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:

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.

Tape or
Pacing
Clinometer Compass Additional Comments
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:


COMPASS USE

COMPASS: Heading shown as Blue Line


CLINOMETER CONSTRUCTION

CLINOMETER: Attach Straw and Hanging Weight