NavList:

The recent discussion of constructing a scaled plotting sheet has reminded me once again of one big error that has become entrenched in American navigation culture. The standard "universal plotting sheet" which has been published for years says right at the top "Mercator", but this is wrong. The projection is merely a small scale "conformal" map. This means that shapes and angles are "preserved" by the projection by making the longitude scale match the latitude scale at the center of the plot (simply by multiplying by cos Lat). On a conformal map, a right angle (between any pair of compass headings) plots out as a right angle on paper, and as a result islands and other geographic features have their "bird's eye view" shapes. This "conformality" is a desriable, even essential, property for any navigational plotting that uses compass bearings including DR plots and plots of the celestial "intercept method". The description of this type of plot as a "Mercator" plot is erroneous. Presumably this error came about because the Mercator projection is locally conformal: if you zoom in on an actual Mercator projection of the globe to a region a few miles across, it is conformal and the scale is nearly constant. But this is true of a majority of map projections since local conformality is a very popular feature of map projections. It is in no way unique to the Mercator projection, though the Mercator projection's local conformality is the most famous example of it. Plotting sheets that have been scaled so that longitude and latitude are properly scaled are not Mercator plots.

Normally, I do not quibble over changing meanings of words. In most contexts of English language usage, it's fair to say that usage defines words. Technical fields, including navigation, are different. Words and names for things have nearly fixed meanings, by common agreement among the practitioners, and we respect those meanings "conservatively" because they are critical to the continuity of the subject. It is time to ditch this historical error. Common navigational plotting sheets are not Mercator projections.

-FER