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    Re: DR plotting techniques
    From: Steven Wepster
    Date: 2003 Oct 20, 16:20 +0200

    When in estuaries etc with enough buoyage around, I follow the buoys. That
    is considered as bad practice (never rely on a single means of navigation,
    especially if it is floating) and one day, when fog comes rolling in, I
    might be caught out. But on fine days with stable weather I trust the buoys
    as long as they correspond to the chart.
    
    When out at sea I maintain a DR position. I have the impression that across
    the ocean people have different views as to what a DR position is: here, in
    Europe, it is what the Americans call EP, inclusive of estimated effect of
    current. The American DR is not something that we have a separate term for.
    Please correct me if I've got it wrong.
    
    I use as instruments: magnetic compass to steer and bear, Walker log,
    plotting on paper charts with soft pencil making wide lines. I use one or
    two triangles, marked along the edge in degrees, instead of a protractor.
    The echosounder may on occasion also provide useful information.
    
    I cross-check by taking one or more bearings, or a fix from a hand-held
    GPS. I use the GPS waypoint function exactly like Arthur does: plot bearing
    and distance to obvious points. That has the added benefit that no
    corrections need be applied in case my voyage continues on a chart with a
    different horizontal datum (waypoint and fix are affected in very nearly
    the same measure by the datum change, so their relative positions stay the
    same). The interval of fixes depends on the circumstances.
    
    When beating to windward I see no use in protracting course lines on the
    chart, and even on other courses the situation changes faster sometimes
    than that I traverse the courses.
    
    Currents in the North Sea are very much influenced by meteorological
    conditions. Published predictions are avaraged data (related, of course, to
    time of HW in a standard port) with no indication of its variation. At slow
    vessel speed I think this variation has a detrimental effect on DR.
    
    In practice I find myself somewhere between George's position and some kind
    of formalised DR routine. No laptop; but I did programme a calculator to do
    "a day's work" or any part thereof, which I seldom use.
    
    My compass seems to suffer from exceptionally large (heeling) errors.
    Before I restore my confidence in it, I hardly venture out to sea.
    
    I do not correct my charts anymore because I get almost-new ones for free
    on a pretty regular basis. But before a prolonged trip I would check up.
    To the electronic corrections: I tried Stacy Hanna's way of inputting the
    chart numbers but unfortunately the UKHO site takes only a single chart at
    a time. People here running a Dutch website are looking into the chart
    correction problem. No solution found yet.
    
    Arthur, when you take speed from the GPS then you break the independence
    between DR and GPS fixes. You can not be sure anymore that the one provides
    an independent check of the other.
    
    Steven
    
    
    

       
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