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> *From: *Brian Walton
> *Date: *2025 Aug 31, 00:31 -0700
>
> Boeings on autopilot fly great circle tracks; it's all they can do.

When a Soviet interceptor shot down a KAL 747, wasn't it off course
because the autopilot was flying a rhumb line?

I'll say this for the Mercator projection: it's conformal. In simple
terms, that means a square of linoleum lying flat on the ground is a
tiny perfect square on the map, if you could see something that small.
But although it's conformal, Mercator is not equal scale. If you move
that linoleum to different places on the ground, its size on the map varies.

The maps shown on TV news (in the US, at least) are neither conformal
nor equal scale. Apparently, TV people find distortion as natural as
breathing. CNN slants their news to the political left, Fox is slanted
to the right, and everyone slants their maps to the south. Scale is
expanded in the south and contracted in the north. Mississippi and
Alabama look about the same size as Minnesota, though the latter
actually has much more area.

We can do better. A Lambert conformal conic projection of the entire
CONUS can maintain constant scale within 2% and depict all the right
angle state boundaries as true right angles. Long ago the Coast &
Geodetic Survey selected Lambert for their aeronautical charts for those
reasons and others, explained in their Practical Air Navigation from 1945:

https://archive.org/details/in.ernet.dli.2015.46242/page/n48/mode/1up?view=theater

However, they no longer use the same two standard parallels for the
entire CONUS. On a 2009 sectional chart (1:500 000 scale) I see the
standard parallels are latitudes on the chart itself. Probably that
change occurred because machine computation makes it easy to optimize
the projection for each chart.

Current charts online at the FAA don't seem to include any statement
about the projection. Knowledge of the projection parameters is helpful
to reconstruct the distances and angles someone would have measured on a
chart not in your possession. With the Geotrans program or something
similar you can convert between lat/lon and Lambert grid coordinates.
The latter are rectangular coordinates of points on the map if it were
printed perfectly.

Some years ago I used that method to analyze the navigation exercise
flown by Flight 19. There were discrepancies in course and distance
compared to the accident report. But I may have been using the wrong
projection. I assumed the same Lambert grid parameters as the present
day Miami sectional chart, which covers the whole route. But back in
1945 it did not extend so far east. The designer of the exercise would
have plotted it on some other chart.

--
Paul Hirose
sofajpl.com

   

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