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    Re: Silicon Sea: Leg 88
    From: Dan Hogan
    Date: 2003 Jul 1, 07:15 -0700

    On 30 Jun 2003 at 23:48, Joe Schultz wrote:
    > Well, a dozen years and the rust has removed all of the paint.  Here
    > are my answers to the first part, followed by questions for the
    > experts.  I used a calculator with trig functions for the
    > calculations and home-made sheets for the graphical solution (after
    > locating my Speedbow, C-Thru ruler, and a metric scale).
    > 1) L2 = 39d 39.0'N     Lo2 = 154d 43.0'W   via Mid Latitude Sailings
    >    L2 = 39d 39.0'N     Lo2 = 154d 42.1'W   via Mercator Sailings
    >    L2 = 39d 38.5'N     Lo2 = 154d 43.9'W   via three plotting sheets
    > 2) 0830:20 ZT 10SEP2002 (1830:20 UT 10SEP2002 +10).  Intermediate DRs
    > were plotted every four hours starting 0400 UT 09SEP2002, since we
    > are in the open sea.
    OK. Find a technique that works for you and stick with it so that it becomes
    routine for you.
    > Questions:
    > 1. Are there rules-of-thumb in terms of distance limits for which I
    > decide to use Plane, Mid Latitude, or Mercator Sailings?
    Yes. Plane sailing use for about 50 miles and under. Mid-Latitude is accurate
    for 500-600 Nmi. Mercator Sailing is accurate N & S and picks up an error E &
    W when you cross the celestial quadrant of Lo 1.
    > 2. Plotting sheet dL = 0.5 Nm to calculated L2, which shows my
    > current ineptitude in plotting (used 10 sheets of paper to get three
    > "good" plots).  How close to calculated Lo2 can I reasonably expect
    > to arrive with a correct plot? Note that I cheated and used dL = 2d
    > 20' for the middle plotting sheet.
    There will be an argument on this, but if you can plot it to within +/- one
    minute of arc you are accurate. In actual practice it will vary within a 4'
    circle of error. The calculated number will vary depending on the formula
    Also consider the width of the pencil line in relation to the size of the
    plotting sheet.

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