A Compass generally consists of a (a) Magnetic Needle (b)Graduated Circle(c) A Line of Sight
Types of Compass
It is very valuable instrument and is commonly used for rough survey, when speed and accuracy is not the criteria, as for example rough traverse or preliminary survey for a road, it is less accurate then a theodolite, it is unreliable in magnetic rocks or iron ore. It is used for finding whole circle bearing least count is 30min.
The surveyor’s compass was formally much used in land surveying, but now it is little used. In generally it is similar to a prismatic compass except that it has another sight having a narrow vertical slit in place of the prism, and that it carries an edge bar needle on a pivot.
Sources of Error in Compass Traversing.
The needle not perfectly straight.
The pivot not being the centre of the graduated circle.
The needle lost its magnetism.
The picot being dull.
Irregular motion of needle due to the dip of needle.
The plane of sight not being vertical.
The graduated circle not being horizontal.
The line of sight not passing through the centre of graduated rings.
The vertical hair being too thick or loose.
Error of Manipulation and Sighting
Inaccurate centering of the compass over the station occupied.
Inaccurate leveling of the compass box when the instrument is set up.
Imperfect bisection of the ranging rods at the stations or other object.
Carelessness in reading the needle or in reading the graduate circle through the prism in a wrong direction.
Carelessness in recording the observed reading
Errors due to External Influences
Magnetic changes in the atmosphere no a cloudy or stormy day.
Irregular variations due to magnetic storms, earthquakes, sunspots, lunar perturbations etc.
Variations in declination, viz secular, annual and diurnal.
Local attraction due to proximity of steel structure, electric lines, etc.