Join Date: Oct 2006
Location: Deep Cove, North Van, BC
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Interestingly I had the same question as you. I have taught land navigation in a military environment, but I haven't "plotted" on a chart in a long time. So I was walking around wondering why the need for a reciprocal; and then the proverbial light bulb went off.
In land navigation using map and compass, you don't have those nifty rolling rulers, walking rulers, or what have you to carry the course from the compass rose over to the sighted bearing. Usually you are just using a standard ruler so you need the reciprocal from the church steeple. With boating charts, you can carry the the steeple angle over from the compass rose to the the church steeple and with at least one other bearing your good to go. If you have radar, you may not need another bearing if you can get a hit off the church; the church will give you distance down your bearing line, and that is all you need.
A reciprocal on a course is handy if you go out into an area that is fog prone, like BC Coastal areas in the fall. So if you are leaving an area, you might take reciprocals so you can get back if fog should roll in at an inopportune time. Remember the reciprocal bearing isn't enough as you might have to take wind, current and tide into account as well.