What is Trigonometry? | Introduction to Trigonometry | Don't Memorise

Tacheometry

* Tacheometry is a method of surveying in which horizontal distances and differences in elevations

are determined by measuring intercepts on a graduated scale and angles by using theodolite.

* This method is not tedious and speed of surveying is very high. However, accuracy is less.

* If interval between stadia hairs is i, intercept on staff is s, f is focal length and d is the horizontal

distance between the objective and the vertical axis of the instrument, then the distance between

instrument station and the position where the staff is held is

D = s + (f + d)

= ks + C

where k = and C = f + d

k is known as multiplying constant and C is additive constant.

Usually, in all instruments-manufacturers keep k = 100 and C varies from 0.3 to 0.6. These values can

be determined by carrying out measurements over known distances. Manufacturers write these

values on the instrument also.

* With simple geometric relations, the elevations and distances can be found with inclined sights also.

* Truly speaking C is not constant but varies slightly from sight to sight. By using anallatic lens this

value may be made zero. In internal focussing telescopes the value of C can be made as low as

0.05 m to 0.15 m and hence may be taken as zero.

* In movables/substance method, the stadia hairs are made movable and staff intercepts are made

constant by using target staffs. The stadia hair interval is read with micrometer screws.

* Since a tacheometry survey involves large observations, instead of calculating each time to get

distances and differences in elevations, one can use stadia tables, stadia diagram or stadia slide

rules.