Modified transverse Mercator
The modified transverse Mercator (MTM) coordinate system is a metric grid-based method of specifying geographic locations, similar to the Universal Transverse Mercator coordinate system (UTM). However, MTM uses a transverse Mercator projection with zones spaced 3° of longitude apart, one half of the spacing of UTM zones, to further reduce distortion.
Source: Wikipedia — Modified transverse Mercator (CC BY-SA 4.0)