It’s used to determine the correct difference of elevations of the two points which are too far to be measured by putting the instrument mid-way between the two points to balance the foresight and backsight.
It’s the indirect method of levelling in which the elevation of a point is determined from the vertical angle measured with a theodolite and horizontal distance measured with a tape or a chain.
All the columns are same as the height of the instrument method except one- Two columns of rising and fall will replace the HI column of the height of the instrument method.
This error occurs due to the inaccuracy of graduations of the staff.
If the bottom of staff has worn out then it will five wrong height of instrument but the error in foresight is balanced by the back sight and thus the calculated elevation is correct.
This error occurs due to fault in focusing tube of the external focusing telescope due to which the objective moves in an inclined direction when tuce is moved for focusing.
It can be eliminated by taking out the defective tube and aligning it properly.
Due to the high temperature, liquid of the level tube expands and bubble gets shorten, leading to the adjustments of the instruments getting disturbed.
Arbitrary benchmarks are assumed to be equal to 100 meters generally and then the elevations with respect to assumed benchmark are determined.
It is commonly practised by engineering students. For GTS surveys of the country, surveyors use permanent benchmarks to calculate the elevations of different points.
Establishing a reference point: The first step in leveling is to establish a reference point with a known elevation, such as a benchmark or a control point. This reference point serves as a starting point for all subsequent measurements.
Setting up the leveling instrument: Once the reference point is established, the leveling instrument is set up over it. The instrument is usually mounted on a tripod and leveled using its built-in spirit level.
Measuring the height of the leveling rod: The surveyor then takes a measurement of the height of the leveling rod held vertically at the point being surveyed. The leveling rod is a graduated staff, typically made of wood or aluminum, with marks indicating the height above the ground.
Calculating the elevation difference: The difference in elevation between the reference point and the surveyed point is then calculated using the height of the leveling rod, the distance from the instrument to the rod, and the height of the instrument.
Repeating the process: The process of measuring and calculating the elevation difference is repeated for all points of interest, creating a series of height measurements.
Plotting the results: The results of the leveling measurements are then plotted on a topographic map or used to create contour lines, which represent the height of the terrain at different locations.