Bob Murdoch

23rd December 2011, 12:59

Found the following explanation of Dead Reckoning recently. Hope you dont mind a Sparks inputing to the Bridge Forum.

Dead reckoning can tell you where you are at any given time, and it operates on the principle that if you know where you were, and if you know where you have gone, then by extrapolating from where you were and where you have gone, you can calculate where you are, or at least where you were when you originally started calculating. Since the position where you are now is not the position that you calculated, and the position that you were in then is not the position you are in now, it follows that you have introduced the error. (This is defined as the difference between where you are and where you aren't, but calculated you should be.)

This error can be minimised by calculating your next position from the position where you weren't but calculated you were, to the position that you now aren't, because you had just moved. Unfortunately, you still have an error but by this method it does not compound itself, giving rise to an accurate position of where you are not.

Unfortunately, things now get complicated. If the wind and tide are not equal and opposite then a calculation must be made which takes into account their cumulative force and direction. It is important to remember, however that the time interval is the time from where you were to where you no longer are. This calculation results in a correction that is to be applied to the position where you are not in order to accurately ascertain that you still are not.

This is the same method as used by Christopher Columbus when he set out to establish the existence of Australasia but discovered the Americas.

No wonder the Mates always had a slightly puzzled look.

Cheers and a Merry Christmas to you all (Pint)

Bob

Dead reckoning can tell you where you are at any given time, and it operates on the principle that if you know where you were, and if you know where you have gone, then by extrapolating from where you were and where you have gone, you can calculate where you are, or at least where you were when you originally started calculating. Since the position where you are now is not the position that you calculated, and the position that you were in then is not the position you are in now, it follows that you have introduced the error. (This is defined as the difference between where you are and where you aren't, but calculated you should be.)

This error can be minimised by calculating your next position from the position where you weren't but calculated you were, to the position that you now aren't, because you had just moved. Unfortunately, you still have an error but by this method it does not compound itself, giving rise to an accurate position of where you are not.

Unfortunately, things now get complicated. If the wind and tide are not equal and opposite then a calculation must be made which takes into account their cumulative force and direction. It is important to remember, however that the time interval is the time from where you were to where you no longer are. This calculation results in a correction that is to be applied to the position where you are not in order to accurately ascertain that you still are not.

This is the same method as used by Christopher Columbus when he set out to establish the existence of Australasia but discovered the Americas.

No wonder the Mates always had a slightly puzzled look.

Cheers and a Merry Christmas to you all (Pint)

Bob