Chandlery December 2002 Issue

Weems & Plath GPS Plotter

We regularly point out that chart plotters, GPS receivers, and navigation software should be thought of as supplements to—and not replacements for—paper charts, even if you spend most of your time consulting the electronics instead of the paper. We haven't noted, however, something we all recognize: These new electronic marvels have made a radical change in the way that a paper chart is used. 

The GPS Plotter is made in England by Weems &
Plath. It's a simple, well-made tool that speeds
up the 'reverse plotting' we all do today. It can also
perform all the usual functions of parallel rules.

A few years back, you would take a sight on a landmark or buoy, then draw a line on the paper chart corresponding to the compass bearing. Then you'd add another line from a different location (as close to 90 degrees as possible from the first) to get a rough idea of your location; and then, if you could find one, you'd add a third bearing to get a reliable fix: The intersection of the two or three lines identified the boat's position on the chart.

Latitude and longitude could then be derived by using a pair of parallel rules or a rolling plotter to "walk" that position over to the edges of the chart, where you could pick up the latitude and longitude data.

Nowadays, it's backwards: It's much more common to get latitude and longitude information directly from a GPS, and then have to mark the fix on the chart. The good folks at Weems & Plath, who work hard to keep us supplied with instruments that aid the navigator (as opposed to the "data clerk"), are now importing a neat gadget to simplify translating that information to the right position on the paper chart.

It's called a GPS Plotter. Essentially, it consists of a pair of transparent plastic rules, 15- 1/2" x 1-1/2", connected with a clever pantograph-type linkage that allows them to be opened to 9" while remaining parallel without offsetting the two rules, as is the case with conventional parallel rules. There are seven pairs of small holes drilled in the outer edges of the rules.

To mark a position on a chart, you simply place one of the rules against the latitude scale at the chart's edge, with a portion of the rule overhanging the longitude scale, and one of the holes lined up with the correct latitude. If you then open the rules until the outer edge of the far rule lines up with the correct longitude, you can use that rule to guide you in drawing a short pencil line on your chart near the hole that corresponds to the starting one. You can then insert the pencil point into that corresponding hole, and open and close the rule slightly to draw a line at right angles to the first one. That's it.

The GPS Plotter is big enough to handle just about any printed chart without undue fuss. It can also be used to measure latitude and longitude of any point on the chart using a similar procedure, or it can be used like a conventional set of parallel rules to lay out headings and bearings (there's even a small protractor printed on one of the rules, though we were happier "walking" the rule over to the compass rose on the chart.

The GPS Plotter isn't exactly cheap, at $49.99 ($40 as of this writing at Landfall Navigation ( It's a well-made, handy device though, that should be extremely useful to both casual and serious navigators.

Contact - Weems & Plath, 410/263-6700,

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