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## 6.3.3 Universal Transverse Mercator (UTM) projection (-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°S and 84°N, most of which are 6° wide. Each of these UTM zones have their unique central meridian. Furthermore, each zone is divided into latitude bands but these are not needed to specify the projection for most cases. See Figure 6.17 for all zone designations.

Figure 6.17: Universal Transverse Mercator zone layout.

GMT implements both the transverse Mercator and the UTM projection. When selecting UTM you must specify:

- UTM zone (A, B, 1-60, Y, Z). Use negative values for numerical zones in the southern hemisphere or append the latitude modifiers C-H, J-N, P-X) to specify an exact UTM grid zone.
- 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. (although 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.

Next: 6.3.4 Oblique Mercator projection Up: 6.3 Cylindrical projections Previous: 6.3.2 Transverse Mercator projection Contents Index