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post #34 of Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Originally Posted by RichH View Post
Originally Posted by Maine Sail
Can one even draw a line on a chart, even with a very sharp pencil, that is less than 24' wide when adjusted for scale..?
Touché . Can one even enter a course-line to next waypoint or 'route line' on a chartplotter, even with a very high magnification, that is less than 24-100' wide?

Damn those stacked up 'tolerances' and 'resolution' problems.

Even more importantly, that applies to the cartographers creating the charts, to begin with...

From Nigel Calder's article cited above:


The finest line that can be drawn is about 0.1 mm wide, but such a thin line is hard to see, and as such, it is not recommended, sometimes forbidden, to be used for drawing features such as coastlines and other critical objects. As a result, various hydrographic offices have adopted 0.2 mm as the finest line to be used on a chart. Let's say the hydrographer decides to plot the smooth sheet at a scale of 1:20,000 (in other words, one millimeter or inch on the smooth sheet represents 20,000 mm or inches on the ground). At 1:20,000, a line that is 0.2 mm wide represents 20,000 x 0.2 = 4,000 mm on the ground. This is four meters. Even if the survey is accurate to within inches, this plotting accuracy has now become the limiting condition in the accuracy of the final product. If the smooth sheet is plotted at 1:50,000, the plotting accuracy is ± 10 meters. If the pencil used to plot the data is not sharp and draws a line that is 0.5 mm wide, the plotting accuracy at 1:50,000 goes down to 25 meters, or 27 yards!
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