1.14.2.1. Prismatic Compass

The magnetic compasses used in surveying may be classified as:

1. Prismatic Compass
2. Surveyor's Compass
3. Trough Compass
4. Tubular Compass The compass works on the principle that a freely suspended magnetic needle takes the direction of the magnetic force at a place. It provides a reference direction with respect to which all angles can be measured. A magnetic needle is generally made perfectly symmetrical and is supported on a hard, pointed pivot so that it rotates freely. The north pole of a magnetic needle freely suspended gives the direction of the magnetic lines of force, and is used as a reference direction in compass surveying.

A magnetic compass is a tool that helps surveyors determine direction based on magnetic forces. It consists of a magnetic needle that spins freely, indicating the direction of the Earth's magnetic field. There are different types of compasses, including the prismatic compass, which is very popular among surveyors, and the surveyor's compass, which is not commonly used. The prismatic compass has specific features that make it suitable for surveying, which we will discuss further.

Think of a magnetic compass like a friend who always knows where North is. Just like you might ask a friend for directions, surveyors use compasses to find their way and measure angles during their work.

A prismatic compass is very popular for survey work, and the Surveyor’s compass is rarely used. However, the principle of operation of both the compasses is the same, but they are made differently. In Surveyor's compass, the graduations are done in quadrantal system, there is no prism attachment and the reading are taken with respect to north end. Trough and tubular compasses are used as add-ons to other instruments. They are not complete surveying instruments by themselves but are used only to indicate magnetic meridian direction to set the instrument in order to derive the magnetic bearing of the lines.

The prismatic compass stands out due to its unique design, which includes a prism that allows surveyors to take precise readings and sight at distant objects. Unlike the surveyor's compass, which uses a quadrantal system and lacks a prism, the prismatic compass offers clearer visibility and accuracy in measurements due to how it displays graduated angles. Trough and tubular compasses are simpler tools that support surveying tasks but do not act as standalone instruments.

Imagine the prismatic compass as a high-tech smartphone camera versus an old film camera. The smartphone lets you capture clearer and sharper images (or in this case, accurate measurements), while the film camera can still take pictures but isn’t as effective in certain situations.

A prismatic compass consists of an eye vane from where sighting is done is to the other station. An object vane is attached to metal frame, diametrically opposite to the eye vane. It is hinged at the bottom for folding over the glass cover when the compass is not in use. A fine silk thread or hair is fitted vertically in the centre of object vane frame, which is used to bisect the other objects/points.

The prismatic compass is designed with specific components such as an eye vane for taking sights at distant points and an object vane for alignment. When the surveyor looks through the eye vane, he can easily sight and align the crosshair with a target object. This optical setup facilitates precise angle measurement, which is crucial in surveying.

Consider how an archer aims at a target with a bow and arrow. Just like an archer uses sights on their bow to line up their shot accurately, surveyors use the sighting features of the prismatic compass to ensure they are measuring angles correctly.

The following steps are required to use the Prismatic compass:

1. Setting up and centering: Fix the Prismatic compass on the tripod, and place the tripod over the station. Since, there is no centering device in the compass, it is centred over the station either with a plumb bob or by dropping a small piece of stone from the centre of the tripod. Tripod legs are moved to carry out proper centring over the station.
2. Levelling: Level the compass by approximation with the help of ball and socket arrangement so that the magnetic needle can move freely in a horizontal plane, after unfolding the objective vane and eye vane. There is no bubble tube attached to the compass for accurate levelling, hence it is carefully done with eye judgement only.
3. Sighting the object: Direct the object vane towards the object/station whose bearing is to be measured. Bisect the object/station with the vertical hair on object vane while looking through the hole of the eye vane. The metal circular plate of compass is rotated clockwise or anticlockwise so that the line of sight is passing through the object/station sighted and the cross hair of the object vane and also the view hole of eye vane.
4. Taking readings: The prism near the eye vane has to be adjusted with the help of a screw for a clear vision of the graduated ring readings. Once the magnetic needle comes to rest, record the reading at the point on the ring corresponding to the vertical hair seen directly through the slit in the prism holder. The prismatic compass will give the whole circle bearings of the lines.

Using a prismatic compass involves several clear steps. First, it is important to set it up properly on a stable tripod to ensure accurate readings. Next, leveling is crucial to ensure the compass needle moves freely. Once it is properly aligned, the surveyor directs the object vane towards the target object, carefully centering it through the eyepiece. Finally, once the needle stabilizes, readings can be taken and recorded for angle measurement.

Setting up a prismatic compass can be compared to setting up a camera for a photo shoot. Just as you must carefully balance and focus your camera to capture a great shot, surveyors have to level and align their compass to take accurate measurements.

Disturb the line of sight, again align the line of slight on the same object/station, and take second reading. Repeat it at least one more time to take third reading. Take the average of at least three readings to improve the accuracy of observations.

In order to achieve the highest accuracy in measurements when using the prismatic compass, it is essential to take multiple readings of the same angle. This method helps identify any potential errors or fluctuations in readings, allowing the surveyor to calculate an average value which is much more reliable.

Imagine you are trying to guess how tall a tree is by measuring the shadow it casts at different times of the day. Each time you measure, the angle of the shadow might slightly change due to the sun's movement. By taking multiple measurements and averaging them, you can get a more accurate estimate of the tree's height.

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