Working in the field

This section describes how to work in the field using a magnetic compass and calculated alignment bearings like those described in the previous section.



You don’t need to get an expensive compass but it must have a bezel that can be twizzeled around and lined up with the floating needle which always points to north.  A sighting mirror will improve the accuracy of the reading although it is not vital.
The usual way of using a magnetic compass in the field is to measure a potential alignment and compare this to theoretical values.  One must take into consideration the effects of magnetic variation and the effects elevation. 

Magnetic variation

Calculated alignment bearings refer to true north, or the axis upon which the earth spins.  A magnetic compass points to magnetic north which is usually within a few degrees of true north and varies considerably depending where you are on the earth and when you take the measurement.  To make accurate measurements you will need to find out what the difference is at the site you are measuring when you take the measurement.  This information can often be found on local maps (like Ordinance Survey 1:25,000) or on the Internet.  Make sure that once you know what the variance is, you convert your magnetic north based measurements to true north based measurements.

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