[Psml] DR Position

[Doug Thomas]

A lot of discussion has been concerend with definitions: SOG, SMG, SOA, knot log, knot meter, log. That was not the question. The question was, which speed (SOA, SMG, SOG) should be used to compute the location of a DR on a DR track. You answered the question with:

"The speed determined when using the 60D = ST is the SOA, Speed of advance. Intended or expected speed along the track."

If I correct my course from a second fix, I can compute my SMG from the previous two fixes. Therefore, wouldn't my "intended or expected speed" (SOA) for the next leg of my track be the SMG computed at my last fix (unless I expected a change in set/drift during my next leg)? Also, this would make the reading of my knotmeter (speedometer) irrelevent, since this would have nothing to do with my SMG.
  ----- Original Message ----- 
  From: Edward Kydd 
  To: dthomas at neo.rr.com 
  Cc: United States Power Squadrons Mailing List 
  Sent: Sunday, November 23, 2008 4:54 PM
  Subject: Re: [Psml] DR Position






  The USPS Glossary, Appendix F in the 99/01 JN Manual defines


  SOG, Speed over the ground as "Actual speed being achieved relative to the bottom.  


  SMG, Speed made good as "The resultant speed from a point of departure to a point of arrival.


  The speed determined when using the 60D = ST is the SOA, Speed of advance. Intended or expected speed along the track. 


  Dead reckoning (DR). Calculating a boat's position based on its course, speed, and time run from a previous position.


  The DR position is a position determined by dead reckoning. The presumed position of a vessel on a DR track (See dead reckoning).


  Log. Instrument to measure distance or speed through the water.




  Note that the knot meter/log gives speed over the water, not speed over the bottom.
  as well as the distance through the water.


  Current, drift, steering variations, instrument errors and sea conditions all contribute to differences between a DR and a Fix at any given time.


  Hope this is of help.


  P/C Edward M. Kydd, SN


  Offshore Navigation Committee




  On Nov 22, 2008, at 7:42 AM, Doug Thomas wrote:


    When a DR position is being plotted using 60D=ST, is S (speed) the SOG or the SMG?

    SOG is taken from the ship's knotmeter.
    SMG is computed from the ship's log (ie. log at T1 = 1325 nm, log at T2 = 1350 nm, therefore, D = 25 over that period of time.)


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