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» Online Classroom   » Tech Support   » StarPilot   » DR Update at 090 and 270

Author Topic: DR Update at 090 and 270

 - posted November 29, 2002 02:16 PM      Profile for vjwilliams           Edit/Delete Post 
DR Update at 090 and 270

First some background: The StarPilot DR Update function takes the stored DR position and updates it with your input of either a new DR Time (and Speed) or a new DR Log reading, depending on the DR mode setting. After it computes the new position (by mid-latitude sailings) it does a double-check on the results by computing from scratch the range and bearing from the old position to the new position using the Rhumbline function (mercator sailing using meridonal parts).
Consequently, when you view the range and bearing reported back in the StarPilot with the new position coordinates, you may see results that differ very slightly (few hundredths) from what you entered... you have, however, still computed the best position you can when applying a given distance and course to a particular position... short of an iterative procedure that is not included in any products we know of, nor is it covered in Bowditch.

Issue at hand: Rhumbline computations do not work for courses of precisely 090 and 270 (this is parallel sailing, not mercator sailing). In versions of StarPilot dated 06/06/00 and earlier, we compensated for this by simply changing the double-check routine to a great-circle computation whenever the course was within 0.03° of 090 or 270. In retrospect, this was not a good approach since in some cases the discrepancy reported for even rather short runs due east or west was several tenths of a mile. Consequently, for all versions dated later than 06/06/00, we now abandon the double-check, and simply repeat back to you what you entered for course and distance whenever the course lies within 0.03° of 090 or 270. The updated position is still the same, as it always has been, it is just that the reported range and bearing you see will be exactly what you put in, you are not getting any "double check" for this parallel route. — thanks to Dr. G.P. in Germany for bringing this to our attention.

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