A trough compass consists of a long, narrow rectangular box, covered with glass. Inside the box, at its centre, there is a magnetic needle resting on the pivot. At the extremities of the trough compass, there are graduated scales with zero at the centre and marking up to 5° on either side of the zero line. Figure 33.6 shows a Trough compass.
Figure 33.6 Trough Compass
The trough compass is used for marking the magnetic north line on the drawing sheet of the plane table. In this case, the magnetic needle point to 0° - 0° of the graduated scale and a line drawn parallel to the edge of the trough compass is along the magnetic meridian. A trough compass is also used to orient the plane table with respect to the magnetic meridian.