UTM zones run from -60 to 60, excluding 0. Negative zones are for the Southern hemisphere. The UTM Northing value ranges from 0 to 10,000 in the Northern hemisphere, but from 10,000 to 0 in the Southern, starting from the Equator. Note the original definition of the meter was the distance from the equator to the North Pole as 10,000 kilometers.
The equation for determining the UTM zone from a longitude is abs(zone) = longitude/6 + 31 using integer math.A square grid is superimposed on each zone and aligned so its vertical lines are parallel to the center of the zone. This centerline is called the central meridian, and is three degrees of longitude from each zone boundary.
The UTM coordinates (measures of distance) are arranged so they always read from left-to-right and from bottom-to-top. This is done as follows. In the northern hemisphere the origin, or zero point, of the horizontal lines is at the equator, while in the southern hemisphere the origin is at the south pole.
Establishing coordinates for the vertical lines was done differently. The vertical line at the center of each zone (central meridian) was arbitrarily assigned the value of 500 km to avoid having negative coordinate values. Assigning the value of 500 km to the center of each zone causes the zero point to fall in another zone - the one to its left (west). For this reason, you will never see a zero value for an east-west coordinate. The smallest value is 160 km and it is at the equator. As one moves away from the equator, the UTM east coordinate at the zone's western edge has larger and larger values. At 84 degrees north latitude it is 465 km. Likewise, the eastern (right) zone boundaries will have coordinates of 834 km at the equator and 515 km at 84 degrees north latitude.
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