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Surveyor Compass
Surveyor compass is a very popular nautical instrument. Usually used with a chain when traversing, the surveyor's compass is an instrument for determining the horizontal direction of a line with reference to the direction of a magnetic needle. This compass is considerd to be an accurate instrument for surveying, and is generally used on a stand or tripod.
The compass consists of a long, thin, pointed needle, made of magnetized steel and there is a small conical-shaped bearing of agate material at the centre. The end of this needle which points north is the north end and the other end is the south end. This is differentiated by a small metal pin which passes horizontally through the needle near its north end.
How does it work?
The compass is set over a point on the line of the bearing which is desired and the compass is leveled. The peep sights are directed to some point along the line, and the needle is lowered on to the pivot. The bearing is determined by observing the angular value on the graduated scale opposite the end of the needle, and the quadrant is determined by observing between which letters on the compass-box the end of the needle lies.
In the surveyor instrument, the needle remains in a fixed position (the position of the magnetic south to north line). The two upper graduated circles, together with the line of sight, rotate about the vertical axis.
Varieties of surveyor compass
Although there are many varieties of surveyor compasses, they all fall into two main categories: either a "plain compass" or a "vernier compass". A surveyor plain compass has no adjustment and always reads magnetic north. A surveyor vernier compass has an adjustable scale that allows for the "setting off" of the magnetic declination and the compass can then directly read true north.
