A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 May 6, 12:36 -0700
A common "universal plotting sheet" is not a "Mercator" chart. It's simply "conformal" or "longitude-scaled". The "plotting sheet" projection is a trivial map projection with no connection to the actual Mercator Projection and its descendants (except the very generic property that they are locally conformal, as are many map projections). The fact that navigators have labeled these things "mercator" is an old error and a bad habit.
By the way, there's yet another possibility for small plotting charts that was difficult decades ago, but is relatively easy today. Leave the latitude and longitude scales equal or "square" and modify the compass rose based on latitude. Unfortunately, this is not a conformal projection, so shapes and angles are not preserved. That means that when you plot an LOP from an intercept, you can't just draw the LOP perpendicular to the intercept vector. Instead you have to take the azimuth of the intercept vector and add 90° to it. Certainly not difficult but it's less visually obvious. Or you can plot from lat/lon pairs and skip azimuth entirely. There's little reason to prefer a system like this except, maybe?, that it saves printing in the long term. Suppose you know you're going to be sailing between 40° and 50° latitude. You print out ten transparencies with modified compass roses, one for each degree of latitude, or possibly five for every two degrees. Then you can do your plotting on common graph paper with each square having a "height" of one minute of latitude and also a "width" of one minute of longitude.