It is angular surveying in which the horizontal distance from the instrument station to the staff and a vertical distance of a point are determined from instrumental observation.

Principle

Principle of tachometry based on the property of isosceles triangle. Where the ratio of distance of the base from the apex and the length of base is always constant.

In a fig $O_1, a_1, a_2, O_1, b_1, b_2 , O_1, c_1, c_2$ are all isosceles triangle where $D_1 ,D_2 and D_2$ are the distance of the bases from the apices and $S_1 ,S_2 and S_3$ are the length of the base (staff intercept)

So according to the stated principle $D_1S_1 =D_2S_2 =D_3S_3$ = fi (constant)

The constant f/I is known as the multiple constant.

f = focal length of object and i = stadia intercept

Following are the objective of tachometry survey

1. Prepare a contour map or plan.
2. used in hydrographic survey.
3. Location survey for road railway reservoir etc.
4. It can be used for checking more precise measurement