Surveying may be defined as the method of making measurements of the relative positions of natural & man-made features on the earth's surface and the presentation of this information either graphically or numerically. In other words, Surveying is the art of determining the relative positions of objects on the surface of the earth by taking measurements in the horizontal and vertical planes. The surveying may primarily be divided into two main categories: Plane Surveying, in which the curvature of the earth is not taken into account and Geodetic Surveying, in which the curvature of the earth is taken into consideration. Geodetic Surveying is also known as Trigonometrical Surveying. It should be noted that generally, for an area less than , Plane Surveying is employed. 

Prismatic Compass Surveying is very important step in creating the ecosystem that surrounds us as humans. This technique or science does measure and determines the 3D positions of particular points and distances on a surface, as well as the angles between them. Compass Surveying is essential when it comes to measuring angles.

In the following article, we will be going to study Prismatic Compass Surveying as a technique to measure angles. This article will be designed in such a way that it will be helpful for your preparation of various examinations like GATE CE, RRB JE Civil and SSC JE CE. 

What is Prismatic Compass Surveying?

Prismatic Compass surveying is a significant part of surveying that is frequently used to locate an item using both angular and linear measurements. In this case, a compass is used to measure angles, while a chain or tape is used to measure lengths.

Some priorities for prismatic compass surveying are as follows-

• If a huge area needs to be surveyed, such as the river's course line or the coastal regions.
• Chain surveying becomes impractical when there are too many details and impediments in the area to allow for triangulation.
• If the surveyor is working within a strict time frame to complete the extensive area.

But there are also some limitations to prismatic compass surveying. It is not recommended for the areas that avoid public attention and have the presence of iron ore deposits, magnetic substances like metallic or steel structures, electric cables conveying current, etc.

Principle of Compass Surveying

• Compass surveying works on the principle of measuring magnetic bearings using a prismatic or surveyor compass.
• Linear distances between survey points are measured using a chain or tape.
• The survey lines connect in a series to form a traverse, which can be either open or closed.
• It eliminates the need for creating a full triangle network, unlike traditional triangulation methods.
• This method is ideal for moderate-accuracy surveys where quick work is required, especially in uneven or obstructed terrain.

• Prismatic compass surveying relies on a technique called Traversing, which uses a network of interconnected lines. 
• The distances between the lines are measured using a chain or tape, and the magnetic bearings of the lines are measured using a surveyor compass. The creation of a network of triangles is not necessary for this survey.
• By deriving offsets from the primary survey lines, interior details are identified. Subsidiary lines may occasionally be used to locate these facts. 
• Traversing can be done by various methods like compass traverse, plane table traverse, stadia traverse, theodolite traverse, etc. 

Prismatic Compass Surveying Instruments

The various instruments in Compass Surveying are:

• Prismatic compass
• Tape
• Ranging rods
• Tripod
• Arrows
• Plumb Bob

Types of Compass in Surveying

There are mainly two kinds of Compass used in Surveying, namely Prismatic Compass, Level Compass and Surveyor Compass. 

Prismatic Compass

The prismatic compass is a magnetic compass in which there is a prism for taking observations. It consists of a circular box about 100 mm in diameter. The magnetic needle used in a prismatic compass is broad in shape. The prismatic compass is used in the calculation of whole circle bearings of the lines. Sighting of the object and reading the bearing are done simultaneously in the case of a prismatic compass. 

Parts of Prismatic Compass 

Parts of a prismatic compass are as follows: 

• Cylinder shaped metal box
• Pivot
• Magnetic needle
• Prism
• Graduation circle
• Object vane
• Eye vane
• Eye silt
• Eye hole
• Sunglasses
• Spring break
• Reflector mirror
• Focussing stud
• Lifting pin
• Lifting lever

Fig.1. Prismatic Compass

Prismatic Compass Adjustments

There are two types of adjustments of a prismatic compass: temporary and permanent. 

Temporary Adjustments 

Temporary adjustments include the following: 

• Centering 
• Levelling
• Focussing the prism

Permanent Adjustments

Permanent adjustment of a prismatic compass includes the following: 

• Adjustment of levels
• Adjustment of sight vanes
• Adjustment for sensitivity of needles
• Adjustment for straightness of needles
• Adjustment of pivot

Prismatic Compass Bearing

Fig.2. Prismatic compass bearing

Surveyor Compass

A surveyor compass is an instrument used for the measurement of horizontal angles and the bearing of a line of sight. The Surveyor compass is commonly referred to as the Circumferentor. It essentially consists of a graduated horizontal circle, a pivoted magnetic needle, and a sighting device. The size of the surveyor compass is defined by the diameter of the reading edge of the graduated ring. It generally ranges from 50 mm to 200 mm. The surveyor compass is used to determine the reduced bearings or quadrantal bearings of lines. 

Parts of Surveyor Compass 

The parts of the surveyor compass are listed below: 

• Box
• Lifting lever
• Pivot
• Glass top
• Circular graduated arc
• Sight vane
• Jewel bearing
• Lifting pin
• Magnetic needle 
• Rider
• Metal pin 
• Object vane 

Fig.3. Surveyor Compass

Surveyor Compass Bearing

The readings from a surveyor's compass are expressed in terms of bearings, which are angular measurements relative to the magnetic north or south.

Fig.4. Surveyor Compass Bearing

Compass Traverse and Its Types

A compass traverse is a series of connected survey lines measured with a compass and chain/tape to plot boundaries or features. It is used when terrain prevents full triangulation. There are two main types of compass traverse:

Level Compass 

A "level compass" typically refers to a compass that incorporates a bubble level, also known as a spirit level. The bubble level is a small, transparent tube partially filled with liquid and an air bubble.

Bearings in Compass Survey

Bearings are used to express the direction of survey lines or features. A bearing is the angle between a survey line and a reference direction, typically measured clockwise from the north direction.

Local Attraction in Compass Surveying

Local attraction refers to the deflection of the magnetic needle due to nearby metallic objects, underground cables, or iron ores. This causes incorrect bearing readings, leading to errors in plotting. Surveyors must detect and correct local attraction by comparing fore bearings and back bearings of survey lines. If the difference deviates from 180°, local attraction is present.

Common Causes of Local Attraction:

• Steel structures or rails near the compass
• Overhead electric cables
• Use of metallic instruments during reading
• Iron ores or mineral deposits in the area

Correction methods include adjusting readings using corrected bearings or avoiding the affected station altogether.

Errors in Compass Surveying

Measurements of angles and distances are made in various surveying operations. It is impossible to determine the true values of these quantities because some type of errors always creep in every measurement. The errors occur due to imperfection in instruments, due to human limitations, due to environmental changes or due to carelessness. As quality surveyors, our aim should be to minimise these errors while taking observations. Still, the errors which creep in should be eliminated, or their effects should be corrected. Errors can arise due to various reasons stated above during the process of surveying are classified as:

• Instrumental errors
• Personal errors
• Natural errors

Instrumental Errors

• Instrumental errors arise due to wrong adjustments of the instruments. 
• If the line of sight is not proper, there will be faults in sighting and reading. 
• If the magnetic needle is not properly straight, there will be faulty readings.

Personal Errors

Personal errors, as the name suggests, arise because of the carelessness of the surveyor. Carelessness here signifies; 

• Incorrect setting
• Incorrect levelling
• Incorrect centering 

Natural Errors

Natural errors are occurring due to the various natural causes which affect the working of the compass. It is not within the limit of the surveyor to minimise or eliminate them. They are:

• Local attraction
• Proximity to the magnetic storms
• Declination of needle

Limit of Accuracy in Prismatic Compass Surveying

The accuracy of a set of repeated observations is defined as the amount of closeness of

their mean to the population or distribution mean, i.e., the closeness of the mean of

observations to the true value. 

For linear measurement:

The degree of accuracy of linear measurement is usually expressed as the ratio of the

probable error and the measured distance.

For angular measurement:

For angular measurements, the degree of accuracy is usually expressed as k× √N, where

N = Number of angles measured

The angular measurement taken in compass surveying has the error should not be more than 5 minutes. Hence, the limit of accuracy is not more than 5 minutes.

Unit of Measurement in a Compass

The unit of measurement typically used in compass surveying is the degree (°). A compass can measure angles in degrees, and bearings are expressed in degrees as well.

Booking Field Notes in Compass Surveying

Accurate field note booking is crucial in compass surveying. It involves recording the name of lines, observed bearings, and measured distances in a tabular format for plotting and calculations. Below is a basic format of a compass field book:

Difference Between the Prismatic Compass and Surveyor Compass

The difference between prismatic compass and surveyor compass is discussed below. 

Advantages and Disadvantages of Prismatic Compass Surveying

Compass Surveying comes with several advantages and disadvantages of its own. Let's look at them; 

Advantages of Prismatic Compass Surveying

• They are portable and lightweight.
• They have fewer settings to fix it on a station
• The error in direction produced in a single survey line does not affect other lines.
• It is suitable to retrace old surveys.
• No electric power is required for the operation of compass surveying. 

Disadvantages of Prismatic Compass Surveying

• It is less precise compared to other advanced methods of surveying.
• It is easily subjected to various errors such as errors adjoining to magnetic meridian, local attraction etc.
• Imperfect sighting of the ranging rods and inaccurate levelling also cause an error.

Application of Prismatic Compass Surveying

The application of compass surveying are as follows: 

• It helps to measure the bearings of lines.
• It helps to measure the included angles.
• Declination and dip can be measured using compass surveying.
• Differences in elevations can be computed using the angle measurements.

Conclusion

We sincerely hope that the article was well formatted in accordance with the syllabus and was clear enough to deliver you the information regarding Compass Surveying in detail. For more such study notes, please download our Testbook App. 

We at Testbook have also launched GATE Civil Coaching and AE/JE Civil Coaching. Enrol now and reinforce your preparation.