Prismatic compass is a type of compass and in prismatic compass, there is a light and simple instrument used for rough surveys where too much accuracy is not required and prismatic compass is used to the fine bearing of the traversing and included angles between them.
This compass can be used as a hand instrument or can be fitted on a tripod and is a portable magnetic compass and the greatest advantage of the prismatic compass is without changing the position the sighting and reading can be done simultaneously.
This type of compass is made into a box that is circular and is pointed steel pivot is used in the center of the compass box.
To run a traverse line the compass is generally used and with respect to magnetic north compass calculates bearings of lines. Prismatic compass essentially consists of a prism which is used for more accurate observations.
The Least Count of this compass is 30 minutes that means the instrument can show readings only in multiples of 30 minutes and Least Count is a minimum value that any instrument can read.
There is also a compass known as Liquid Prismatic Compass and this is a military-based compass because in World War 2 it was first introduced in service for the Allied Forces and the liquid used in it is purified kerosene and construction is made of brass.
Adjustments Of Prismatic Compass:
There are two types of this compass such as;
Fixing the compass exactly over the station centering is the process of fixing and by adjusting the tripod legs centering is done usually. To judge the accurate centering of instruments over the station a plumb-bob is used.
if the instrument is used as in hand or mounted over a tripod the instrument has to be leveled. The graduated disc should swing freely and appears to be completely level in reference to the top edge of the case if it is used as in hand and they usually have a ball and socket arrangement for leveling purpose if the tripod is used.
Focusing the prism;
To make the readings clear and readable prism can be slide up or down.
In circumstances where the internal parts of the prism is disturbed or damaged the permanent adjustment are done and there are some points in permanent adjustment such as;
Adjustments in levels.
Adjustment of pivot point.
Adjustment of sight vanes.
Adjustment of needle.
Major Parts Of Prismatic Compass:
There are following major parts used in this compass such as;
Sliding arrangement for mirror.
Procedure Of Prismatic Compass:
There are following points in the procedure of survey on this compass as given below:
For taking a reading with this compass, in hand the same may be held and turned till the ranging rod on the next station and when seen through the eye-vane the next station is bisected by the cross-hair.
As nearly level as can be judged the instrument should be held near and if the needle does not oscillate it means that it is touching the glass lid and not been vertically held.
The reading is taken when it should be held properly and the broken knob pressed lightly and this gives the bearing of the line joining the point vertically. The compass is fixed on the tripod which carries a ball and socket device.
The compass can be turned in a horizontal plane and clamped in any position with the help o ball and socket.
The compass is leveled and for taking a reading the compass is fixed on the tripod and centered over the station and the compass is centered on station A to take the bearing of line. The compass box is turned so that the raging rod at station B is bisected.
Then the needle comes to rest and to cut the image of the graduated ring is noted by the reading at which the hairline produced appears.
Up to 30 minutes of readings may be accurately taken and in 15 minutes estimate may be taken.
30 minutes is the least count of the prismatic compass.
Errors In Prismatic Compass:
There are following errors in the compass surveying such as;
There are some instrumental errors such as;
The errors are caused in sighting and reading if the plane of sight not being vertical.
Reading may not be accurate if the magnetic needle is not perfectly straight.
There are some personal errors such as;
There are also some natural errors such as;
Proximity to the magnetic storms.
Advantages Of Compass Surveying:
There are following advantages of the compass surveying such as;
They are lightweight and portable.
To fix it on a station they have fewer settings.
The error produced in a single survey does not affect other lines.
To retrace surveys it is suitable.
Disadvantages Of Compass Surveying:
There are also some disadvantages of compass surveying such as;
As compared to other advantages of surveying it is less precise.
To various errors like errors adjoining to magnetic meridian, local attraction etc, it is easily subjected.
The errors are also caused by imperfect sighting of ranging rods and inaccurate leveling.