A particular subset of the transverse Mercator is the Universal Transverse Mercator (UTM) which was adopted by the US Army for large-scale military maps. Here, the globe is divided into 60 zones between 84S and 84N, most of which are 6 wide. Each of these UTM zones have their unique central meridian. GMT implements both the transverse Mercator and the UTM projection. When selecting UTM you must specify:
In order to minimize the distortion in any given zone, a scale factor of 0.9996 has been factored into the formulae. (allthough a standard, you can change this with MAP_SCALE_FACTOR). This makes the UTM projection a secant projection and not a tangent projection like the transverse Mercator above. The scale only varies by 1 part in 1,000 from true scale at equator. The ellipsoidal projection expressions are accurate for map areas that extend less than 10 away from the central meridian. For larger regions we use the conformal latitude in the general spherical formulae instead.