One-liner Surveying
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Compass surveying -
The principle of compass surveying is traversing which involves a series of connected lines.
Compass surveying - It measures the angle between the magnetic meridian and a given line.
Compass are classified into two types :-
- Prismatic compass
- Surveyors compass
1. Prismatic compass :-
➤ The angles are measured from 0° to 360°. This system is called whole circle bearing system. [Civil engg competitive exam]
➤ Bearing of line measured in clockwise direction from North.
➤ Sighting the object and taking readings are done simultaneously.
2. Surveyors compass :-
➤ The angle are measured ranges from 0° to 90°. This system is called Reduced bearing or quadrantal bearing system
➤ Bearing of line measured east or west from North or south which ever nearer.
Local attraction Compass surveying :-
- The magnetic needle is deflected from its normal position when it is place near to external attractive forces such as steel structure and magnet such as disturbing influence is known as local acceleration.
- If the difference between fore bearing and back bearing is 180°. There is no local attraction of both the station.
Sources of errors in compass :-
Instrumental error - It is related to needle and graduated circle.
Errors of manipulation and sighting - Sighting, levelling and readings.
Errors due to external influences - These is related to local attraction and declination.
Fore bearing - The bearing of a line in the direction of progress of survey is called fore bearing or forward bearing. [Concrete Technology]
Back bearing - The bearing of a line opposite in direction is known as back bearing. It is also known as reverse bearing.
Types of meridian :-
- True meridian - The true meridian at a point is passing through that point and geographical North and South poles of earth surface.
- Magnetic meridian - Magnetic meridian at a point is the direction of a freely floating and balanced magnetic needle free from all other attractive forces.
- Grid meridian - For a survey of a state the true meridian of central place is taken as reference meridian for whole circle bearing is called grid meridian.
- Arbitrary meridian - Meridian taken in any convenient direction towards a permanent and prominent mark or signal is known as arbitrary meridian.
Types of bearing :-
- True bearing - The horizontal angle between the true meridian and a given line is called true bearing or Azimuth of a line.
- Magnetic bearing - The angle between the magnetic meridian and a given line is called magnetic bearing or bearing of a line.
- Grid bearing - Horizontal angle between grid meridian and a given line is called grid bearing. [Civil engg competitive exam]
- Arbitrary bearing - Horizontal angle between arbitrary bearing and a given line is called grid bearing.
Magnetic Declination - The horizontal angle between the magnetic meridian and true meridian is called declination or magnetic declination.
Dip - Dip is the inclination of the magnetic needle with the horizontal. It is 0° at the equator and 90° at the poles.
Isogonic line - Line joining the same points of declination.
Agonic line - Line joining points of zero dip. [surveying]
Q. Classification of Survey ?Types of declination :-
The declination at a place does not remains constant but it changes from time to time.
- Secular variation
- Annual variation
- Diurnal variation
- Irregular variation
1. Secular variation :-
- secular variation of declination occur over a long period of time 100 year or 150 year.
- The annual rate of change of secular variation is generally vary between 5' to 10'.
2. Annual variation :-
- The variation of declination in one year is called annual variation.
- These value changes from 1' to 2'
3. Diurnal variation :-
- The variation of declination in one day is called diurnal variation.
- It ranges from 3' to 12'
4. Irregular variation :-
- The variation of declination occur due to natural phenomena like Earthquake, Volcanic eruptions. [Building Material]
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