In reality, the method we are going to explicate is not only extraordinary for levelling machine. Obviously, this technique can be used using tachometer and any other device like a theodolite, etc. In case there are two points on earth surface like point A and point B and we require the distance between these two points.
Here the distance signifies the horizontal distance between these two points. In order to find the distance, we just set the instrument at one of these points in the graduated staff rod. Furthermore, we require upper hair and lower hair readings.
Upper hair reading is equal to 1.112 and the lower hair reading is equal to 0.654 meters Now, we can make use of the horizontal distance determination formula.
Distance = (top reading – bottom reading) * constant
[Constant will be the horizontal distance between Point A and Point B]
Distance = (1.112 – 0.654) x 100
Distance = 45.8 meters
[The constant may be dissimilar for each and every type of instrument, however mostly we use 100. The constant can be found in the manual of the levelling machine. ]
for more, By viewing this video one can easily learn the technique and calculation