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.

 

 

The prismatic compass consists of a compact cylindrical metal box (8–12 cm diameter) with the following main parts:

Component	Description
Compass Box	A non-magnetic aluminum or brass circular container housing all elements,
Magnetic Needle	A lightweight, magnetized aluminum strip (broad at ends) pivoted at the center, aligning with the magnetic meridian.
Graduated Ring	Attached to the needle, marked in degrees (least count: 30 minutes or 0.5°), enabling WCB readings.
Prism	Triangular glass prism at the eye vane for reflecting and inverting the scale image for easy reading while sighting.
Sighting Vanes	Objective vane (front sight with slit) and eye vane (rear sight with prism and peep hole) for aligning the line of sight.
Lifting System	Includes a lifting lever, pin, and brake to lock the needle during transport.
Tripod Stand	Optional ball-and-socket joint for mounting, ensuring quick leveling over survey stations.
Accessories	Focusing stud for prism adjustment, sun shade, and dust cover.

 

Product Overview

Surveying is the art and science of determining the relative positions of points on the Earth’s surface by measuring horizontal & vertical distances, angles, and details — then preparing accurate maps or plans to scale.

The Prismatic Compass is the core instrument for compass surveying. It measures bearings (angles) while distances are recorded using a measuring tape — enabling precise determination of object positions through angular + linear measurements.

Why This Prismatic Compass Stands Out

Feature	Advantage
Simultaneous Sighting & Reading	Prism reflects the scale — read bearings without moving your eye
Whole Circle Bearing (WCB)	Clockwise from 0° (South) → 90° (West) → 180° (North) → 270° (East) → 360°
Handheld or Tripod Mount	Use in rough terrain or for high-precision station work
Portable & Lightweight	Easy to carry in field surveys
High Accuracy	Least Count: 30′ (0.5°)

Complete Survey Kit Included

Item	Description
Prismatic Compass	Aluminum body, sharp prism, magnetic needle, graduated ring
Adjustable Tripod Stand	Ball & socket joint for quick leveling
30m Measuring Tape	Fiberglass, waterproof, non-stretch
Ranging Rods (2 pcs)	2m each, red/white bands
Plumb Bob + Plumbing Fork	For accurate centering
Survey Arrows (10 pcs)	Mark chain points
Field Book	100 pages, water-resistant
Protective Carrying Case	Shockproof, branded

How to Use – Step-by-Step (Temporary Adjustments)

1. Centering

Place the compass exactly over the station point using:

• Tripod legs adjustment
• Plumb bob (or drop a pebble to confirm alignment)

2. Leveling

Ensure the graduated disc swings freely and appears level from the top edge. Use tripod for stable support.

3. Focusing the Prism

Slide the prism up or down until the scale appears sharp and clear.

4. Taking a Bearing

• Sight the next station through Object Vane (slit) and Eye Vane (prism)
• Magnetic needle aligns with magnetic meridian
• Read bearing directly from prism reflection (clockwise)

Types of Surveys You Can Perform

Survey Type	Use Case
Open Traverse	Linear projects (roads, pipelines)
Closed Traverse	Boundary surveys (returns to starting point)

Traverse = Series of connected lines with measured lengths & directions

Ideal For

• Civil Engineering Students (BUET, KUET, RUET, Diploma)
• Professional Surveyors (LGED, RHD, SOB, BWDB)
• Land Developers & Contractors
• Infrastructure Projects (bridges, highways, canals)
• Floodplain & Delta Surveys (Padma, Jamuna, Meghna regions)

Technical Specifications

Specification	Detail
Diameter	100–120 mm
Material	Non-magnetic aluminum
Graduation	0°–360° (WCB)
Least Count	30 minutes (0.5°)
Prism	High-clarity optical glass, adjustable
Weight	~800g (with tripod)
Standard	IS 2172:1985 compliant