Construction
The plotter consists of a plotting surface impregnated with fine wires and a moveable "mouse" containing a sensing ring. The mouse's location on the surface can be read by the system at all times.
To use the plotter, a conventional chart is first fixed in position on the surface (the method of fixing varies between the different models of plotter; see below). It is then "registered" - the mouse is placed over three known points on the chart and a button pressed at each. Knowing the location on the surface of these points, the system can interpolate the mouse's position anywhere on the chart. Many charts now have plotter reference points printed on them, and the coordinates of these points are pre-programmed in the Yeoman unit, but this is not essential. Adding an unknown or "user" chart to the plotter's memory is a simple procedure taking around a minute, and many such charts can be stored for later use.
Although the device can provide some useful data on its own (positions of charted objects, distances and bearings between points), in practice it is invariably linked to a source of position data such as GPS. This enables many useful features, the most important of which is the fixing on the chart of the vessel's current position. In position mode, four illuminated arrows around a transparent area on the mouse are used. The device is pushed across the chart, following the arrows, until all four of them are extinguished. This indicates that the mouse is directly over the vessel's current position on the chart; if required, a pencil can be poked through a small hole to mark the fix on the chart. After a couple of goes, this procedure becomes instinctive and very fast.
The mouse can also be used to obtain the range and bearing from the vessel's current position to any other object, simply by placing it over that object on the chart and reading the figures from the display. If a suitable radar is fitted it can be combined with the Yeoman plotter for navigation in poor visibility. In one mode, the position of the mouse on the chart is reflected in a cursor on the radar display - the mouse can be placed over a charted object to identify its echo on the radar, which is very useful for objects whose reflection may not be obvious. Similarly, a position from the radar screen can be sent to the Yeoman - the mouse is moved by hand following the illuminated arrows as for plotting a fix, and is guided to the location of the radar return.
Read more about this topic: Yeoman Plotter
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