Elevations – The basics
- Bench Mark (BM) = A permanent point of known elevation.
- Temporary Bench Mark (TBM) = A point of known elevation.
- Turning Point (TP) = An intervening point between BMs or TBMs upon which a backsight and a foresight are taken.
- Backsight (BS) = A rod reading taken by “looking back” at a point of known elevation such as a BM or TP.
- Foresight (FS) = A rod reading taken when “looking ahead” at a point where you want to determine its elevation, such as a TP, TBM or BM.
Height of Instrument (HI) = The elevation of the line of sight in the telescope of the level.
Height of Instrument (HI) = Known elevation + Backsight (BS)
Turning Point (TP) = Height of Instruction (HI) – Foresight (FS)
1. Elevation are all relative to known benchmarks.
2. So from below, the bench mark is known to be at 100ft
3. When surveyed the pole reading at the benchmark is 7.4ft
4. So the elevation line of sight is 100ft + 7.4ft = 107.4 ft
- Now obtain the new reading at point A below, the pole reading = 10.9
6. The elevation at PT. A = Line of sight elevation – Pole reading at PT.A
= 107.4 ft – 10.9 ft = 96.5 ft