So, over the holidays I reconditioned a second survey compass for my Colonial Surveyor impression and was reminded of a question posed to me several months ago. 

Why does the rose on a surveyor’s compass reverse east and west?

Many people look at the reversed compass rose and are confused.  The compass is correct, but they are confused with how to read it.  You see there are two compass headings we are familiar with 1) bearing and 2) heading.  Most people are familiar with heading as this is what is read from a map.  The surveyor’s compass (and your GPS by the way) report bearing relative to north.

Heading is the direction you are facing.  When you stand at a point and face the setting sun, your heading is west(ish).  When you look at a printed map, by convention we put this to the left because when your heading is north, west is to your left.  Bearing, on the other hand, is angle formed between your heading and north.  This is the direction the compass needle points when you head in a specific direction.  When you face the setting sun, the compass now points to your right which is north.  North and west are still in the same direction, it’s just the measurement system that switched on us.  The surveyor’s compass is set up so that the observer looks through the two sights and the compass rotates as the sights are lined up to a specific heading as defined by the survey reference point and a range pole held up along the line to be surveyed.  Since the needle stands still and compass turns under it, the letters E and W must be reversed from their natural position so that the direct reading of the needle will give not only the angle but also the proper quadrant.

I know this confuses people as we also bring out navigational compasses (some with sights) for the plane table.  The thing to remember about this is that on the plane table the map is fixed and as the map is drawn rather than rotated to face the direction of the vectors mapped.  Maps should always point north but when they rotate to the direction of travel (like a modern GPS) they are easier to read in the small scale but its important to remember that the map is rotating and north stays in the same direction.

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