Closed Loop Survey Calculator
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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:
The direction of travel lies at an angle of 69 degrees east of north
The direction of travel lies at an angle of 22 degrees west of north
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
Distance Traveled Slope (degrees) Angle to North (Landmarks, Difficulties,...)

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 USE

COMPASS: Heading shown as Blue Line