Training - UTM

The Meaning of UTM 12S
By Kathy Baas

What does the term UTM mean?

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).

What is a Zone?

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 UTM zones are numbered at the top of the below chart.  Our zone is 12.

What do the leading numbers and letters mean on my GPS UTM coordinate display (e.g. 12S)?

The number refers to the UTM zone as described above; the letter refers to an 8-degree band of latitude within the specific UTM zone.  The southernmost band from 80 degrees South to 72 degrees South is letter "C".  The letters increment to the North ending in "X" which is a 12 degree band from 72 degrees North to 84 degrees North.  The system skips certain letters that may be confused with numbers, e.g. the letters “O” and “I” are not used.  Our band is S.

If your GPS is set to the UTM coordinates you may see a location like 12S 305008 3785800 when it is turned on.  Last month an article in the TRSAR newsletter titled Some Basics of UTM detailed what the 12 S stood for.  In that article the 60 zones and 20 bands of the UTM coordinate system were described.  The majority of Arizona is located in zone 12, band S. 

Any UTM waypoint includes two sets of numbers to better define the actual physical location within the zone and band.  Today’s article will discuss what the two sets of numbers that follow the zone and band stand for.  

Eastings

The first number following the 12 S is called the Easting.  It is a 6-digit number to designate the EAST/WEST position in the grid.

Because of the round shape of the earth the 120 grids created by the zone lines and band lines do not make perfectly shaped squares of the same width.  The grids are the widest at the equator. The further north or south you go from the equator, the grids become less wide.  Since each grid has a different width, a system was created to measure the distance across a grid starting from the vertical center-line of each grid as opposed to starting from the left or right edge of a grid. 

The central line of each grid is called the central meridian.   The central meridian is always assigned an easting value of 500,000 meters East. It is expressed as 500,000 mE.  As you move west of the central meridian the easting will be a number less than 500,000.  As you move to the east of the central meridian the easting will be a number greater than 500,000. 

An easting of zero will never occur, since a 6° wide zone is never more than 674,000 meters wide.  Minimum and maximum easting values are 160,000 mE and 834,000 mE at the equator.  In the 12 S grid our minimum easting is approximate 225070 mE and the maximum easting is approximately 774475 mE. 

Northings

The second number following the 12 S is called the Northing.  It is a 7-digit number that designates how many meters you are north or south of the equator.  Payson lies approximately 3,787,850 meters north of the equator. 

There is an interesting phenomenon of UTM.  If you are on the Western edge of the 12 S grid you may have a UTM location of 12 S 225071 3787850.  If you move one meter to the West your location could then be 11 S 774474 3787850.  You have changed grids to 11 S.  You are on the eastern edge of grid 11 S.  But you are still in the same northing location.  Don’t be confused when you move to a new grid. 

The chart below shows 4 UTM grids made up of zones and bands.  One of the below stars has coordinates that are erroneous.  Can you determine which one?  Answer below.

The left most star in grid 12S has an erroneous Easting coordinate. Since the star is located to the west of the Central Meridian, it should have a easting coordinate less that 500,000. Its correct coordinate should be:

12S 376360mE 3906210mN