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    Re: Circle charts
    From: Nicol�s de Hilster
    Date: 2009 Jan 28, 08:46 +0100

    Richard M. Pisko wrote:
    > On Thu, 15 Jan 2009 01:04:44 -0700, Nicol�s de Hilster  
    >  wrote:
    >
    >   
    >> These circle charts were made on the job by the surveyor and showed only
    >> what the navigator needed, in most cases that were a few known points, a
    >> bit of coast line, an outline of the working area and of course the
    >> circles. The charts were used for small area's of only a few kilometres
    >> in diameter and usually made of paper (A1 or A0 size).
    >>     
    >
    > As a practical method, did you calculate the centers of the circles (on  
    > the perpendicular bisectors of each line joining pairs of shore stations)  
    > for drawing the chart of current interest?  Or did you use an adjustable  
    > triangle, set to the double angle of the "observation angle", and find the  
    > center for the circle of that observation angle graphically?   Or  
    > possibly, put pins in your beacon positions, set an adjustable triangle  
    > (made of two pivoting straight edges) and use a (mechanical) pencil in the  
    > vertex to trace out a circle.  Also, what would be the average degree  
    > spacing of the chart's circles?
    >
    >   
    The company I used to work for in those days had dedicated in-house 
    software to create these plots and plotted them using an A1 or A0 pen 
    plotter, but I do recall that at some jobs they were even made by hand, 
    something I never did. I do not recall the degree spacing any more, 
    neither do I have surviving charts to check. I remember that what did 
    matter was the physical spacing of the circles on the chart itself. For 
    accuracy and ease of use reasons you did not want that to be more than a 
    few centimetres at maximum, but also not smaller than about one 
    centimetre at the widest spacing as this reduced plotting time and 
    costs. So the spacing depended on geometry, location in reference to the 
    beacons and scale. The final map was often a product of trial and error; 
    simply re-plotting the chart until a satisfactory result was obtained.
    > It seems as though there is a fair amount of work to set up a chart like  
    > that without modern aids such as a computer, and you would have to make a  
    > large number of observations to make it worth while.  
    For this reason circle charts were only hand made on jobs with cheap 
    labour available. How they produced them, I do not know, but you could 
    use a station pointer for it. The Warren Knight station pointer has a 
    special inset that allows you to position a 0.5mm pencil in the middle 
    of the circle. Usually computers were flown in to do the job, even in 
    the time before pc's were generally used. At those days Hewlett 
    Packard's were used running on what they called technical basic.
    > I wish I were  
    > skilled enough to use something like Gnuplot for doing a circle chart of  
    > our river-bottom flats, using some reference points 200 to 300 feet above  
    > the somewhat uneven work area.
    >
    > Easier to locate your position using a plane table or theodolite than with  
    > a sextant; and many software packages for total stations include the three  
    > point problem solutions.
    >
    >   
    For what I understood Cook charted New Zealand using a plane table from 
    a ship and still did quite a proper job. He must have used similar 
    techniques to get his positions right.
    
    Nicol�s
    
    
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