NavList:

Frank, you speak of ‘longitude by compass variation’.  I suggest that this particular expression has over the years been responsible for putting the suggestion into the same category ‘the Earth is flat’ and other similar notions.  However, if you say ‘using isogonals as position lines’, the idea is worthy of consideration.  This should bring a blush to many air navigators’ cheeks, because in the days when we were required as part of our preparations to update the position of the isogonals printed on our chart in blue or similar crayons, there can be few who at least once in their training accidentally did not include an updated isogonal as part of a three-position-line fix.  

In theory, if you have an accurate up to date chart of isogonals and an accurate source of latitude then there might be places on the Earth where you can plot your position in lat and long.  There are of course many prerequisites and corrections to be made, and you must be careful not to make your corrections using a knowledge of time, because with time there are much simpler ways of calculating longitude.  You would need isogonals lying within 30 degrees of the north-south line in a place where variation is changing rapidly.  You’d need accurate knowledge of your compass’ deviation, and it will be noted that the South Atlantic where the isogonal conditions look best is not the best area for finding compass declination or magnetic variation for making an isogonal master chart in the first place.  It’s all starting to look like a giant nth order differential equation. DaveP