A compass is an instrument which is used in survey to measure the direction of object. The compass measures the direction of object with reference to magnetic meridian.
There are two types of meridians one is true meridian and other is magnetic meridian.
A true meridian is a line of intersection of plane intersects the earth surface through north and south geographical poles. The true meridian always remains constant.
The magnetic meridian is a line which shows the direction of meridian on freely suspended magnetic needle uninfluenced by local attractive forces. The magnetic meridian is not constant, and it is affected due to the earth magnetic force and its value changes in different times and in different regions of earth.
Types of Compasses
- Prismatic Compass
- Surveyor Compass
Prismatic compass was invented in 1814.It is a small circular box of 85mm to 110 mm diameter with readings written on it. Prismatic compass is used during compass survey in order to find the position of objects on earth surface. The angular measurement is found by prismatic compass and linear measurement is determined by using chain or tape. It can either be used as hand instrument or mounted on tripod. Prismatic compass is very beneficial when rough angular measurements are needed where speed is more important consideration than accuracy.
Parts of Prismatic compass
Main parts of Prismatic compass are as follows
- Compass box
- Lifting lever
- Agate cover
- Magnetic needle
- Graduated ring
- Focusing screw
- Reflecting mirror
- Glass cover
In the center of compass box there is a pointed steel pivot. A magnetic needle of broad form is balanced over the pivot and a light aluminum ring is fixed on which graduations are marked in degrees and half degrees. The graduations are written such that zero is at south and 180 degrees at north. Similarly, 270 degrees at east and 90 degrees at west. The bearing observed with prismatic compass is whole circle bearing. All the readings are written on aluminum ring and compass box is protected by glass lid cover.
Also Read: Instruments used in chain surveying
Procedure of Survey on Prismatic compass
The observations are taken by prismatic compass by keep it straight in hand or by adjusting it on tripod stand and turned it till ranging rod on the station is bisected by cross hairs when surveyor see through eye vane. The instrument should be level and it can be judged. If the needle does not oscillate it means that it is touching the glass lid and it is not level. Prismatic compass should be held properly, and brake knob pressed lightly. Then reading is taken which is accurate without chances of error. This gives the bearing of line with reference to magnetic meridian.
Least count of Prismatic compass is 30 minutes it means that it can measure readings which are multiple of 30’
Uses of Prismatic compass
The prismatic compass is light and simple, and it is used for rough traverse surveys, preliminary surveys of roads etc. where time and accuracy are not the main consideration.
Surveyor compass was originally a colonial instrument designed for use in places such as America and Ireland where land was plentiful. The first instrument of this type appeared in 1610 in William Folkingham, Art of Surveying London. The surveyor compass is also used to find the bearing of line with reference to magnetic meridian. The major parts and functions of surveyor compass are almost same as prismatic compass. There are small differences between them.
- The prismatic compass consists of reflecting prism along with eye vane while in surveyor compass prism is replaced by narrow vertical slit.
- The magnetic needle of prismatic compass is of broad form. While in Surveyor compass it is of edge bar type.
- The prismatic compass consists of graduations from zero at south end and reading increase in clockwise direction up to 360 degrees. While in surveyor compass the ring is divided into four quadrants and each quadrant consists of graduations from 0 to 90 degree.
- The bearing observed in prismatic compass is whole circle bearing while the bearing observed in surveyor compass in quadrantal bearing.
Also Read: What is Compass Surveying?
Procedure of Survey on Surveyor compass
The procedure of survey on surveyor compass is same as discussed above for prismatic compass.
Least count of Surveyor compass is 15 minutes.
Difference Between Prismatic Compass and Surveyors Compass
|It consists of magnetic needle of broad form.
|It consists of magnetic needle of edge bar type.
|The graduated ring is attached with magnetic needle in prismatic compass, and it does not rotate when box is rotate for line of sight. The graduations have 0° at South end 90° at West, 180° at North and 270° at East. When the needle is in north direction the reading under the prism should be zero. It is so because the prism is placed exactly opposite the object vane and the south end will be under the prism while the needle points north. Hence, the Zero is placed at the south end then the ring is graduated clockwise from it
|The graduated ring in surveyor compass is attached with the box and it rotates when we sight an object for reading. The graduations have 0° at North and South. Similarly, 90° at East and West. The letters E and W are interchanged from their true positions to read the bearing in its proper quadrant. As the graduated ring is attached to the box, it moves with the sight. If the hearing of a line in the first quadrant is to be measured, since the letters E and W are reversed from their natural positions, the proper quadrant NE will be read.
|A prism is attached with eye vane. The readings are observed while viewing through prism is case of prismatic compass. In prismatic compass surveyor can see the reading while sighting .it means that sighting and reading can be done at the same time.
|There is no prism in surveyor compass and for taking reading surveyor will see through top of glass box. In surveyor compass readings are taken through glass box so sighting and reading cannot be taken at the same time.
|Prismatic compass survey can also be done by adjusting the instrument properly in hand without tripod stand.
|Surveyor compass survey is done by fixing it at tripod stand and then object is sighted for reading.
|The bearing observed in prismatic compass is whole circle bearing.
|The bearing observed in surveyor compass is quadrantal bearing.
|The least count of prismatic compass is 30 minutes.
|The least count of surveyor compass is 15 minutes.
|Graduations in prismatic compass are inverted and prism is attached to read the readings correctly.
|Graduations in surveyor compass are not inverted.