What is Compass Surveying?

Saad Iqbal | 🗓️Modified: December 15, 2017 | ⏳Read Time: 7 min | 👁Post Views: 1063

Compass surveying can be defined as the method of surveying in which the directions of the survey lines is determined by means of a compass and also by using tape or a chain. This measurement is directly done on the surface of the earth. The use of compass as a navigator is a practice followed for centuries. This was the only means through which directions and the horizontal measurements are made, before the invention of a theodolite or sextant.

What is Compass Surveying?

Even after different findings in the field of surveying, this method of compass surveying is still used for preliminary measurements in engineering surveying. Most of the land surveying is conducted by compass surveying by the civil engineers and surveyors which helps the foresters, geologists etc.
Initially, the theodolites where equipped with a compass which made the same heavy. New theodolite does not have a compass in it.

Mainly the compass surveying method make use of a compass to carry out the angular measurements. The compass surveying procedure is done through traversing.
Magnetic Compass in Compass Surveying

The magnetic meridian is the basis on which the magnetic compass measures the directions. This will require the magnetized needle, a line of sight and a graduated circle. The important types of compass used in compass surveying are the:

• Prismatic Compass
• Surveyor’s Compass
• Transit compass

Principle of Magnetic Compass: The basic principle of working of a magnetic compass is that a magnetized steel or an iron that is suspended on a pivot at its center which lets it to oscillate freely about the vertical axis, will take the direction of the magnetic meridian of with respect to that place.
The basic parts of any magnetic compass used in surveying are the magnetic needle, Graduated ring, sighting vanes, reading system and a tripod to support the compass.

Principle of Compass Surveying

The main principle of compass surveying is traversing. Traverse, a series of connected lines is measures by a prismatic compass. Compass surveying is mainly employed in areas that is free from local attraction. This can be used for surveying the land area with undulations which is crowded and large.

Every bearing at each of station can be observed by conducting the compass survey by letting the needle to float. This approach is known as loose-needle or free surveying. But the same can be carried out with a compass with a Vernier fitted that is connected with the line of sight and moves over a fixed graduated circle. This method is called as fixed-needle or fast – needle surveying. The use of compass in compass surveying is found easy and portable which makes it suitable for exploratory surveys and reconnaissance. This bring a great application for the preliminary route surveys of lakes, streams, survey of rivers and for topographical works.

Traversing in Compass Survey 

One of the main applications of compass surveying is traversing. Traverse can be defined as a series of connected straight lines each will join at two points on the ground. The length and directions of these lines are determined from the field measurements. Compass surveying is one of the widely practiced method to determine the relative locations of the points.

The traverse can be of two types:

• Open Traverse
• Closed Traverse

Open Traverse: A traverse that never ends or returns to the starting point or the starting station or neither ends at a known station is called as an open traverse. The figure-2 below shows the pathway of an open traverse.

Fig.2. Open Traverse

Route surveys mainly employ open traverse. This method does not have error checking methods, so measurements are repeated to avoid mistakes.

Closed Traverse: This type of terrace will end the work at the starting point itself hence forming a polygon. This is closed both geometrically and mathematically. Ending the survey at a point that is known before will also make it a closed traverse. The figure -3 below shows examples of closed traverse.

Fig.3. (a) A closed traverse that is geometrically and mathematically closed (b) Closed traverse that is geometrically open but closed mathematically.

The closed traverse will provide a check on the angles and the distances. This method will help in the property surveys, construction surveys, topographic surveys and control surveys. 

The traverse surveys will determine the interior angles, the deflection angles, the angles to the right, bearing and azimuths.

Compass Traversing – Fieldwork

The two fieldworks employed in compass surveying are:

1. Measurement of the Bearing of all the Survey Lines
2. Measurement of length of all the lines using a tape or a chain

The prismatic compass is mainly employed for compass surveying in order to measure the angles. This will measure the forward and the backward bearings. This measurement will help in eliminating the errors that is caused due to local attraction. 

While conducting survey, the traverse stations have to be selected carefully in order to satisfy the usual conditions. A field notebook is kept in order to take the observations.

Fore Bearing and Back Bearing in Compass Surveying

Every line in a traverse is defined by two bearings. They are the Fore bearing and the back bearing. The bearing measured is represented in the Whole Bearing System (WCB) and these differ by 180 degrees. 

Fore or forward bearing (F.B) is the bearing of the respective line in the direction of progress of the survey. The bearing that is measured in the opposite direction of survey is called as the back bearing (B.B).

Fig.4.The fore Bearing and Back bearing conducted on a line AB.

As shown in fig-4 above, the bearing taken in the direction of AB is called as Fore bearing and the measurement taken in the direction of BA forms the back bearing as shown in the figure-4. 

Temporary Adjustments in Compass Survey –  Using a Prismatic Compass

The figure-5 below shows the schematic diagram of a prismatic compass. The adjustment of the prismatic compass in tripod will involve the following procedure:

• Centering
• Levelling
• Focusing

Centering: The procedure of centering will involve the placing of a compass over the station where the bearing has to be determined. Dropping a pebble from the bottom side of the compass will help to know the correct position. If the pebble falls correctly over the station then the centering is correct. If not correct, the adjustment of the tripod will help in correcting the position.

Fig.5.Prismatic Compass

Levelling: Free swing of the graduated circular ring in the prismatic compass is achieved by the leveling procedure. The tripod possesses a ball and socket arrangement that helps to achieve a proper level of the compass.

Focusing: Until we see the aluminum ring clearly, the prism is moved up or down. The vision of the observer is controlled by the position of the prism.