6.3.3 Universal Transverse Mercator UTM (-Ju -JU)

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 84$^{o}$S and 84$^{o}$N, most of which are 6$^{o}$ 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:

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UTM zone (1-60). Use negative value for zones in the southern hemisphere

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Scale along the equator in inch/degree or 1:xxxxx (-Ju), or map width (-JU)

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$^{o}$ away from the central meridian. For larger regions we use the conformal latitude in the general spherical formulae instead.