INSTRUMENTS AND PROCEDURES
The compass essentially consists of a freely suspended magnetic needle mounted on a smooth pointed pivot. The needle can freely move over a graduated scale. Two slit vanes are provided on the frame – one as the object vane and other as eye vane – placed at 180 o to provide the line of sight. A tripod stand is provided on which the compass can be mounted and positioned over the survey station, while taking observations. A circular metal box, approximately 100 mm diameter, is used with a hardened steel pivot at the centre. The magnetic needle, graduated aluminium ring and vanes etc. are other parts of the compass. Design of these parts and their placement vary in different types of compass.
The two types of compass – prismatic compass and surveyors compass – are currently used in practice.
It is the commonly used compass for engineering surveys and is suitable for surveys where speed is more important than accuracy, for example, the preliminary surveys of road, railway line or pipe line alignments and rough traversing etc. Figure shows the different constituents of a prismatic compass in their final assembled form.
The aluminium ring of prismatic compass has a magnetic needle marked with N-S along the diameter of the ring. The graduations are itched from 0o to 360o in clockwise direction with zero marked at south end of needle and 180o at the north end (Figure . The itching is marked in inverted fashion so that they are read in correct way when viewed through the reflecting prism. Each degree in graduation is divided into half to give a least count of 30'.
The object vane has a vertical hair thin wire bisecting the object under observation. The observation vane (or eye vane) consists of a reflecting prism. Both the vanes are collapsible to be folded to lie on compass cover when not in use. A plane mirror is hinged to object vane to sight the object which is too high or too low to be sighted directly. The indication of mirror can be adjusted to facilitate this process. In case of sun glare, when making the measurements become difficult, sun screen of tinted glasses can be used by placing them in the line of sight between prism and object vane.
To dampen the oscillation of magnetic needle and providing stability to measurement process, a brake pin is provided on the side of the compass box. A lifting pin is also provided to lift the needle and to keep it pressed against glass cover when the object vane is folded and the compass is not in use. This prevents the pivot from excessive wear and tear.
System of Graduation
This instrument is more or less obsolete these days and not often used for land surveying. Its construction is somewhat similar to prismatic compass except that it has plane sight vane with a narrow vertical slit in place in prism. The graduations on scale vary from 0o to 90 o with 0 o at North and South and 90 o at East and West positions marked (Figure ). The magnetic needle is edge bar type while the circular graduated scale is fixed with the box. Thus, here, instead of whole circle bearing, reduced bearings are recorded.