US SPCS Zones
The US SPCS zones show the decisions made in picking a projection, in this
case between the transverse Mercator and the Lambert conformal conic, both
conformal projections.

Lambert conformal conic
Transvere Mercator 
 The US has a little over 120 state plane coordinate
system zones, for 50 states.
 SPCS coordinates allow "plane surveying" techniques, instead of much
more complicated geodetic surveying. It works provided the region of
interest is sufficiently small.
 Some states mandate that SPCS be used for all state mapping.
 Some states use feet instead of meters, and there are two varieties of
feet, international and US, and despite the difference appearing to be
extremely small, it accumulates over millions of feet which is the magnitude
of the coordinates.
 Smaller states have one zone, and larger
states up to 10 (Alaska) and 6 for California. This allow minimation of
distortion, which is less than with the UTM projection with its larger
zones, 60 for the world.

States that are tall use a transverse Mercator projection.

States that are wide use a Lambert conformal conic projection.

The zone boundaries are irregular, because they match internal political
boundaries in the states. This is for the same reason that time zone
boundaries are not straight lines every 15 degrees.

The large state of Montana has only a single zone. They might have
prioritized simplicity over further minimizing distortion.

Two states have a mixed system
 Alaska has 9 Transverse Mercator zones, and one Lambert
conformal conic for the Aleutian Islands
 Florida has 2 Transverse Mercators zones, and one Lambert conformal
conic for the pan handle
Other countries have made the same decision about the projection to use for
their national maps, based on the shape of the country to choose either the
Lambert Conformal Conic or a transverse Mercator.
Last revision 8/29/2018