Compass Surveying:

Previously, you have learnt that the general objective of surveying is to determine the relative positions of distinctive features on ground (in the area under study) in order to prepare maps which can be used in future for various engineering applications, e.g. designing, planning and executing civil engineering projects. This, in general, will consist of measuring distances, both horizontal and vertical, and angles. Previous, you have studied different methods of measuring horizontal distance along the survey lines and offset distances of ground features from survey lines.

However, if the area to be surveyed is comparatively large it is rather difficult and inaccurate to obtain all the required distances by measuring horizontal distances only. In real life situations, there are many obstructions and difficulties in direct ranging the survey lines. It becomes extremely tedious and inconvenient, even impossible, to fix the directions of survey lines by linear horizontal measurements alone. These problems can be conveniently solved by making angular measurements. The directions of survey lines could be simply fixed by measuring the angles between two survey lines meeting at a station or angle of a line with reference to some fixed axis. The direction of a line associative to a given fixed axis (or meridian) is known as its bearing. Traversing becomes quite convenient by carrying out the measurements along a series of interconnected survey lines either to form a closed or an open traverse. The lengths and offsets are measured through chain or tape although directions of survey lines are measured using compass or any angle measuring instrument. This process is termed as chain and compass survey.

Objectives

After studying this unit, you should be able to

• Know various important terms and instruments used in compass surveying,
• Know the procedure of compass surveying,
• Compute bearing of survey lines and find the angle between these lines, and
• Understand various types of error involved in compass surveying, their source and methods of correction.