Brunton Compass

Brunton compass is a popular instrument used by geologists for surveying since ages. The instrument was invented in 1894 by Canadian mining engineer D. W. Brunton, and hence the name. Brunton compass was later commercially develped by Brunton, Inc. of Riverton, Wyoming . Today however, the name "Brunton compass" is referred to any type of pocket transit compass even if they are manufactured by other companies. Brunton compass, inspite of being expensive are considered indispensible tool for the structural geologist.

Components and sizes of Brunton compass:-

This compass is composed of delicate mirror and glass components which are vulnerable to shock and moisture thus, requiring care and periodic maintenance for proper application. Brunton compasses are available in a number of sizes which ranges from 8.5 cm(4 inches) in diameter and 5 cm thickness, 7cm(3 inches) in diameter and 4.5 cm thickness, 5cm(2 inches) in diameter and 4cm thickness. Brunton compasses have three main parts, box, sighting arm, and lid. The box contains most of the components: the needle; bull's eye level (round level to read horizontal angles); clinometer level (barrel-shaped) and clinometer scale (for reading vertical angles); damping mechanism (to more efficiently stabilizing the needle); lift pin (to lock the needle); side brass screw and index pin (to set and display the declination); graduated circle (to read the bearing). The needle has two ends: the north-seeking end (commonly white in genuine Brunton compasses, labeled 'N' in others), and the black, south-seeking end. The north-seeking end of the needle is pulled down in the northern hemisphere where the magnetic inclination is downward. An additional small weight attached to the south-seeking end of the needle provides proper balancing of the needle. The weight needs to be reversed if using the compass in the southern hemisphere where the magnetic inclination is upward.

The lid, attached to the box with a hinge, contains the mirror with the axial line and oval sighting window (for waist- and eye-level sighting), and the sight. The long sighting arm, attached to the box with a hinge, has a long, oval rectangular cutout or slot (for reading linear objects), and a tiltable sighting tip, which is used for aligning the line of sight. The circle card of the Brunton compass is designed in two traditional scales. The azimuth scale uses three digits, with north at 0⁰ or 360⁰, and south at 180⁰. The quadrant scale uses an alphanumeric notation (e.g., N60⁰ E, S20⁰ W) with the card graduated in four 90o quadrants (NE, SE, SW, NW); north and south lie at the two upper and lower 0⁰ marks, respectively.

The direction of a line on the ground is given by the bearing of the line, which is the horizontal angle between the line and a reference, commonly north in the quadrant scale, or 0⁰ (marked as 0⁰ on the card) in the azimuth scale. The reference, however, can also be the south (S) in the quadrant scale, when reading the bearing (i.e., trend) of south-trending linear objects. The position of 'E' and 'W' are reversed on the circular card; 'E' lies left of the 0⁰ mark and 'W' is to the right of the 0⁰ mark on the card. The reversal is designed to make the correct reading of the bearing possible. To appreciate this fact, notice that the north-seeking end of the needle always stays pointing north even when the compass dial is rotated. For example, to read a bearing of 45⁰, we level the dial and then turn right of north, but the north-seeking end of the needle turns to the left of 0⁰, which is actually east on the dial; so we read a correct bearing.
Uses and Importance of Brunton compass:-

Geologists use this as an instrument for measurement of the altitudes of structural features. This compass helps in the visualization of lines and planes in three-dimensional space. There is a natural sine scale on the cover of this compass for calculations of different readings during a survey.

Reflective readings from the dial are acquired by the mirror placed under the cover of the compass while the level inside this tells us when the base of the compass is horizontal. The lever stituated inside the compass helps in adjusting the levels. The entire mechanism allows measurements of dip angles.

The Brunton compass is commonly used for the following:

• Field mapping of geological objects since they provide a precise sighting-clinometer and hand level capability and can be used both at waist and eye levels.
• It is used in mapping and measurement of stratigraphic sections.
• It is used to measure azimuth (compass bearing).
• The compass is used in mapping and measurement of vertical angles, percent grade, slopes, inclination of objects, height of objects and for leveling.

The brunton compass has been the geologists most trusted tool and it is considered an ideal instrument for carrying out surveys. This is still the finest and most respected field instrument for geologists, surveyors, miners, civil engineers, environmental engineers and forestors world wide.