Measuring Direction
A compass is a device that uses the Earth's magnetic field as a
reference for measuring and setting direction. The compass is essentially
a
magnetic needle that must be
able to freely rotate in the horizontal plane to align with the Earth's
magnetic field.
This field does not align exactly with the geographic North and South
Poles of the Earth -
"true North" and "true South".
A correction called the magnetic declination must be added to the compass
direction
to reference directions to true North. We will not be concerned with
referencing
our measurements to the geographic poles here. This step is necessary for
accurate map
making and for surveying/navigating over regions where the magnetic
declination varies.
The compass provides a measure of the angle between your intended
direction of travel and
the horizontal component of the Earth's magnetic field lines. This angle
is called the
magnetic bearing. It is typically recorded as an angle referenced
to
North. Bearings are recorded in three parts with:
- the reference direction (N)
- the acute angle between this reference direction and the intended
direction of travel
- the east or west orientation of the direction of travel compared to
the reference direction
(E/clockwise or W/counterclockwise). Angles measured clockwise from North are recorded as
positive values and angles measured counterclockwise from North are recorded as negative values.
Examples follow:
- Heading = 69
- The direction of travel lies at an angle of 69 degrees east of north
- Heading = -22
- The direction of travel lies at an angle of 22 degrees west of north
- Heading = 243
- The direction of travel lies at an angle of 43 degrees east of north
The method for measuring the magnetic bearing of a direction of travel
will vary somewhat depending on compass construction. Many have a
360 degree rotating dial that facilitates measurement.
- Face in the direction of travel. For this, it may be useful to
site on a distant landmark in the proper direction. This will
help you to stay on track when you actually travel.
- Hold the compass horizontal
- Measure the acute (<90 degrees) angle between magnetic N and
your intended
direction
of travel. Ideally, one person should hold the compass and a
second person should read the bearing by sighting along the line of the
compass needle to the measurement scale. Make sure no magnetic influences
(including other compasses!) are nearby.
- Record your heading angle measured from North (clockwise from N is positive, counterclockwise from N is negative).
Record data into a
notebook with bound pages. Do not erase. Cross out
mistakes with a single line.
- Ideally this sighting of direction should be done in two
directions: sighting
along the path already traveled as well as along the path to be
traveled. The purpose
of making two sightings is to have two bearing values for every segment
traveled that
can be averaged together.
Tape or
Pacing |
Clinometer |
Compass |
Additional Comments |
| Distance Traveled |
Slope (degrees) |
Angle to North |
(Landmarks, Difficulties,...) |
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Store the compass so that it is secure and not subject to frequent
mechanical shock or stress.
Early explorers would test the precision of their surveying
by recording measurements over a "closed loop" path. They would
go to an objective via one path or river and come back to their
starting point via another route. If there were few or small errors
in the survey, the numbers should show that, at the end,
the explorers would be at zero distance from their start location.
This method of checking provided an excellent method for
helping surveyors improve technique and avoid poor habits that
would cause even small, systematic errors.
To do the activity, you will need:
- A Clinometer: This will serve to measure slope angles. Students
will need to build a clinometer using the template below.
- A Compass: This will serve to measure the angle between
the explorer's heading (intended direction of travel) and magnetic
North. In case students are working on a surface that is gridded (with
floor tiles, for example), they can measure heading angles with
reference to this grid. Use of a compass is preferable, however.
- A Tape Measure or Rulers: This will serve to measure distance.
- A Closed Loop Course: This can be a real course outside or an
"artificial" course set up with books, landmarks and furniture inside
the classroom or even on a single desktop.
COMPASS: Heading shown as Blue Line
- Decide on your heading or direction of travel.
- Orient the compass so that the North pointing end of the
compass points at 0 degrees.
- Measure the angle between your heading direction and magnetic
North. Angles East of North are recorded as positive values; angles West
of North are recorded as negative. The North direction is 0 degrees.
- Cut out the clinometer template and paste or tape it onto a stiff
piece of cardboard or heavy paper.
- Get a 12 inch (30 cm) piece of string and tie a nut or other small
weight to one end.
- Punch a small hole through the cross in the upper left corner
of the template and thread the free end of the string through this hole.
Attach the string so that the weight hangs just slightly below the angle
scale on the template.
- Securly tape a straw (or similar sighting device) horizontally to the
top of the clinometer. Be careful that the straw does not interfere
with the free swinging motion of the string and hanging weight.
CLINOMETER: Attach Straw and Hanging Weight