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    Re: Pilot charts
    From: Gary LaPook
    Date: 2008 Apr 16, 22:39 -0700
    Gary LaPook writes:

    My original post was "tongue-in-cheek." The chart I copied was from the first edition of Pub. 107 from 1981. My pilot charts for the North Atlantic from 1985 have the correct current descriptions in knots. I figured out then that the current on the pacific charts was "drift", nautical miles per day. Apparently some layout artist at the DMA didn't know the difference between knots and drift and applied the same legend to both series of charts. I guess by 1998 they had caught the error : )   

    I made this post to remind us all that even government printed "official" charts and other publications may contain errors such as this one and the one I pointed out several months ago with the online, "official" version of H.O 249 (which has still not been corrected) so a prudent navigator must always double check all available information that may affect the safe navigation of his vessel.

    gl
    Marc Bernstein wrote:
    You are right. The legend for the Pub. 107 Atlas of Pilot Charts of the South Pacific dated 1998 states:

    "the green arrows on the chart indicate the prevailing direction, and the numerals indicate the minimum and maximum drift in miles per day".

    But they are not all the same. In Pub. 106 Atlas of Pilot Charts of the North Atlantic dated 2002:

    "the numerals show the mean current speed in knots". Decimal points are included.

    On Wed, Apr 16, 2008 at 9:56 AM, David K. Bainbridge <davidk.bainbridge@gmail.com> wrote:
    I am by no means knowledgeable in this area, but are not the roses on the chart "wind roses" with the center number indicating % calm, and length of the segment indicating % prevailing from each direction, and the number of feathers indicating the force of the wind (Beaufort scale)? I am having a little trouble with the chart because the color of the scan seems a bit shifted.

    The current is indicated by the 'curved' arrows with a number like 10-20 near it. According to the charts I have seen, and it has not been many, the numbers indicate the minimum-maximum drift in miles per day. So the current is 10-20 knots, but instead the drift is 10-20 miles per day.

    Or am I just completely off?

    cheers,
    /dkb

    Andres Ruiz wrote:
    Dear Gary,
    Such strong currents no exist. 30 is not the speed, is the percentage of occurrence in current direction.
    
    Reading Current Roses:
    Like the wind rose, the current rose graphically, (attached pic), depicts numerous current characteristics such as percentage occurrence of current in eight compass directions, and speed in knots of the current.  The length of the 8 lines radiating out from the circle indicates the percent occurrence of current FLOWING TOWARDS each compass direction.  The longer the line the higher the occurrence in that direction.  If the length of the line exceeds the area available to display it (approximately 30 percent), the actual percentage is displayed.  The number of feathers on the line indicates the speed in knots of the current in that direction with each feather representing 0.2 knots.
    
    
    -----Mensaje original-----
    De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de Gary J. LaPook
    Enviado el: miércoles, 16 de abril de 2008 1:43
    Para: NavList@fer3.com
    Asunto: [NavList 4842] Re: Pilot charts
    
    glapook@pacbell.net wrote:
    
    
    I have attached a portion of the January Pilot Chart of the South
    Pacific Ocean published by the Defense Mapping Agency. Until I looked at
    this chart I had no idea that currents in the South Pacific could be so
    strong. The green arrows show the current  direction and speed in knots.
    For example, the current shown passing the southern end of New Zealand
    is 10 to 20 knots! And in the Gilberts it shows up to 30 knots! How do
    ships manage to maneuver against such strong currents?
    
    gl
    
    
    
    
      





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