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post #1 of 14 Old 10-04-2006 Thread Starter
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Reciprocal Bearings

This has been bothering me for a while... Can anyone provide a good explanation as to why we should bother with reciprocal bearings? I understand what they are and how to get them, but I fail to see their use.
If I have a true bearing of 240 on a church steeple and a bearing of 300 on a lighthouse, I know where I am. Why do I need to know the reciprocals of 60 and 120 respectively?
Thanks!
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post #2 of 14 Old 10-04-2006
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Reciprocal compass bearings are useful for reversing the route, when plotting routes on paper charts or electronic plotters.

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Last edited by TrueBlue; 10-06-2006 at 04:12 PM.
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post #3 of 14 Old 10-04-2006
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Reciprocals are used primarily to figure out where you are. When plotting a position on a map or chart you find that a point bears 10 degrees from you. Where do you plot that line from? You don't know your position exactly, that is what you are trying to find. So, you go to that point on the chart and draw a line on a reciprocal bearing. After you find a few points and draw the reciprocal bearings, where they cross is where you are.
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post #4 of 14 Old 10-04-2006
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Quote:
Originally Posted by SteveCox
Reciprocals are used primarily to figure out where you are. When plotting a position on a map or chart you find that a point bears 10 degrees from you. Where do you plot that line from? You don't know your position exactly, that is what you are trying to find. So, you go to that point on the chart and draw a line on a reciprocal bearing. After you find a few points and draw the reciprocal bearings, where they cross is where you are.
Exactly. You can't draw the line TO the steeple because you don't know were it begins. You have to draw the line FROM the steeple which requires mentally placing yourself on the steeple and looking back at your boat. The reciprocal heading is what you would get looking over a compass pointed at your boat while standing on the steeple.

Plot a bearing from the steeple, from the light house and from one other point and you are located within the triange the lines form.
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post #5 of 14 Old 10-06-2006
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I can think of 2 situations where you need to know reciprocals.

Firstly, lead markers are often marked on a chart with the bearing to run in on but that bearing is no good if you're outbound - to safely steer the course outbound you would steer the reciprocal of what is on the chart.

Secondly, to use your example, when you take the bearings, they are bearings from YOU to the object. If you needed to give someone your position using an object as the reference, you would say you are 120 FROM the lighthouse at a range of , say, 2 miles, or 060 FROM the church at a range of 2 miles.

BTW, it is convention for bearings to be represented by 3 numbers. For example, 60 should be represented as 060. Also, just to clarify, taking a fix on 2 points will not give you an accurate position; always use a minimum of 3 bearings . . . I'm sure you knew that already :-)

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post #6 of 14 Old 10-06-2006
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Beyond those two reasons and as previously mentioned, reversing a course with plotted routes utilizes reciprocals. Most of my planned routes are drawn directly on paper charts, recorded on my chrat plotters and (as anal as it may be) formatted on 5 x 8 index cards - as a convenient backup of information.

Here's a typical course which shows forward/reverse bearings, the reverse being the reciprocal:


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post #7 of 14 Old 10-06-2006
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Another good reason for it is MOB situations. If you know what your heading is and someone falls off at night, in the chaos of dropping the sails, starting the motor and getting turned around, you have some indication of what direction someone should be at.
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post #8 of 14 Old 10-06-2006
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I find the whole discussion silly (notwithstanding the serious and excellent responses).

Asking why reciprocals are important is like asking, "Gee, if I know my way TO the grocery store, why should I be concerned with the way FROM the grocery store?".

Silliness.

Or, maybe just an idle thought, not thunk thru :-)

Bill
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post #9 of 14 Old 10-08-2006
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Quote:
Originally Posted by btrayfors
I find the whole discussion silly (notwithstanding the serious and excellent responses).

Asking why reciprocals are important is like asking, "Gee, if I know my way TO the grocery store, why should I be concerned with the way FROM the grocery store?".

Silliness.

Or, maybe just an idle thought, not thunk thru :-)

Bill
Bill, I don't disagree with you, but if sailingman has a genuine question then we need to respect he needs the help. I'd be interested in feedback from sailingman. . . .

Sailingman - has your question been answered?

Graham
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post #10 of 14 Old 10-09-2006
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sailingman

The only silly question is the one not asked.

To "fix" your position you take a bearing from where you are to the object, the church steeple or the lighthouse. On the chart, you place the line of position over that object and draw a line away from it. Where the two lines cross is your visual fix. To convert the bearing to its reciprocal only adds another step and another chance for error.

As mentioned above, noting the reciprocal course simlpy makes it easier on the way back.
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