Looking through some old books last night I came across A Dictionary of Civil Engineering published by Penguin in 1958. Some good words in there, but most probably of little interest to most people. One entry caught my attention:

Agonic line – a line on a map of the earth along which the magnetic declination is zero. From the Greek; agonios, without angle.

To find this line I guess you draw a line between the magnetic North Pole and the geographic North Pole and then project that line both ways around the globe. The idea is new to me and I cannot see that it has any use, although the dictionary says that it is a surveying term. I’ve done my share of surveying for roads and railways in years past, but not once did I think ‘Damn, I wish I knew where the line of zero declination was’. Does anyone know how surveyors use it, or why they might need to know it?