You have studied about the compass and its use for bearings of measuring survey lines. The angular measurements are carried out with prismatic compass that uses a magnetic needle and the bearings of survey lines are measured with reference to magnetic meridian. Such bearings are called magnetic bearings. However, on earth the true north (geographical north) is different from magnetic north. The difference between true N-S axis and magnetic N-S axis is called declination. This declination varies from place to place on surface of earth and also on time of observation. Hence, the correct measurements of angles would be to obtain bearing of survey lines with reference to true N-S axis. Such bearings are termed as true bearings. However, true bearing can only be made with the help of astronomical surveying which is tedious, time consuming and costly for engineering applications. Magnetic bearings which can be obtained easily at much reduced cost with the help of simple instruments like magnetic compass are considered accurate enough for survey of small areas.
Procedures of making angular measurements, the errors involved and method of corrections are also discussed in detail. You can now conduct a simple chain and compass survey of the area, which is fast and sufficiently accurate method of surveying for most of the engineering applications.