Ever wonder why it is called the State PLANE Coordinate System, one of the reasons is that the XY coordinates values were designed to be on the same PLANE (elevation). Once a point was moved to another PLANE (elevation) it became something other than a State PLANE Coordinate.
This system has worked well for a long time but with today’s technology we easily collect data with the X,Y, and Z value, adding a whole new dimension to the usefulness of the point.
Most surveyors have a method to bring a State PLANE Coordinate to the more useful “GROUND” value by including the Z value and scaling the XY from the origin (0,0) by some scale factor. This works OK but the determination of what scale factor to use is random and that creates the need for metadata to be included with the point. Another drawback is that the resulting coordinate value is normally within a few hundred feet of the actual State Plane coordinate which can create confusion and mixed coordinates.
How could (if you think we should) we go about examing our current State Plane Coordinate System and adopting a standard for a XYZT coordinate system. Might as well go ahead and add the fourth dimension of time since the future value of the point will be dependent on when it was collected.