[Psml] DR Position

[Edward Kydd]

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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