Do state plane coordinates always begin as longitude and latitude?
In other words, they are not another system that can be calculated without Long. and Lat., correct?
I guess I'm saying, back in 1930+ they came up with the State Plane system. To make that work, however, they had to start with L&L. Today, I am guessing I can simply compare it to a database. But if for example, I have a GPS device and I want to know the Plane Coordinate, I have to convert it.
Almost all software for doing coordinate projection use geographic coordinates as a standard system in the middle and defines "forward" (from geographic to projected) and "inverse" (from projected to geographic) routines for projections.
So, to go from a UTM projection (say "UTM 10 N") to a stateplane projection (say "Washington Stateplane North") you'd first run the inverse projection from UTM to geographic, then run the forward projection from geographic to stateplane.
There are algorithms that will transform from a grid system to another grid system without going without requiring a conversion to lat-long. So the best answer I can come up with is not always, but often they use lat-long.