at a setup, used to pick up detail levels. A run may have none, which is why the arithmetic checks are built from backsights and foresights.
- Collimation error
- The error arising when the line of sight is not truly horizontal. It is proportional to the distance to the staff, so equalising backsight and foresight distances makes the two errors equal and cancels them in the subtraction.
- Height of collimation
- The reduced level of the line of sight itself at a given instrument setup, computed by adding the backsight to the known level of the point it was taken on.
- Misclosure
- The difference between the level computed on returning to a known benchmark and its published value. It is distributed within the allowance specified for the job, or the run is re observed.
Levelling and Booking Reduced Levels FAQ
Which booking method should I use in an exam?
Use whichever the question names, and if you have a choice on a run with intermediate sights, prefer rise and fall. It gives three quantities that must agree rather than two, and it draws every reading including the intermediate sights into the check. The collimation method is faster to compute, which is its own advantage under time pressure, but an arithmetic slip on an intermediate sight escapes it.
Why must backsight and foresight distances be equal?
Because the height difference between two points is computed by subtracting one reading from the other. If the line of sight is not exactly horizontal, each reading is wrong by an amount proportional to its distance, so at equal distances the two errors are equal and cancel in that subtraction. The instruction is an error cancelling design rather than a matter of neatness.
What is a change point actually for?
It carries the height forward when the instrument moves. The point is read as a foresight from the old setup, fixing its reduced level, and then as a backsight from the new setup, which establishes the new height of collimation. It must be firm and well defined, because if it settles between the two readings the run is wrong and no arithmetic check will reveal it.
How is a misclosure handled?
If it lies within the allowance specified for the job it is distributed across the run, commonly in proportion to the number of setups, since each setup is an equal opportunity for error. If it lies outside the allowance the run is re observed rather than adjusted. The permitted misclosure depends on the class of survey specified for the work, so quote the principle rather than inventing a tolerance.
Exam move
Do this chapter with a pen, never by reading. Take any set of readings, book them both ways, and force yourself to write the sums row before computing any check. Learn the three way check as a single line and be able to say which of its three terms the collimation method loses.
Rehearse the two habits that protect the arithmetic: recompute the height of collimation after every instrument move, and treat a change point as a reading taken twice. Then practise the diagnosis question, which is the one that separates understanding from table filling: given a run whose checks close but whose closure fails, name the physical causes and say which of them the booking method cannot see.
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