Graeme J W Smith

So I’m preparing cross country flights and laying my plotter on the sectional chart. To my seagoing navigation eye it looks like the parallels of latitude are slightly curved instead of the straight I’m used to. I look in the title box on the chart. No mention of the projection used. I dig around and find it is Lambert’s Conical. A little more reading.

The Mercator projection that used to hang on the elementary school room wall – latitude and longitude lines are straight, parallel and at right angles to each other. The distortion at the poles is horrific and a Great Circle route is drawn as a curved line. Charles Lindeburg figured this out in the week before his flight across the Atlantic while studying navigation for his trip in the local library. He prepared a series of courses on his school room map of the Atlantic and flew for an hour at each heading – adjusting slightly on his chart every hour and (rather amazingly) found Ireland and then Paris. This is the practice at sea – where you move relatively slowly and have time to figure this stuff out and alter course on the distorted paper chart every few hours. There are some other useful sides to Mercator projection for plotting star and sun sights which is why it is still used at sea.

But as flying speeds increased this method became a PIA and the Lambert conical projection was used for flying. A line drawn between two points in a straight line is essentially a Great Circle route and this avoids the need to fly constantly changing courses across the distorted Mercator chart. But there IS a gotcha – because of the way the projection is laid on the paper – Yep – the LATITUDE lines on sectionals ARE slightly curved. Lambert Conical Projection at work. But an upside to Lambert (as with Mercator) is that you can use the LATITUDE (but not Lon) scale as a Nautical Mile scale.

It actually gets alluded to on page 4-42 of the Jeppsen Private Pilot Book book without going into detail.

More about it – two fifths of the way down this page where the Civil Air Patrol discuss how to draw the correct slightly curved lines to grid a sectional for Search and Rescue Operations.

and more at Wikipedia

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