FUNDAMENTALS OF SURVEYING : Anallactic Lens

1)   Anallactic Lens

 In externally focusing telescope a convex lens is fitted between the diaphragm and the objective at

a fixed distance from objective.

 By the provision of anallactic lens, the vertex is formed at the vertical axis and its position is always

fixed irrespective ofthe staffposition.

 Multiply constant and additive constant becomes 100 and 0 respectively when anallactic lens is

used.

 In case of internally focusing modern telescopes the additive constant is very small and can be

taken equal to zero. Therefore, internal focusing telescope is vertually anallatic.

 Externally focusing telescope are commounly used.

 In case of internally focusing modern telescopes the additive constant is very small and can be

taken equal to zero. Therefore, internal focusing telescope is virtually anallactic.

 Externally focusing telescope are commonly used.

Movable hair method (subtevse method )

 This is slow method.

 Vertical subtense method is almost obsolete.

 Only horizontal base subtense method is in use.

 A subtense bar of 2 to 3 m length is use.

S



   2
tan 

2

D	S	/ 2



tan(_ )

2

 AOB is mesured by theodolite preferably by the method of repetition.

Tangential method:

 In tangential method horizontal and vertical distances from the instrument to the staff station are comuted from the observed vertical angles to the vanes fixed at a constant distance apart on the staff.

 Thus, stadia hairs are not used and vane is bisceted every time with the axial hair.

 The method is used when theodolite is not fitted with stadia hairs.

Case a) Both the angles are angles of elevation: From AMBQ ,

'                         V =D.tan oc ............................................ .1
1

AMBQ ,

'                                                                      V+S=D.tanoc — tan oc .................................. .2

2                        1

From equantion 2 – equation 1 S = D(tan oc 2 — tan oc₁)

D=	S

a2	>  (tan al

V D tan = . oc=	S. tan	oc1

(tan oc 2 — tan oc₁)

Elevⁿof Q= Elevⁿof P+h+V—r

Case b) Both the angles are angles of depressid

V =D. tan oc 2.............................................1

V — S = D. tan oc₁.......................2 Equation 1 – equation 2

D=	S

(tan oc 2 — tan oc₁)

Elevⁿof Q= Elevⁿof P+h—V —r

Case c) One angle of elevation and other of depression.

V =D. tan oc 2.............................................1

S —V =D. tan oc₁.......................2 Equation 1 + equation 2

S =D.(tanoc₁ — tan oc ₂ )

D=	S

(tan oc₁ + tan oc ₂)

V	S. tan	oc 2

(tan oc₁ + tan oc ₂)

Elev of Q= Elev of P+h—V —r

n                                                                                   n

Sine Rule:

sin A sin B sin C

=