A compass surveying is performed by means of a magnetic compass which helps to determine the angles and the direction of the survey lines. The main two types of magnetic compass employed are a prismatic compass and a surveyor compass.
The theory behind the magnetic compass and its types are briefly explained.
Principle of Magnetic Compass
The magnetic bearings of a line can be determined by a magnetic compass. The bearings can be measured either in the whole bearing system (WBS) or Quadrantal Bearing System (QBS). This is merely dependent on the type of compass employed for the measurement.
When a narrow strip of steel or iron is magnetized and suspended about its center such that it can freely oscillate about the vertical axis, then the strip establishes itself in the magnetic meridian at the place of observation. This is the working principle of a magnetic compass.
Features of a Magnetic Compass
The main features of a Magnetic compass are:
- Magnetic Needle
- Line of Sight
- Graduated Circle
- Compass Box
The purpose of the magnetic needle is to establish the magnetic meridian. A line of sight helps to sight the other end of the survey line through the compass. A graduated circle is employed to read the directions of the lines. It can be attached either to the box or to the needle. In order to house the above parts, a compass box is used. The whole housing is then supported by means of a tripod or a suitable stand.
Types of Compass Used in Surveying
The main types of compasses that are used in compass surveying are:
- Prismatic Compass
- Surveyor’s Compass
1. Prismatic Compass
Figure-2 below shows the parts of a prismatic compass. It is one of the most convenient and portable forms of the magnetic compass. It can be held in hand or in a tripod stand for carrying out the measurement.
The line of sight is defined by the object vane and the eye vane. A prismatic compass helps to conduct both sightings and reading simultaneously. The figure-3 below shows the system of graduation in a prismatic compass.
The compass is initially held over the starting station of the survey line and the adjustments are provided. The magnetic meridian is thus obtained and then starts to take the measurements by sighting to the next station. As shown in figure-3 above, the readings increase in clockwise direction i.e. from the south ( 0 degrees) to West (90 degrees) and North (180 degrees) and East (270 degrees)
Temporary Adjustments for Prismatic Compass
The temporary adjustments usually followed for prismatic compass are:
- Focusing the Prism
1. Centering: In this step, the instrument is kept exactly over the station point. This can be done either by adjusting the tripod stands or by using a plumb-bob. Sometimes, a pebble can be freely dropped from this center to the bottom of the instrument to check the centering.
2. Levelling: The instrument must be held such that the graduated disc swings freely and when viewed from the top edge it must appear level. If it is not used as a hand instrument, a tripod is used to support the instrument for levelling.
3. Focusing the Prism: Till the readings are observed sharp and clear, the prism attachment is slid up and down for proper focusing.
2. The Surveyor’s Compass
The figure-4 below shows the sectional view of a surveyor’s compass. It consists of a circular brass box housing a magnetic needle. This needle swings over a bass circle which is divided into 360 degrees.
It consists of a pair of sights which is used to measure the horizontal angle. This is located in the north-south axis. The surveyor’s compass is usually mounted over a tripod and leveled by using ball and socket mechanism.
The temporary adjustments for the surveyor’s compass are the same as that of the prismatic compass. The permanent adjustments are sometimes necessary for the surveyor’s compass-like:
- Adjustments in Levels
- Adjustment of needle
- Adjustment of Sight vanes
- Adjustment of the Pivot