The first step of any construction work is surveying and thus it is an important branch of civil engineering. In general, surveying can be defined as the art of determining the relative position of various features above, on or beneath the surface of the earth utilizing different instruments and preparing the maps. One of the important types of such survey instrument is the compass. 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. Surveyor compass has been briefly discussed below.
Surveyor compass is an instrument used for the measurement of horizontal angles and 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, surveyor compass was used as a Colonial instrument, 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.
Use of Surveyor Compass
Surveyor compass is used extensively for the measurement of angles and surveying of the region.
Components of Surveyor Compass
A surveyor compass essentially consists of a graduated circle or disc fitted to a box. A magnetic needle is provided which can freely rotate above it. Unlike a prismatic compass, it has a narrow slit at the viewing end. Then, the sight of the line is fixed and the reading is directly taken from the top of the glass cover. The graduated circle is free to rotate with the line of sight and the magnetic needle is always towards the north. In this compass graduations are from 0 to 90 degree, 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 degree E and an angle of 40 degrees to east from the south is written as S 40 degree E. The reading is taken at the tip of the needle.
Temporary Adjustment of Surveyor Compass
The temporary adjustment of 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.
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.