Theory
The instrument are commonly employed in are
1. A techenometer and stadia rod theodolite fitted with a stadia and telescope i.e. a telescope fitted with stadia diaphragm is known as tachenometer where this instrument is used for survey it is know as tachnemometer survey. In the stadia diaphragm two additional horizontal cross hair are fixed one above the other below the middle horizontal heuz equally spaced. The types of stadia diaphragm two additional horizontal cross hair are fixed one above the other below the middle horizontal hair equally spaced. The types of stadia diaphragm commonly used are shown in fig. The winds of telescope fitted in tachenometer are
a) The external focusing telescope
b) The internal focusing telescope
c) The external focusing anallatic telescope is used as a technometer
Diagram
Stadia rod
For small sights ordinary levelling staff is used for long sight aspecially designed graduted rod is used. It is usually of one piece but for easy trancefort it may be of folding type. It is 10 cm to 15 cm wide 1 cm in thickness and 3 m to 4 m in length. The stadia rods used for small sights may have 1 count of 5 mm (0.005m) and for long sight as 1 cm (0.01m). The graductions should be bad and distinct. The figures shows some common patterens of stadia rods.
For precise work the staff is held vertically by suspending a plot or by mean of folding circular level as shown in fig. which is attached to the case side of the staff and perpendicular to it, so that the staff is vertical when the bubble is in the centre.
Characteristics of a tachnometer
1. The value of constant (Æ’/i) = 100
2. The telescope should be fitted with an analliclens to have the value of (Æ’/i) =
3. The telescope should be powerful the magnification should be 20 to 30 times the diameter.
Tachenometric method
The tacherometric method are classified as follows
1. Stadia method
a) Fixed hair method
b) Movable hair method
c) Tangential method
Principle of stadia method
It is based on the geometric principle that into similar triangle corresponding sides and altitudes are proportional. In practis the distance of point froman instrument is found by holding a stadia rod in vertical position on the point and by reading the portion of rod which appears to be intercepted between the upper and lower stadia hairs.
Theory involved
Let,
O = Optical centre of object glass
a,b,c = The top axial bottom hair reading
A B C = The top axial and bottom hair reading
AB = É» = Length of image (Stadia hair interval)
