Description

Prismatic Compass for Survey

Surveying is the art and science of determining the relative positions of various points or stations on the surface of the earth by measuring the horizontal and vertical distances, angles and taking the details of these points and by preparing a map or plan to any suitable scale.
The survey is based on the instrument known as prismatic compass. In this survey prismatic
compass is used to measure the angles known as bearings and the distances are measured with the measuring tape. Thus the position of the object is determined by measuring angular and linear distances.
A prismatic compass is one of the most convenient and portable forms of the magnetic compass. It can be held in hand or in a tripod stand for carrying out the measurement. The line of sight is defined by the object vane and the eye vane. A prismatic compass helps to conduct both sightings and reading simultaneously. The compass is initially held over the starting station of the survey line and the adjustments are provided. The magnetic meridian is thus obtained and then starts to take the measurements by sighting to the next station. The readings increase in clockwise direction i.e. from the south (0 degrees) to West (90 degrees) and North (180 degrees) and East (270 degrees)
Instruments Required For Prismatic Compass Survey

Following instruments are required for conducting a prismatic compass survey
Prismatic compass with tripod stand
⦁ Measuring Tape
⦁ Arrows
⦁ Ranging Rods
⦁ Plumbing Fork or Plumb Bob
⦁ Field Book

Temporary Adjustments for Prismatic Compass
The temporary adjustments usually followed for prismatic compass are:
1. Centering
2. Leveling
3. Focusing the Prism

Centering: In this step, the instrument is kept exactly over the station point. This can be done
either by adjusting the tripod stands or by using a plumb-bob. Sometimes, a pebble can be freely
dropped from this center to the bottom of the instrument to check the centering.
Leveling: The instrument must be held such that the graduated disc swings freely and when
viewed from the top edge it must appear level. If it is not used as a hand instrument, a tripod is
used to support the instrument for leveling.
Focusing the Prism: Till the readings are observed sharp and clear, the prism attachment is
slid up and down for proper focusing.
Prismatic Compass survey is carried on in two ways:
⦁ Open Traverse Survey
⦁ Closed Traverse Survey

A traverse is a series of connected lines whose lengths and directions are to be measured and the
process of surveying to find such measurements is known as traversing.