Absolute lowest altitude you can fly (legally)

Watson writes:

If I recall correctly, 1 min of latitude = 1 NM everywhere (not just the
equator), aren't lines of latitude also called parallels and thus maintain
there equidistant relationship all the way to the poles?

The size of one minute of longitude contracts along each of the
latitude parallels as they get closer to the poles. At the equator,
one minute is one nautical mile (roughly); at the poles, one minute is
zero distance. At U.S. latitudes, one minute of longitude is
considerably smaller than a nautical mile--you can see this if you
look at sectionals.

And don't lines of longitude meet at the poles and therefore their angles
maintian the same
angular relationship yet the distances decrease the further one travels from
the equator?

Right--so one minute in an east-west direction gets smaller and
smaller as you move towards the poles. It's only one nautical mile at
the equator.

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