NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Why Not To Teach Running Fixes
From: Lu Abel
Date: 2009 Dec 13, 21:32 -0800
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From: Lu Abel
Date: 2009 Dec 13, 21:32 -0800
John:
A very short answer to your query is to look at the diagram you provide. The LENGTH of the three lines from EP1 to "Advanced LOP1" are DIFFERENT.
In a bit more detail: when advancing a LOP, the navigator traditionally draws a DR or EP corresponding to the estimated position of the vessel at the time the second LOP is taken. A line drawn between the original location (DR or EP) to the advanced one is a VECTOR, ie, it has both length and direction. You are absolutely right, only the component of the vector perpendicular to the first LOP controls its advance. But the size of that component is controlled by the direction between the first and second positions. You would not get the same perpendicular component with different course directions. Take the three lines emanating from the first position in your example and make them the same length!! The advanced LOP would then not be in the same position.
And, oh yeah, if you just happen to sail in the direction of the first LOP (or its reciprocal) the LOP will not advance.
Lu
John Karl wrote:
A very short answer to your query is to look at the diagram you provide. The LENGTH of the three lines from EP1 to "Advanced LOP1" are DIFFERENT.
In a bit more detail: when advancing a LOP, the navigator traditionally draws a DR or EP corresponding to the estimated position of the vessel at the time the second LOP is taken. A line drawn between the original location (DR or EP) to the advanced one is a VECTOR, ie, it has both length and direction. You are absolutely right, only the component of the vector perpendicular to the first LOP controls its advance. But the size of that component is controlled by the direction between the first and second positions. You would not get the same perpendicular component with different course directions. Take the three lines emanating from the first position in your example and make them the same length!! The advanced LOP would then not be in the same position.
And, oh yeah, if you just happen to sail in the direction of the first LOP (or its reciprocal) the LOP will not advance.
Lu
John Karl wrote:
My argument against running fixes has nothing to do with how they're taught, but it does challenge why they're taught -- at all. It has nothing to do with how the location DR2 was estimated. As long as DR2 is found by combining relatively inaccurate data, and not by forming a fix of a third LOP with LOP2, it is irrelevant what (or which) estimates are included in DR2: speed, time, logged distance, drift, current, averaged headings, the flight of birds, etc. I'm talking about arriving at DR2 without a bona fide fix. I pointed out that the concept behind the traditional running fix is based on two ridiculous assumptions: the assumption that the estimated DR track perpendicular to LOP1 is completely accurate while the DR component parallel to LOP1 is completely without value. I ask again, can anyone on the List refute these these two assumptions?? Can anyone justify them?? Ah, the traditions of the sea. JK
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