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Even at the beginning of the first chapter a major change between 1914
and 1938 is obvious. The 1914 edition says, "The Earth is an oblate
sphereoid whose greatest and least diameters are 3963 and 3,950 statute
miles respectively." It continues with an exposition on the non-constant
radius of curvature of a meridian and a formula for the length of a
nautical mile as a function of latitude.

Eventually on the fifth page it admits that "although the Earth is an
oblate spheroid, for nearly all purposes of navigation it is
sufficiently accurate to assume it to be a sphere whose radius is the
mean of earth's greatest and least radii. The errors involved in this
assumption are very small and entirely lost in practice amongst the many
other errors incidental to navigation." In other words, disregard the
previous pages!

The 1938 edition is more practical. It omits the spheroid stuff and
simply says, "Although the Earth is not a perfect sphere, it may be
considered one for the purposes of navigation because the errors which
result from the assumption are usually negligible."

https://archive.org/details/admiraltymanualo00grea/page/n19/mode/2up?view=theater

https://archive.org/details/dli.ernet.205299/page/1/mode/2up?view=theater

Thinking back half a lifetime ago, when I was a maintenance man on the
ASB-9A system in the B-52H, I recall it compensated for the
non-spherocity of Earth. Velocity over ground was resolved into two
components (400 Hz sine waves) whose amplitudes were proportional to
north and east velocity. North velocity was slightly modified by a
potentiometer whose shaft was positioned by latitude. That made the
voltage proportional to latitude rate instead knots of northing.
Latitude rate was integrated into latitude by a precision amplifier and
motor. It was remarkably sophisticated for a vacuum tube analog system.

--
Paul Hirose
sofajpl.com

   

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