Compass is a survey instrument designed for the measurement of direction concerning the magnetic meridian. Two types of compass i.e. prismatic compass and surveyor compass are most commonly used. The Surveyor compass has been briefly discussed below.
Surveyor compass is an instrument used for the measurement of horizontal angles and the bearing of a line of sight. It is commonly referred to as the Circumferentor.
It essentially consists of a graduated horizontal circle, a pivoted magnetic needle, and a sighting device.
Originally, the surveyor compass was used as a Colonial instrument and designed for use in countries such as America and Ireland where the land was plentiful.
Surveyor compass essentially consists of a circular box with a magnetic needle that swings freely over a brass circle.
The brass circle is divided into 360 degrees. The measurement of the horizontal angle is accomplished using a pair of sights located on the north-south axis of the compass.
It is equipped with vertical sights for aiming at a distant object. It is levelled using a ball and socket mechanism and mounted over a tripod.
In this compass, graduations are from 0 to 90 degrees; 0 being to north or south and 90 degrees being to east and west.
An angle of 20 degrees to the north direction to the east is written as N 20° E and an angle of 40 degrees to east from the south is written as S 40 ° E. The reading is taken at the tip of the needle.
1.2. Use of Surveyor Compass
a. It is used to measure horizontal angles.
b. To determine the bearing of the surveying line of sight.
1.3. Parts of Surveyor Compass
A surveyor compass essentially consists of a graduated circle or disc fitted to a box. A magnetic needle is provided which freely rotates over a brass circle.
Unlike a prismatic compass, it has a narrow slit at the viewing end. The sight of the line is fixed and the reading is directly taken from the top of the glass cover.
a. Graduated disc or ring:
It is fixed with the compass box. It moves along with the movement of the box but the magnetic needle in it remains stationary with poles of North-South.
The ring of the surveyor’s compass Is divided into 4 quadrants and graduations from 0° to 90° in each quadrant. North and South points have 0° graduations while East and West points are marked at 90°.
Graduations are not inverted in the surveyor compass. So, its readings can be directly taken from a glass cover having a narrow slit at the viewing end.
b. Magnetic needle:
The magnetic needle of the surveyor compass remains stationary in the North-South direction. While taking readings, the ends of the needle act as an index.
It is placed at the centre of the box to support the movement of the magnetic needle.
It is connected with jewel bearing as a hinge.
e. Lifting lever:
It rises the magnetic needle to stick to the glass cover to stop its damp oscillations.
f. Circular box:
It is the circular part attached to the graduation circle.
g. Top glass:
Surveyor compass has top covering made up of glass known as Top glass.
h. Eye vane and object vane:
Eye vane is used to observe the line of sight while on object vane image of the object is formed.
i. Bubbling Tube
There is a bubbling tube for accurate levelling of the compass.
1.4. Temporary Adjustment of Surveyor Compass
The temporary adjustment of the surveyor’s compass involves the following:
The process of fixing the compass exactly over the station is known as centring. Centring is done by adjusting the legs of the tripod. The accuracy of the centring of the instrument over the station is checked using a plumb bob.
The compass has to be levelled so that the measurement taken is accurate. When the compass is used on hands, the graduated disc should swing freely and appear to be completely levelled regarding the top edge of the case. If a tripod is used, then a ball and socket arrangement may be used for levelling.
Focusing has to be then done for the completion of the temporary adjustment of the compass.
1.5. Permanent Adjustment of Surveyor Compass
Permanent adjustment of the compass is done only when the internal parts of the compass are defective or damaged. Permanent adjustment involves the following:
1. Adjustment in level.
2. Adjustment of the pivot point.
3. Adjustment of sight vanes.
4. Adjustment of the needle.
1.6. Sources of Errors in Surveyor Compass
Some sources of errors in this compass are:
a. Local attraction ( i.e. disturbance to the magnetic property of magnetic needle due to the presence of magnetic & electrical substances near the compass).
b. Improper levelling and centring of the tripod.
1.7. Precautions To Be Taken
a. Stop the damp oscillations of a magnetic needle by pressing the knob containing the spring brake inside to observe readings.
b. Local attraction should be avoided at the location.
c. While taking readings from the top cover glass of compass, it should be dust-free. While cleaning, take care of static force that may cause deflection of the magnetic needle.
1.8. Advantages of Surveyor Compass
a. Easy to handle.
b. Portable & Lightweight.
c. It has fewer angle or position errors.
What do you mean by Quadrantal Bearing System?
In this system bearing of the survey, lines are measured eastward or westward from North and South whichever is nearer.
In this system, both north and south directions are used as reference meridians and bearings are reckoned either clock or anticlockwise, depending upon the position of the line.
The quadrant in which a line lies is mentioned, to specify the location of the line. Surveyor’s compass is graduated in the QB system. It may vary from 0⁰ to 90⁰. Referring to the figure:
Reduced Bearing of lines OA= NαE
Reduced Bearing of lines OB= SβE
Reduced Bearing of lines OC= SγW
Reduced Bearing of lines OD= NδW
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