Compass Surveying| Types of Compass | Advantages & Disadvantages of Compass Surveying |

 Compass surveying

Compass surveying is the branch of surveying in which the position of an object is located using angular measurements determined by a compass and linear measurements using a chain or tape. Compass surveying is used in the following circumstances:

• If the surveying area is large, chain surveying is not adopted for surveying rather compass surveying is employed.
• If the plot for surveying has numerous obstacles and undulations which prevents chaining.
• If there is a time limit for surveying, compass surveying is usually adopted

Compass surveying is not used in places that contain iron core, power lines etc which usually attracts magnets due to their natural properties and electromagnetic properties respectively. Compass surveying is done by using traversing. A traverse is formed by connecting the points in the plot by means of a series of straight lines.

Magnetic compass

The magnetic compass is used to find out the magnetic bearing of survey lines. The bearings may either measured in the Whole Circle Bearing (W.C.B) system or in the Quadrantal Bearing (Q.B) system based on the type of compass used. The basic principle of the magnetic compass is if a strip of steel or iron is magnetized and pivoted exactly at centre so that it can swing freely, then it will establish itself in the magnetic meridian at the place of arrangement.

Major types of the magnetic compass are:

1. Prismatic compass
2. Surveyor’s compass
3. Level compass

Prismatic compass

A prismatic compass is a portable magnetic compass that can be either used as a hand instrument or can be fitted on a tripod. It contains a prism that is used for accurate measurement of readings. The greatest advantage of this compass is both sighting and reading can be done simultaneously without changing the position.

Major parts of a Prismatic Compass are:

• Magnetic needle
• Graduated ring
• Adjustable mirror
• Sliding arrangement for mirror
• Object vane
• Eye vane
• Metal box
• Glass cover
• Horse hair

Adjustments of prismatic compass

Two types of adjustments:

1. Temporary adjustment
2. Permanent adjustment

Temporary adjustments

• Centering: it is the process of fixing the compass exactly over the station. Centering is usually done by adjusting the tripod legs. Also a plumb-bob is used to judge the accurate centering of instruments over the station.
• Leveling:  the instrument has to be leveled if it is used as in hand or mounted over a tripod. If it is used as in hand, the graduated disc should swing freely and appears to be completely level in reference to the top edge of the case. If the tripod is used, they usually have a ball and socket arrangement for leveling purpose.
• Focusing the prism: Prism can be slide up or down for focusing to make the readings clear and readable.

Permanent adjustments are same as in the Surveyor’s compass.

Surveyor’s compass

Surveyor’s compass consists of a circular brass box containing a magnetic needle which swings freely over a brass circle which is divided into 360 degrees. The horizontal angle is measured using a pair of sights located on north – south axis of the compass. They are usually mounted over a tripod and leveled using a ball and socket mechanism.

They also have two types of adjustments, temporary and permanent. Temporary adjustments are same as described in prismatic compass.

Permanent adjustments

They are done only in the circumstances where the internal parts of the prism is disturbed or damaged. They are:

• Adjustments in levels
• Adjustment of pivot point
• Adjustment of sight vanes
• Adjustment of needle

Errors in compass survey

Errors can be arising due to various reasons during the process of surveying, they are classified as:

• Instrumental errors
• Personal errors
• Natural errors

Instrumental errors

As the name suggests they are arise due to the wrong adjustments of the instruments. Some other reasons are:

• If the plane of sight not being vertical, it causes error in sighting and reading.
• If the magnetic needle is not perfectly straight or if it is sluggish, readings may not be accurate.

Personal errors

They arise mainly due to the carelessness of the surveyor. They are:

• Inaccurate leveling
• Inaccurate reading
• Inaccurate centering

Natural errors

Natural errors are occurring due to the various natural causes which affect the working of compass. It has nothing to do with the surveyor and to minimize them, some corrections in calculations applied. They are:

• Local attraction
• Proximity to the magnetic storms
• declination

Difference btwn prismatic and surveyor compass

whole circle bearing

In whole circle bearing (WCB) the bearing of a line at any point is measured with respect to a meridian. Its value varies from zero to 360°, increasing in clockwise direction. Zero is north direction, 90° is east,180° is south and 270° is west .This type of bearing is used in prismatic compass.

In reduced bearing (RB) system, bearings are measured from north or south direction towards east or west. Hence, angles are from 0 to 90° .. This system of measuring bearings is used in Surveyor’s compass and it is also known as Quadrantal Bearing (QB). The bearing measured is designated with letter N or S in the beginning to indicate whether it is from north or south. The letter E or W written after the angle indicates whether the bearing read is towards east or west, respectively. The conversion of the bearing from one system to the other system can be easily carried out by drawing a sketch to indicate WCB or RB .It may be observed that conversion table

is as given below: