# Optical measurement method

## What is optical measurement method?

Optical measurement method is also known as tacheometry method or stadia method. Optical measurement is inspection of measurement with the help of telescope and then calculations are solved using triangulation method to find out the distance. The instrument tacheometer is considered as important in this method. Other than these, level staff which is a graduated wooden or aluminium rod needed to carry out optical measurement. Before the electronic distant measurement method this method is more famous ad accurate for a particular distance. This is a suitable method to survey a hill country and counter lines.

## Level staff

Levelling staff is wooden or aluminium made rectangular rod, graduated as to easy the reading. The levelling staffs are available in both metric and imperial graduation and sometimes even single staff contains both graduation on two sides. The level staff helps to determine the height difference between two points on a survey. The sliding joints of a levelling staff help to adjust the length of staff according to the need of use. A staff can be divided into two depending on its use as, self reading staff and target staff. The only difference between self reading staff and target staff is an attachment of metallic circular plate with a vernier, which called as target.

Target staff

## Tacheometer

Tacheometer is an instrument used to measure the vertical and horizontal distance between two points. Tacheometer is more similar to theodolite but an optical lens is available in a telescope and two stadia line are available other than central cross hair. The telescope used to measure the distance between two points using the level rod. Then the distance between instrument and levelling staff is obtained by multiplying levelling staff interval by a constant.  The common equation to obtain the distance, d between the instrument and levelling staff is,

d = k × s + c

Here, s is the levelling staff interval; k and c are multiplicative and additive constants. In most of the instrument the k is 100 and c is 0.

There are some methods available in tachemoetric survey such as stadia hair method and tangential method. Stadia Hair Method is a method in which a theodolite with the stadia diaphragm is used to find out the staff intercept between the lower and upper hairs and also the central hair reading is noted. The stadia hair method can be further classified into two as, fixed hair method and movable hair method. In fixed hair method the cross hairs of the diaphragm are kept at a constant distance apart and the staff intercept varies with the horizontal and vertical position of the staff with respect to the Theodolite. But in movable hair method the staff intercepts between the lower hair and the upper hair is kept constant by moving the horizontal cross hairs in the vertical plane.