Transverse Mercator is similar to the Mercator projection except that the axis of the projection cylinder is rotated 90° from the vertical (polar) axis. The contact line is then a chosen meridian instead of the Equator, and this central meridian runs from pole to pole. It loses the properties of straight meridians and straight parallels of the standard Mercator projection (except for the central meridian, the two meridians 90° away, and the Equator).

Construction | Cylinder |

Property | Conformal |

Meridians | Meridians are complex curves concave toward a straight central meridian that is tangent to the globe. The straight central meridian intersects the Equator and one meridian at a 90° angle. |

Parallels | Parallels are complex curves concave toward the nearest pole; the Equator is straight. |

Graticule spacing | Parallels are spaced at their true distances on the straight central meridian. Graticule spacing increases away from the tangent meridian. The graticule retains the property of conformality. |

Linear scale | Linear scale is true along the line of tangency, or along two lines equidistant from, and parallel to, the line of tangency. |

Uses | Where the north-south dimension is greater than the east west dimension. Used as base for USGS 1:250,000-scale series, and for some of 7.5-minute and 15-minute quadrangles of National Topographic Map Series. |

Transverse Mercator also loses the straight rhumb lines of the Mercator map, but it is a conformal projection. Scale is true along the central meridian or along two straight lines equidistant from, and parallel to, the central meridian. It cannot be edge-joined in an east-west direction if each sheet has its own central meridian.

In the United States, Transverse Mercator is the projection used in the State Plane coordinate system for states with predominant north-south extent. The entire Earth from 84°N to 80°S is mapped with a system of projections called the Universal Transverse Mercator.

This projection contains this unique parameter:

Scale factor at central meridian

Designate the desired scale factor at the central meridian. This parameter is used to modify scale distortion. A value of one indicates true scale only along the central meridian. It may be desirable to have true scale along two lines equidistant from and parallel to the central meridian, or to lessen scale distortion away from the central meridian. A factor of less than, but close to one is often used.