Collecting data in the field, especially spatial data in the form of mapping, cannot always be done without knowing direction. Instruments used to determine direction have evolved over the years. Two centuries ago at the age of 19, George Washington, who went on to become the first president of the United States, was a professional land surveyor and used instruments that at once look simple yet complex. The most basic instrument for determining direction is, of course, the compass. For mapping, there is one type of compass that is simultaneously simple but complex, and particularly useful--the Brunton Pocket Transit. Note that the name implies that it is far more than a compass, but a sophisticated scientific instrument. At approximately $450 Brunton's are expensive by compass standards but inexpensive by scientific instrument standards.
There are two types of Brunton's, azimuthal and quadrant. Azimuthal Bruntons are calibrated in 360 degrees (North = 0 and 360 degrees, East = 90 degrees, South = 180 degrees, and West = 270 degrees). Quadrant, also known as a Bearing, Brunton 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." Personal preference usually dictates which type of Brunton field workers use.
The first thing that needs to be said about Brunton's is that they are both rugged and delicate. On one hand, they are durable and can tolerate hard use. But, on the other hand, the parts cannot tolerate mistreatment. Special care must be taken when closing. Always be sure that the large site is laid-out straight, not at a right angle, before closing. If it isn't, the glass will break when the lid (the part containing the mirror) is folded down.
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 the NOAA Declanation Website 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 (be careful that your belt buckle doesn't affect the magnetism) with the lid opened toward the field worker at approximately 45 degrees.[example] 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 (the white end) then points to the azimuth or bearing on the graduated circle. Note, some field workers prefer to have the front sight laid-out horizontal.[example]
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.[example] 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 azimuth or bearing is read at the South (black) 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.[example] 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.[example] 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 the azimuth or bearing determined. Sighting is performed from the north end of the instrument so the south end of the needle must be read.