Student No. 617265
The Automatic Level
An Automatic level is an optical instrument that provides a height reference.
The height reference is the horizontal plane measured through the axis of the level. This is also known as the Height of Plane of
Backsight reading to OBM.
These are commonly found carved into old buildings. 0.0
The staff is to be held vertically parallel above the OBM.
The OBM will also have a known height – example:
Centre of height reference Benchmarks(2)
The Temporary Benchmark (TBM) is another known point which can be related to the OBM.
It is common to set up several TBM’s with known elevations from the OBM.
If the OBM was disturbed through on site works, the
TBM’s act as insurance. They also imporve accuracy of recordings. This steel pin is an example of a TBM.
Other references are used such as the marking on the drain cover adjacent to
Ashley Down Rd.
Height of Collimation
The HPC is found by taking a back sight to a known point of elevation such as a Temporary
Ordnance Benchmark (TBM).
Placement of the automatic level must be higher than the OBM.
The staff reading is added to the benchmark value to find the HPC.
HPC = TBM + Staff Reading
You can find the reduced level once you have the height of collimation.
The reduced level is the real ground height above ordnance datum.
Any ground level can be found below the
HPC – by subtracting the staff reading from the HPC.
Reduced Level = HPC – Staff Reading
This method works by allowing the horizontal plane to intersect through the level’s axis.
Height of Collimation
HPC booking is commonly used where the instrument is stationary and lots of readings are taken.
This booking method is ideal for grid of levels. You begin with a backsight from the TBM.
Intermediate readings are recorded while the instrument is stationary. You close with a foresight onto the TBM.
HPC = TBM (50.856) + Backsight (3.670)
Cut or Fill depths can be calculated for construction purposes. This also determines the amount of Topsoil which is required to be preserved.
Arithmetic checks are applied on the booking sheet to determine error.
Rise and Fall Booking
This method works out one level from the previous by calculating the difference, then determine whether it’s a rise or fall.
You must always read a backsight first. In most cases the backsight is measured against a staff held upright above the
Rise and Fall is the most common method of booking especially when moving from an
OBM to a TBM as practised within my college group.
The booking sheet comprises of backsights and foresights. An arithmetic check is applied for errors. There should be no error when measuring from: OBM > TBM >
2 Peg Test
This method checks the accuracy of instrument. It detects error of collimation.
To ensure accuracy when levelling, this test should be carried out every time before the level is used.
The error of collimation should be noted as mm per metre length.
If there are inaccuracies in the readings, the instrument is not in calibration – so measurements will have to be repeated or the level will have to be sent back to the manufacturer for re-calibration.
Two Peg Testing comprises of 4 readings.
When the instrument is not sighting horizontally, thus looking at an angle – vertical error can occur.
Placing the level in-between two equidistant locations and finding the