# State Plane Coordinates vs. Surface Coordinates, Part 4.

Please note that these projection surfaces intersect the ellipsoid, not
the earth's surface. The secant cone intersects the surface of the ellipsoid along
two parallels of latitude called standard parallels. Specifying these two parallels
defines the cone; specifying a central meridian orients the cone with respect to the
ellipsoid. The transverse secant cylinder intersects the surface of the ellipsoid along
two small ellipses equidistant from the meridian through the center of the zone. The
secant cylinder is defined by specifying this central meridian plus a desired grid scale
factor on the central meridian. The ellipses of intersection are standard lines; their
location is a function of the central meridian scale factor.

Before we get into defining zones and zone constants, letÂ¿s look again at
Figure 2 and ask, "When does one use the Lambert Conformal Conic Projection?"
and "When does one use the transverse Mercator Projection?" (Note: Although the
word "conformal" is not used in naming the transverse Mercator Projection, the
projection is conformal). The Lambert Projection provides the closest approximation to the
datum surface for a rectangular zone longest in the east-west direction. The transverse
Mercator Projection provides the closest approximation to a rectangular zone longest in a
north-south direction. The narrower the strip of the earth's surface desired to be
portrayed onto a plane, the smaller the scale distortion on the projection. As mentioned
in an earlier column, "when the width of an area covered by a single grid is 158
statute miles, the extreme differences between geodetic and grid length will be 1/10,000
of the length of a line." For a state like Connecticut that is somewhat longer in the
east-west direction, the Lambert Projection is ideal. The north-south distance across
Connecticut is less than 158 statute miles; one zone can and does cover the entire state.
New Hampshire, New Jersey and Rhode Island are somewhat longer in the north-south
direction; all three states use the transverse Mercator Projection and, as with
Connecticut, one zone covers each state.