Disadvantage of chain surveying is that, in it only distances are measured and hence area is to be covered with a network of triangles. If the length as well as angle of a line can be measured with respect to a known direction then it is possible to plot a line, independent of length of other lines. Hence, in such cases there is no compulsion of going for a network of triangles only. Compass is an instrument which can be used to measure the direction of a survey line with respect to magnetic north-south. The magnetic north-south direction which is the reference direction is called meridian (reference direction) and the angle between the line and the meridian is called bearing. Use of compass for measuring direction of a line simplifies the surveying to a great extent.
The difference between prismatic and surveyor’s compass are
- Graduation circle is fixed to broad type needle. Hence, it will not rotate with the line of sight.
- There is a prism at viewing end.
- Sighting and reading can be done simultaneously.
- The magnetic needle do not act as an index.
- The graduations are in whole circle bearing.
- Graduations are marked inverted since its reflection is read through prism.
- The reading is taken through a prism.
- Tripod may or may not be used. It can be held on a stretched hand also.
- Graduation circle is fixed to the box. Hence, it rotates with the line of sight.
- At viewing end there is no prism. There is only a slit.
- Sighting and viewing cannot be done simultaneously.
- Magnetic needle acts as index while reading.
- The graduations are in quadrantal system.
- Graduations are marked directly. They are not inverted.
- The reading is taken by directly viewing from top glass.
- Tripod is essential for using it.