NavList:

I recently used the approach of measuring the bearing of prominent landmarks from my known GPS position to check the accuracy of a pair of marine binoculars with a built in compass visible in the eyepiece. I measured the bearings of various radio masts and tall buildings in the centre of Boston and determined their positions using Google Maps. The true bearing can be obtained from the great circle sailing initial course formula

C = atan2( cos(B1) * sin(B2) - sin(B1) * cos(B2) * cos(L2 – L1), cos(B2) * sin(L2 – L1) )

or to a pretty good approximation

C = atan2( B2 – B1, cos(B1) * (L2 – L1) )

In this case B1, L1 are respectively the latitude and longitude of the observer and B2, L2 are those of the target object. The function atan2( x, y ) is the usual 2 argument arctangent. The order in which the arguments need to be supplied may vary between computing platforms,

Robin Stuart

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