A compass is an essential tool for forest
management. Compasses do two things very well, point in a straight
line, and measure angles. You can find a lot of utility in these two
applications.
A compass’ primary use is for you to
“stay found” Even on familiar ground, it is surprisingly
easy to get turned around when you are busy concentrating on
something else. While topography, the sun, and traffic noise can
often lead you in the right direction, a compass is a reliable way to
point the way.
A compass is essential for finding boundary lines, and performing a number of management activities. Most forest mapping projects and inventories make use of a compass to plot straight lines on the ground.
At its simplest, a compass consists of a magnet mounted on some type of pivot mounted in a case. The needle points to magnetic north. Beyond that items like a clear plastic base, rotating bezel, sighting mirror and declination adjustment make the compass easier to use. Figure 1. shows a basic model.
Compasses come in a variety of configurations,
from cheap plastic toys to very accurate pocket transits costing
hundreds of dollars. Most foresters use a compass that costs about
$40 for professional work. Some qualities to look for:
Compasses are graduated in either azimuth or quadrants. Azimuths divide a circle into 360 degrees, starting with 0° as true north, 90° as east 180° south, and 270° as west. The bearing of a line is simply the azimuth expressed as a number between 0° and 360°.
Quadrant compasses break the circle into 4 — 90° quadrants.
North East, South East, South West and North West. The bearing is
expressed as a number and direction away from true North or true
South. A bearing of S 15° E
would be expressed as 15° East of
true South. On an azimuth compass that would equal 180° for true
south minus the 15° East to become 165°. An azimuth of
280° would
represent a westerly direction from true north. It would be written
as N 80° W.
Figure 2 shows how this works. Ether type is simple to use. Many
property surveys use the quadrant notation.
Declination represents the difference between true north, and magnetic north. True north and magnetic north are about 1300 miles apart. In California this translates to a difference of about 14° east of true north in southern California to 18° in northern California. Declination is printed on US Geological Survey (USGS) topographic maps. Declination changes over time. The USGS publishes a chart every 5 years that shows current declination. The easiest way to adjust for declination is to use a compass that has a mechanical adjustment feature built in. Otherwise, you should draw a sketch of the declination for reference.
When comparing a map to the ground it represents,
it is useful to align the map with ground features. Orienting the map
makes it easier to identify distant features. Orienting the map is
useful when following boundary lines, having everything in alignment
reduces the chances of making mistakes. Using a compass makes
orienting a map easy.
The compass is used to follow a straight line
through the woods. It is applied to tracing a boundary line, or
finding a property corner, making a sketch map, or conducting a
forest inventory. Being able to follow a course through the forest
can help you move quickly from one place to another.
The compass can also be used to accurately locate
you current position on a map. Using triangulation you sight two or
more objects that are identified on the map, and visible from the
ground. Mountain tops, radio towers or other distinct features work
well.
You can check this by sighting a third landmark in the same manner. All of the lines should intersect.
One condition to be aware of is local attraction.
In areas with large mineral deposits, power lines, or large metal
objects the compass reading could be off by several degrees. Local
attraction is indicated by taking two bearings – one at the
beginning of the line towards the objective – and the other from
the objective, back to the beginning of the line. If they disagree
then local attraction can be suspected.
Before suspecting local attraction, consider other sources of error.
Most errors of this type come from holding the compass too close to a
metal object on your person. Metal framed eyeglasses, watches,
jewelry, or a metal bound notebook often are the culprit.
Using care and a good quality compass, you can take accurate measurments of direction and angles. Technique is important. Keeping things aligned, using the guidelines will insure that you get accurate readings.
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Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028