UTM COORDINATES

My previous post, titled Number 33, Truman and the Bomb, turned out to be more appropriate to my Alternative Media blog to which I copied it afterwards. However, my very first image in that post featured a reference to UTM COORDINATES that I hadn't heard of before. See Figure 1. I thought an investigation of this system of coordinates would be an appropriate follow-up post to the previous one.

Figure 1

So what is the UTM coordinate system? UTM is an acronym standing for Universal Transverse Mercator. To quote from this source:

UTM is the acronym for Universal Transverse Mercator, a plane coordinate grid system named for the map projection on which it is based (Transverse Mercator). The UTM system consists of 60 zones, each 6-degrees of longitude in width. The zones are numbered 1-60, beginning at 180-degrees longitude and increasing to the east. The military uses their own implementation of the UTM system, called the Military Grid Reference System (MGRS). One system is no more or less accurate than the other. They are just two different ways of positioning a point. Many experienced users prefer UTM over latitude/longitude when using 7.5' topographic quadrangle maps. Ocean-going sailors and other marine users almost always use latitude/longitude because navigation charts are optimized for this method.

A two page PDF file is available for download here and the graphic in Figure 2 is taken from it. Note the nuclear test site is in the 13th zone. From Figure 1 the UTM coordinates are:

 13S 363231.08738201  3727353.1773228

Figure 2

Wikipedia does a better job of explaining it. The S in the 13S does not refer South as one might expect but to a latitude zone, each zone being designated by different letters.

A position on the Earth is given by the UTM zone number and the easting and northing planar coordinate pair in that zone. The point of origin of each UTM zone is the intersection of the equator and the zone's central meridian. To avoid dealing with negative numbers, the central meridian of each zone is defined to coincide with 500000 meters East. In any zone a point that has an easting of 400000 meters is about 100 km west of the central meridian. For most such points, the true distance would be slightly more than 100 km as measured on the surface of the Earth because of the distortion of the projection. UTM eastings range from about 167000 meters to 833000 meters at the equator.

In the northern hemisphere positions are measured northward from zero at the equator. The maximum "northing" value is about 9300000 meters at latitude 84 degrees North, the north end of the UTM zones. In the southern hemisphere northings decrease southward from the equator, set at 10000000 meters, to about 1100000 meters at 80 degrees South, the south end of the UTM zones. For the southern hemisphere, its northing at the equator is set at 10000000 meters so no point has a negative northing value.

The position of the nuclear test site is thus about 3727.353 kilometres north of the equator and about 363.231 kilometres from the western edge of Zone 13 with the central meridian of that zone being located at 500 kilometres. Thus the site is about 126.769 kilometres east of the central meridian. The system certainly takes some getting used to but it's easy enough once you understand it.