[Psml] DR Position

[Harleigh Ewell]

Doug,

 

I believe the technical answer to your question is the expected speed
through the water, or SOA, not SMG, or SOG.  DR, by definition, excludes the
effects of current and leeway.  If you factor in what you think those
factors are, you obtain an estimated position, not a DR position.  Further,
information from a GPS (such as track or SOG) is based on a series of fixes
and is not DR (although it is usually more accurate).  Thus, a DR course is
the course on a chart from point A to point B.  The effects of current and
leeway cause the actual course (the track) to differ from the DR course and
produce a SOG that differs from the speed through the water.  Further, the
speed from the last fix to the destination may not be the same as the SOG
between the last two fixes, such as where a sailboat encounters different
wind conditions.  SMG is determined by the time and straight-line distance
from point A to the destination, regardless of changes of course that take
place enroute (such as a tacking sailboat).  Thus SMG is relevant to this
issue only if the boat maintains a constant heading, in which case the SOG
and SMG are the same..

 

The situation you describe in your last paragraph is plotting an estimated
position based on the track and SOG between your last two fixes.  This may
(or may not) be the most accurate estimate you can come up with without
using instantaneous GPS data, but it is not DR.  Thus, you can use any of
these speeds in 60D=ST so long as you know what it is you are trying to find
out.

 

P/C Harleigh Ewell, JN

 

 

 

-----Original Message-----
From: psml-bounces at usps.org [mailto:psml-bounces at usps.org] On Behalf Of Doug
Thomas
Sent: Monday, November 24, 2008 6:53 PM
To: hewell at comcast.net
Cc: United States Power Squadrons Mailing List
Subject: Re: [Psml] DR Position

 

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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.usps.org/pipermail/psml/attachments/20081125/744049fb/attachment.html