Lessons · Engineering · Levelling: backsight, foresight, reduced level
Backsight, foresight, and the level you actually want
Levelling transfers a known height across a site. You read a staff on a point of known level (the backsight) to find the height of the instrument, then read the staff at the unknown point (the foresight) and subtract. Instrument height equals known level plus backsight; the new level is instrument height minus foresight.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the most-used arithmetic on any site and the easiest to get backwards under pressure. Getting the sign wrong puts a drainage invert above the thing it is meant to drain.
How to think about it
Say it as a sentence every time rather than recalling a formula: add what you read looking back at something known, subtract what you read looking forward at something unknown. Then check the answer is plausible against the ground you can see.
Worked example
Known benchmark 50.000 m, backsight 1.250.Reading on the known point.
Instrument height = 50.000 + 1.250 = 51.250.Add the backsight.
Foresight at the new point = 2.100.Reading on the unknown point.
New level = 51.250 - 2.100 = 49.150.Subtract the foresight.
Your turn
Benchmark 20.000 m, backsight 1.500, foresight 0.800. Write the reduced level in metres.
20.000 + 1.500 - 0.800 = m
Solve one, graded on the server
The trap
Adding the foresight. Backsight up, foresight down -- reversing it produces a level that is wrong by twice the reading and still looks like a number.