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    Plotting Sheets (again)
    From: John Brown
    Date: 2012 Nov 30, 06:15 -0800

    In my pre-GPS seagoing days we constructed simple plotting sheets on the back of folded admiralty charts using a fixed longitude scale. The sheets were drawn with one central parallel, labelled with the whole number of degrees nearest to the EP, AP or DR. Two meridians intersected the parallel, somewhere near the middle of the sheet. Between them, the parallel was divided into 60 minutes of longitude at any convenient scale. These were all inked in for multiple use.

    A simple scale diagram was drawn in a corner of the sheet so as not to clutter up the plot - for right handed navigators usually at the bottom right corner. This consisted of a parallel with three or four meridians at ten minute intervals of longitude erected on it. The first ten minute section was further subdivided into one minute intervals. This was also made permanent in ink.

    To obtain the latitude scale and for measuring distances, a pencil line was drawn across the scale diagram at an angle to the horizontal equal to the latitude of the central parallel. Minutes of latitude (nautical miles) were measured by dividers along this line where it intersected the meridians. This was quick, accurate for all practical purposes and takes much longer to describe than to do, and it is really just a graphical representaion of the relationship: departure = dlon cos lat.

    This is no different in principal to commercial plotting sheets, but it is very easy to do and can be made conveniently on anything - squared notebook paper, for example.
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