# Universal Transverse Mercator (UTM)

Based on the Transverse Mercator projection.

The U.S. Army selected this as the standard for mapping the world in 1947. UTM is Conformal, meaning that local angles are preserved at the expense of distortion of size. The UTM grid divides the world into 60 zones, each zone being 6 degrees wide. The zones are numbered one to sixty, moving eastward from 180th meridian from Greenwich. Letters are assigned moving south to north. For example, Washington, DC falls in grid zone 18N. Each quadrangle on the grid is further subdivided and measured in meters.

Scale reduction factor is set to 0.9996, producing a 1:2500 reduction.

When mapping regions beyond the six degree width, distortion increases at a faster rate as distance from the central longitude increases. Applying a zone of width greater than 15 degrees is not recommended, as the mathematics of transforming Transverse Mercator coordinates back into latitude/longitude values is not as accurate as the transformation of latitude/longitude values into Transverse Mercator values.

See Transverse Mercator (TM) for an example.

Promis.e provides a variation called Universal Transverse Mercator Using BF Calculations. BF stands for Bernard FlaceliÃ¨re, who modified the Transverse Mercator equations. His changes ensure identical results within the 6-degree-wide central area to the ordinary Universal Transverse Mercator but allow use of coordinates beyond the 6-degree zone in a reversible fashion. Stretching and general deformation of the map does occur outside the central area but the conversions are reversible. The transformation is similar to UTM using the ESRI Complex Transverse Mercator in its purpose. Calculations using this variation will be slightly slower to operate.