Introduction and Measurement of Angles in Compass Surveying
Introduction and Measurement of Angles in Compass Surveying, Embarking on a surveying adventure requires a solid foundation in the art of angle measurement with a compass. In this blog post, we unravel the intricacies of Compass Surveying, focusing on the introduction and accurate measurement of angles. Follow along as we provide practical insights and essential tips to sharpen your skills in this crucial aspect of surveying.
Introduction to compass surveying
- A network of control points is made using the linear measurement done by a chain in chain surveying.
- Whereas in compass surveying, these control points are plotted by using both Linear and angular measurements, and this process is called "Traversing".
Difference between chain surveying and compass surveying
| S.No | Chain Surveying | Compass Surveying |
| 1 | Only linear measurement is taken | Both linear and angular measurement are taken |
| 2 | Here framework consists of network of triangles. | Here framework consists of connected lines forming an open or closed polygon. |
| 3 | Accuracy is checked by check lines. | Accuracy is checked by methods of adjustment. |
| 4 | Suitable for small, flat area where high precision is not required. | Suitable for large area where high precision is required. |
Notes:
Traverse survey: It is the type of survey in which a framework is formed by a number of connected survey lines, and the direction and length of the survey line is measured with the help of an angle (or direction) measuring instrument and a tape or chain, respectively.
Traverse survey
The framework consists of a number of connected lines, forming an open or closed polygon. Based on the type of framework, the traverse survey is further classified as:
a) Close traverse:
- A closed traverse starts from a station and closes either on the same station or another station whose location is already known.
- Loop traverse: Closed traverse, which starts and ends at the same point whose location is known.
- Link traverse: Closed traverse, which starts and ends at different points and whose location is not known.
- In closed traverse detection and adjustment or balance of errors is easy.
b) Open traverse:
- An open traverse starts at an established station whose location is known but closes at an unknown or unestablished station.
- Open traverse is used for the survey of roads, railways, canals, tunnels etc.
- Traversing can be done by any of the following methods:
- a) Compass traverse
- b) Plane table traverse
- c) Theodolite traverse
- d) Stadia traverse
NOTES:
Compass traverse: In compass traverse, angles are measured with a magnetic compass and linear measurements are done with chain or tape.
Measurement of angles:
A "MERIDIAN" is the reference line used to show the direction of the traverse line by a horizontal angle between meridian and traverse line.
Types of Meridians
true meridians
- A true meridian passing through a point on the earth's surface is a line which is obtained by the intersection of earth's surface with the plane passing through that point and the geographical north and south poles.
- It can also determine by astronomical observations.
- True meridian at a point is always fixed.
- True meridian through various points/ stations are not parallel to each other but will converge at the poles. However, for the small survey these are considered to be parallel to each other.
magnetic meridians
- Magnetic meridian at a point is the direction indicated by a freely suspended balanced magnetic needle.
- It indicates the direction of magnetic north and magnetic south.
- Magnetic poles are not fixed hence the magnetic meridian is also not fixed and changes with time.
Grid meridian
- For the survey of a country, the true meridian passing through the central place is sometimes used as a reference meridian for the entire country and is termed as grid meridian.
- The meridian of all other places is considered to be parallel to this grid meridian.
arbitrary meridian
- The meridian taken in any convenient arbitrary direction is termed as arbitrary meridian.
- Example: Chimney, Church, Railway Station etc.
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