# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Question about LoP's and Intercepts on a Mercator Chart
From: Frank Reed
Date: 2021 Jan 21, 07:34 -0800

David Iwancio, you wrote:
"Plotting is typically done on Mercator projections (or something 'close enough')"

Small area plotting sheets (also known as "universal plotting sheets") are sometimes erroneously referred to as Mercator projections. They're not. They are only "locally conformal" and all that means is that they are scaled by a factor of cos(Lat) so that nautical miles east-west match nautical miles north-south --same scale. This makes them "shape-perserving" -- buoys arranged in a perfect square in any orientation in the real world, will, when plotted on a conformal plot, make a perfect square on paper.

With that out of the way, the rest of your question still stands. What is the math project you're working on? I understand you don't want to show all your cards (so to speak), but if you can give some idea of the range of scale involved, then we can discuss what projection issues would come into play. With the intercept method, if the DR is used for the AP, the intercepts are generally short --a few miles, maybe ten miles at most-- and there's no significant error introduced from plotting. But if we use the constrained AP system as for short tables like 229/249, then intercepts are normally dozens of miles long, and projection issues are more pronounced.

Frank Reed
Conanicut Island USA

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