![]() The distortion ellipses plotted in red help us visualize the pattern of scale distortion associated with a particular projection. Because of this, map scale varies within projected (plane) UTM coordinate system grids. Similarly, the act of mathematically transforming geographic coordinates to plane coordinates necessarily displaces most (but not all) of the transformed coordinates to some extent. The illustration below (Figure 2.22.2) shows the 60 UTM zones as they appear when projected using a Transverse Mercator map projection formula that is optimized for the UTM zone highlighted in yellow, Zone 30, which spans 6° West to 0° East longitude (the prime meridian).Īs you can imagine, you can't flatten a globe without breaking or tearing it somehow. Such transformations are called map projections. "Transverse Mercator" refers to the manner in which geographic coordinates are transformed into plane coordinates. In fact, since meridians converge toward the poles on the globe, every UTM zone tapers from 666,000 meters in "width" at the Equator (where 1° of longitude is about 111 kilometers in length) to only about 70,000 meters at 84° North and about 116,000 meters at 80° South. The illustration above (Figure 2.22.1) depicts UTM zones as if they were uniformly "wide" from the Equator to their northern and southern limits. These are numbered west to east from 1 to 60, starting at 180° West longitude (roughly coincident with the International Date Line). (Polar coordinate systems are used to specify positions beyond these latitudes.) The UTM system divides the remainder of the Earth's surface into 60 zones, each spanning 6° of longitude. Only polar areas-latitudes higher than 84° North and 80° South-are excluded. The Universal Transverse Mercator system is not really universal, but it does cover nearly the entire Earth surface. Also shown are two polar coordinate systems used to specify positions beyond the northern and southern limits of the UTM system. Figure 2.22.1 A Mercator projection of the world, showing the 60 UTM coordinate system zones, each divided into north and south halves at the equator.
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