The following are the points of differences between prismatic and surveyor’s types of compasses.
Difference # Prismatic Compass:
1. The graduated ring being attached to the magnetic needle remains stationary when the compass-box and the sight vane is rotated.
2. The graduations are marked on the ring in a clock-wise direction with 0° or 360″ at South end of the needle so that 90° is marked at the West. 108° at the North and 270° at the Fast (Fig. 5.3 (a)]. The figures are written upside down as shown in [Fig. 5.3 (b)].
3. The sighting of an object and reading of the bearing are done simultaneously.
ADVERTISEMENTS:
4. This can be used without a stand.
Difference # Surveyor’s Compass:
1. The graduated ring being attached to the compass-box moves with the sights and the needle remains stationary when the box is moved.
2. The graduations are marked both in the clockwise and counter clockwise directions dividing the graduated ring into four quadrants and the graduations are numbered from 0° to 90° in each quadrant. The zero points are marked at north and south and 90° at east and west (Fig. 5.4). The east and west have been inter-changed from their true positions.
3. An object is sighted first and the bearing is then read with the naked eye by going vertically over the middle point.
4. This cannot be used without a stand.