Compass Surveying Guide - With Advantages & Disadvantages

Compass Surveying | Types of Compass | Advantages & Disadvantages of Compass Surveying with Example | In this Article we are discussing what is compass chain surveying , the definition of chain surveying, how many types of chain in surveying,

Compass Surveying or Compass survey – The framework of the compass survey consists of a series of connected lines, the length and direction of which are measured by a chain and compass respectively, The process is known as compass surveying.

What is Compass Surveying?

Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain.

In compass surveying the direction of survey line called the bearing of line is defined as the angle made by the line with the magnetic meridian.

In practice compass is generally employed to run a traverse. Traverse consists of series of straight lines connected together to form an open or closed traverse.

Advantages & Disadvantages of Compass Surveying

We listed some points about advantages and disadvantages of compass surveying. In First section we explained advantages of compass surveying and then we shared disadvantages of compass surveying.

Advantages of Compass Surveying

Compass surveying is a method of surveying that uses a compass to determine the direction of lines and points on the ground. It is a relatively simple and inexpensive method of surveying, and it can be used to survey large areas.

Here are some of the advantages of compass surveying:

Portability: Compasses are relatively small and lightweight, making them easy to transport. This makes them ideal for surveying in remote areas or where access is limited.
Accuracy: Compasses can be used to measure directions with a high degree of accuracy. This is because the magnetic needle in a compass is very sensitive to changes in direction.
Versatility: Compasses can be used for a variety of surveying tasks, including locating points on a site, traversing an area, and determining the magnetic declination.
Cost-effectiveness: Compass surveying is a relatively inexpensive method of surveying. This is because compasses are relatively inexpensive to purchase, and they do not require any special equipment or training to use.

However, there are also some disadvantages to compass surveying:

Accuracy: The accuracy of compass surveying can be affected by magnetic interference from nearby objects. This can be a problem in areas with a lot of metal or electrical equipment.
Precision: Compass surveying is not as precise as other methods of surveying, such as theodolite surveying. This is because the magnetic needle in a compass is not as precise as theodolite sights.
Skill level: Compass surveying requires a certain level of skill to use accurately. This is because the user must be able to level the compass and sight the needle accurately.

Overall, compass surveying is a versatile and cost-effective method of surveying that can be used for a variety of tasks. However, it is important to be aware of the limitations of compass surveying and to use it in conjunction with other methods of surveying to ensure accuracy.

Disadvantages of Compass Surveying

Here are some of the disadvantages of compass surveying:

Accuracy: The accuracy of compass surveying can be affected by magnetic interference from nearby objects. This can be a problem in areas with a lot of metal or electrical equipment.
Precision: Compass surveying is not as precise as other methods of surveying, such as theodolite surveying. This is because the magnetic needle in a compass is not as precise as theodolite sights.
Skill level: Compass surveying requires a certain level of skill to use accurately. This is because the user must be able to level the compass and sight the needle accurately.
Time-consuming: Compass surveying can be time-consuming, especially if you are surveying a large area.
Subject to errors: Compass surveying is subject to a number of errors, including:
- Local attraction: This error is caused by magnetic fields from nearby objects.
- Tilt error: This error is caused by the compass not being level.
- Centering error: This error is caused by the needle not being centered in the compass.
- Reading error: This error is caused by the user not reading the compass correctly.

Here are some tips to improve the accuracy of compass surveying:

Use a compass that is designed for surveying.
Level the compass carefully before using it.
Sight the needle accurately.
Use a compass in a quiet environment, away from sources of magnetic interference.
Take multiple readings and average them.
Use a compass in conjunction with other methods of surveying, such as theodolite surveying.

Objective of Compass Surveying

The objective of compass surveying is to determine the horizontal direction of a line with reference to the direction of a magnetic needle. In civil engineering, compass surveying is used for a variety of purposes, including:

Locating points on a site. Compass surveying can be used to locate points on a site, such as the corners of a building or the intersection of two roads. This information can be used to create a survey map of the site.
Traversing an area. Compass surveying can be used to traverse an area, which means to measure the bearings of a series of lines that connect different points in the area. This information can be used to create a plan of the area.
Determining the magnetic declination. The magnetic declination is the angle between the true north and the magnetic north. This angle can be determined using compass surveying and is used to correct for errors in surveying measurements.
Determining the location of underground utilities. Compass surveying can be used to determine the location of underground utilities, such as pipes and cables. This information is important for avoiding damage to utilities during construction projects.

In addition to these specific purposes, compass surveying can also be used to gather general information about an area, such as the topography of the land and the presence of natural features. This information can be used to plan construction projects and to develop land.

Here are some of the advantages of using compass surveying in civil engineering:

Compute bearing of survey lines and find the angle between these lines, and
Know the procedure of compass surveying,
Know various important terms and instruments used in compass surveying,
Understand various types of error involved in compass surveying, their source and methods of correction.
It is a relatively inexpensive and easy-to-use method of surveying.
It can be used to survey large areas.
It is a versatile method of surveying that can be used for a variety of purposes.

However, there are also some disadvantages to using compass surveying:

It is not as accurate as other methods of surveying, such as theodolite surveying.
It can be affected by magnetic interference from nearby objects.
It can be difficult to use in areas with a lot of vegetation or other obstacles.

Overall, compass surveying is a valuable tool for civil engineers. It is a relatively inexpensive and easy-to-use method of surveying that can be used to gather a variety of information about an area. However, it is important to be aware of the limitations of compass surveying and to use it in conjunction with other methods of surveying to ensure accuracy.

Define Compass in Compass Surveying

Definition – The commonly used instrument for compass surveying is Compass. A compass is small instrument which consists essentially of magnetic needle, a graduated circle and a line of sight.

When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.

Compass consists of cylindrical metal box of about 8-12 cm diameter in the center of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminum ring. The ring is graduated and is read by reflecting prism. Diametrically opposite to the prism is the object vane hinged to the box side carrying a horse hair with which the object in the field is bisected.

Compass Surveying Types Of Compass, Advantages & Disadvantages Of Compass Surveying

Adjustment of Compass (Working)

Compass adjustment is the process of compensating for errors in a compass so that it can be used to accurately measure direction. There are two types of compass adjustment: temporary adjustment and permanent adjustment.

Temporary adjustment is done at each setup of the compass and is used to correct for errors that are caused by the position of the compass. These errors include:

Tilt error: This error is caused by the compass being tilted. It can be corrected by leveling the compass using a spirit level.
Pivot error: This error is caused by the compass needle not being perfectly centered on the pivot. It can be corrected by centering the needle using a centering tool.
Local attraction: This error is caused by magnetic fields from nearby objects. It can be corrected by rotating the compass until the needle is pointing to the magnetic north.

Permanent adjustment is done once and is used to correct for errors that are caused by the compass itself. These errors include:

Hard iron error: This error is caused by the compass being made of magnetic materials. It can be corrected by placing compensating magnets near the compass.
Soft iron error: This error is caused by the compass being surrounded by soft iron. It can be corrected by placing soft iron shims near the compass.

The specific steps involved in compass adjustment will vary depending on the type of compass being used. However, the general principles are the same.

Here are the steps on how to adjust a compass:

Level the compass. Use a spirit level to level the compass.
Center the needle. Use a centering tool to center the needle.
Rotate the compass until the needle is pointing to the magnetic north. This can be done by using a compass rose or by using a map and a compass.
If necessary, adjust the compass for hard iron error. This can be done by placing compensating magnets near the compass.
If necessary, adjust the compass for soft iron error. This can be done by placing soft iron shims near the compass.

Once the compass has been adjusted, it should be accurate within a few degrees.

Here are some tips for adjusting a compass:

Make sure that the compass is in a quiet environment.
Avoid touching the compass needle.
Be patient and careful when adjusting the compass.

If you are not comfortable adjusting the compass yourself, you can take it to a professional compass adjuster.

Working of compass involves three steps:

CENTERING
LEVELLING
OBSERVING THE BEARING

Centering involves to align the compass is such a way the Centre is placed vertically over the station point. It is done with the help of tripod stand and is checked by dropping a small pebble below the Centre of compass.

Levelling is done so that the graduated ring swings quite freely. It is done with the help of ball and socket arrangement and can be checked by rolling a round-pencil on the compass box

Observing the bearing once centering and levelling has been done, raise or lower the prism until the graduations on the ring are clearly visible when looked through the prism. Afterwards turn compass-box until the ranging rod at the station is bisected by horse-hair of objective vane. At this position note down the reading.

COMPASS TRAVERSING

Whenever in traversing compass is used for making angular measurements, it is known as compass traversing or compass surveying.

In compass traversing, the compass is used to determine the direction of survey lines of the framework of the traverse for measuring the angles which these lines make with the magnetic meridian.

The process of chaining and offsetting is the same as in chain surveying and running the check lines is not necessary.

Compass traverse may be closed or open. Close traverse starts from one traverse station and closes either on same or on another traverse station whose location is already known. On the other hand an open traverse starts from one station and closes at other station whose location is neither known nor established.

EQUIPMENT USED IN COMPASS SURVEYING

PRISMATIC COMPASS
MEASURING TAPE
RANGING RODS
PLUMB BOB
CHAIN
CROSS STAFF

MEASUREMENT OF INCLUDED ANGLE

In a compass (prismatic) which uses Whole Circle Bearing (0⁰ to 360⁰) system the included angle is given by

Included angle = F.B of next line — B.B of previous line at same station

PRECAUTIONS

Compass surveying is used in case of rough surveys where speed and not accuracy is main consideration. One of the biggest disadvantages of compass is that magnetic needle which gives bearing of lines is disturbed from its normal position in presence of materials such as iron-pipes , current carrying wires , proximity of steel structures , transmission lines etc. called sources of local attraction.

Principle of Compass Surveying

Principle of compass surveying lies in the fact that the readings or a graduated circle are the angles which a line makes with the magnetic meridian.

Instruments for the direct measurements:-

For Direction:-

Surveyor compass

Prismatic compass

For angle :- Theodolite, Sextant

Bearing and Angles:-

The bearing of a line is its directions relative to a given meridian.

Meridian –

A meridian is any direction such as True Meridian and Magnetic Meridian and Arbitrary Meridian.

Types of Bearing :-

Whole Circle Bearings.

Quadrantal bearings or Reduced Bearings.

Whole Circle Bearings (WCB) – In this system, the bearing of the line is measured with magnetic North or South in the clockwise direction.

Whole circle bearing

Quadrantal Bearings – In this system, the bearing of the line is measured eastward or westward from North or South whichever is near.

Quadrantal bearing

Fore & Back Bearing:-

. If the bearing of a line AB is measured from A towards B , it is known as forward bearing or Fore Bearing.

. If the bearing of the line AB is measured from B towards A it is known as backward bearing or back bearing.

Difference between Prismatic and Surveying Compass :-

S.No.	Item	Prismatic Compass	Surveyor’s Compass
1.	Magnetic Needle	Broad type.	Edge Bar type.
2.	Graduated Card	Based on WCB.	Based on QB or Reduced Bearing.
3.	Sighting Vanes	(i)The Object vane consists of metal vane with a vertical hair.(ii)The eye vane consists of a small metal vane with slit.	(i)The Object vane consists of metal vane with a vertical hair.(ii)The eye vane consist of metal vane with fine slit.
4.	Readings	Reading is taken through prism provided at the eye slit.	Reading is taken directly.
5.	Tripod	Tripod may or may not be provided.	Instrument cannot be used without a Tripod.

Local Attraction :-

Local Attraction denotes the influences which prevents the needle from pointing to the Magnetic North in a given locality. It may be due to electric wired , steel structures, railroads, underground iron pipes, etc.

The Local attraction can be detected by observation of fore bearing and back bearing of each line and finding the difference.

e.g. B.B – F.B = 180 then both stations free from local attraction. No observation errors occur.

Magnetic Declination :-

It is the horizontal angle between the true meridian and the magnetic meridian shown by needle at the time of observation. If the magnetic meridian is to right side(or Eastern Side) of the true meridian than declination is said to be Eastern or positive declination. If it is downward west of true Meridian then it is called negative or westward Meridian.