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.
More UTM Basics
By Kathy Baas
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.
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
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
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.
of UTM locations
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?
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