BRASS BRUNTON SURVEYING COMPASS
BRASS BRUNTON SURVEYING COMPASS
The Brunton Compasses is an instrument used by geologists for measurement of the altitudes of structural features. This compass can help a person develop the ability to visualise lines and planes in three-dimensional space. On the cover of this compass there is a Natural Sine Scale for calculations of different readings during a survey. Under the cover is a mirror used for taking reflecting readings from the dial. It has a level inside, which tells you when the base of the compass is horizontal. The levels are adjusted by moving the lever situated under the compass. The entire mechanism allows measurements of dip or plunge angles. On shutting the compass, a button-like mechanism locks the compass needle. This protects the magnetism when transporting the compass. All sizes are sold in a smart wooden box. These compasses are available with a polished brass finish.
The compass is 7.5 cm (4 inches) in diameter and 5 cm thick.
Brunton Compass Instructions for use.
Collecting data in the field, especially spatial data in the form of maps, cannot always be done without knowing direction. One of the most basic tools for determining direction is, of course, the compass. For mapping, there is one type of compass that is particularly useful-the brunton compass. Note that the name implies that it is far more than a compass, but a sophisticated scientific instrument.
There are different types of bruntons. the one we will be dealing with is the ,quadrant, also known as a bearing brunton, and is divided into four "quadrants." direction is designated first in reference to north or south, then to east or west. For example, from where a field worker is standing, the bearing to some feature slightly east of north, or at about the so-called "one o'clock" position, would be read "north 31 degrees, east." an object at the 7 o'clock position would be described as being at "south 58 degrees west."
The first thing that needs to be said about brunton's is that they are both rugged and delicate. the second thing that must be noted is that as a magnetic instrument, bruntons have to be corrected for magnetic declination. The magnetic north pole is not located at the true north pole. Today, the magnetic north pole is located in northeastern quebec, canada. depending on where one is working, the angle between true north and magnetic north--magnetic declination--varies. When beginning field work this should be corrected by first noting your location on an isogenic chart, and then moving the graduated circle accordingly by means of the slotted adjustment screw on the side of the brunton. One can, however, leave the brunton set a zero, but such must be recorded in the field notes, or all readings must be adjusted before recording.
There are three ways of reading horizontal angles depending on whether the object or feature being tie-in or mapped is above or below the elevation of the field worker. when the object being sighted is between 45 degrees above and 15 degrees below the observer, the brunton is held horizontal, waist high ,with the lid opened toward the field worker at approximately 45 degrees. The large or front sight is set vertically, that is perpendicular to the bottom case, and on the side away from the observer. be sure that the brunton is cupped firmly in both hands with both forearms pressed firmly against the body. the instrument is correctly sighted on the object when the user, looking down into the mirror, sees the black centerline of the mirror bisecting both the front sight and the object. At this time, the bubble in the circular level vial is centered. the north-seeking end of the needle (red or coloured tip) then points to the bearing on the graduated circle. note, some field workers prefer to have the front sight laid-out horizontal.
When the feature or object being sighted is more than 15 degrees below the observer, the brunton is held waist high with the front sight toward the body and the mirror away. the front sight is tilted over the bottom case at 45 degrees and the mirror is tilted back at the same angle. Sight over the front sight and through the window opening in the cover (mirror) near the hinge. the mirror and the front sight are then adjusted so that the image of the front sight can be seen in the mirror bisecting the centerline of the mirror. The tip of the front sight and the mirror centerline are then aligned with the object. the bearing is read at the south (silver or not coloured) end of the needle.
Occasionally, there may be obstacles between the point of observation and the object or feature being sighted, or other circumstances may be encountered which require the field worker to hold the brunton at eye level. The biggest problem with holding the instrument at this height involves stability, or lack of it. when used at eye level, the compass is held horizontally and the front sight is set vertically or slightly away from the face. The tip of the front sight is positioned straight up. some field workers prefer to have the sighting arm laid horizontally, pointing toward their eye, but with the actual site turned upward.in either case, the mirror is angled toward the observer, over the compass body. the object is sighted through the front sight and over the mirror or through the window. Centering of the round level can be accomplished at this time by observing it in the mirror. the needle can be seen in the mirror and bearing determined. Sighting is performed from the north end of the instrument so the south end of the needle must be read.
Please note that the above is only an indication on how to use a brunton. it is in no way an acurte means of calculation. it does not imply that all bruntons will function in the above manor. some bruntons compasses may have errors and hence this is in no way a means of specifying that the instrument, is acurate and it isalso in ,no way a means of soliciting the sale of the instrumenst. for reference only.
revised and edited for avesta & co.tm