Compass survey part1 English

  

What is Compass  

In surveying, a compass is an essential tool used to determine directions. It consists of a magnetic needle that aligns with the Earth's magnetic field, indicating the direction of magnetic north. Surveyors use compasses to establish bearings, which are angles measured clockwise from a reference direction, typically north. By aligning the compass needle with north, surveyors can measure angles accurately and determine the orientation of lines, boundaries, and features on a map or in the field. This information is crucial for creating accurate maps, conducting land surveys, and navigating terrain, making the compass a fundamental instrument in the field of surveying. 

Type of compass 

1) surveyor Compass 

2) prismatic compass 

3) truff compass 

1) surveyor Compass

Surveyor Compass is an old instrument for measuring directions. 

It is a circular box of brass 125 mm in diameter the periphery of which has two vane .while using they are held vertically upright. The eye vane has a vertical slit through which one view the object. The object vane's slit has a horse hair is fitted vertically which is used to aim at the target. 

Needle both the ends of the needle of survey compass are pointed. The needle can move freely on the pivot and stop in the direction of magnetic meridian.

Graduated Ring 

The graduated ring of the surveyor Compass is not attached to the needle but permanently to the box, and moves with the box. The markings on the graduated ring are in quadrant system.

The North (N) and South (S) direction have the zero marking .and the east and west have 90° marked on it .

But the location of East and West on the ring is interchanged.

2) prismatic compass 

 A prismatic compass is an essential tool used for orientation and angular measurement in surveying and construction projects. It consists of a magnetic needle housed in a transparent compass box with a prism that allows simultaneous sighting of objects and reading of bearings. This enables engineers to determine directions and angles accurately, aiding in tasks such as mapping, boundary demarcation, and construction layout.

Principle of Compass survey 

The principles of compass surveying involve ensuring accuracy and reliability in measuring directions. Surveyors must first calibrate the compass to correct for magnetic declination, the angle between magnetic north and true north. They then establish a reference direction, typically north, and measure angles clockwise from this reference. To minimize errors, surveyors must maintain the compass level and steady, avoid magnetic interference from nearby objects, and account for local anomalies in the Earth's magnetic field. Additionally, they often use multiple compass readings and cross-check with other surveying techniques to verify accuracy. These principles are essential for producing precise and reliable survey data.

Compass surveying is recommended when 

1) A large area is to be surveyed 

2) The coastline or the course of a river is to be surveyed 

3) The area is crowded 

Meridian 

The reference direction which a line makes with some reference direction known as meridian . The reference direction may be any of the following.

1) True Meridian 

2) Magnetic Meridian 

3) Arbitrary Meridian 

1- True Meridian 

True Meridian is the line or plane passing through the geographical north pole, geographical south pole and any point on the surface of the earth.

It is also known as geographical meridian the true Meridian at a station is constant.

True Bearing  

The angle between the true Meridian and a line is known as true bearing of the line .It is also known as the azimuth.

2-Magnetic Meridian....

The direction shown by a magnetic needle at any point, when it is absolutely free and uninfluenced by the local attraction, and is hung horizontally ,is called the magnetic meridian of that point. 

-Magnetic bearing 

The horizontal angle that forms between between the magnetic meridian and any survey line is called the magnetic bearing or bearing of that line 

3-Arbitrary Meridian 

The meridian that passes through any fixed point chosen as per convenience and in a favourable direction is known as Arbitrary Meridian.

Arbitrary bearing 

The horizontal angle forming between any Arbitrary Meridian and survey line is called the arbitrary bearing.