COMPASS SURVEY
1.Chain surveying can be used when the are to be
surveyed is comparatively smaller and is fairly flat.
2.But when the area is large in such cases chain
surveying is not possible
3.In Traversing the frame work consists of number of
connected lines
4.The lengths are measured by using tape or chain
and angles are measured by using compass
Type of traverses
1) Open traverse
2) Closed traverse
Principle of compass surveying involves a series of connected lines the magnetic
bearing of the lines are measured by prismatic compass and distance are measured by
using chain surveying
Compass surveying is not recommended for areas where local attraction is suspected
due to local attraction like steel structures, magnetic e..t..c .
Types of compass
1. Prismatic compass surveyor
2.Surveyor compass
Prismatic compass
Prismatic compass is a magnetic compass which consist of following parts
Cylindrical metal box having diameter 8-12cm.
It is protected from dust and rains e .t. c
Pivot
Pivot is provided at the centre of compass and support freely suspended magnetic needle
over it .
Lifting pin and lever arm
lifting arm is provided just below the sight vane
When the sight vane is folded it press the lifting pin
Magnetic needle
Magnetic needle is the heart of the instrument
This needle measures angle of a line from magnetic meridian a the needle always pointed the
north and south at the two ends of the needle when freely suspended on any support
Graduated circle
This is a aluminum graduated ring marked from 0 to 360 to measure all the possible angles of
a line
Prism
Prism is used to read graduations on a ring and to take exact reading by compass
Object vane
Object vane is diametrically opposite to the prism and eye vane the object vane carries a horse
vane to sight the object by eye vane
Eye vane
Eye vane a fine slit is provided with eye hole at the bottom to bisect the object from the slight
and to take reading simultaneously from the eye hole
Glass cover
It covers the instrument from dust and wind e. t. c
Working of prismatic compass
when the needle of compass suspended freeely .its always points towards north ,there fore all
the angles measured now are said to north oriented (magnetic meridian)
The horizontal angle made by a survey line with reference to magnetic meridian in clockwise
direction is called bearing of the line
Temporary adjustments of compass
Centering
Leveling
Focusing
Centering : it is the process of placing the compass exactly over the station where the
bearing is to be determine .
The centering is checked by dropping a small pebble or plum bob
Leveling :leveling of the compass is done aim to free swimming the graduated circular
ring of the prismatic compass
If not ball and socket is should be adjusted
Focusing:
Prism is move up and down in its slide till the graduations of the aluminum ring are clear and
sharp
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