#22 Shovelbums Guide Part 7: The UTM Grid

I’ve surveyed a lot of land in the last few days and every day someone seems to have trouble with the UTM grid.  Admittedly, math is not usually a strong point with most archaeologists but knowing where you are and the ability to read and understand a map are important.  The UTM grid is a tool that, when used properly, can greatly simplify your surveying efforts.  What is the UTM grid and why do we use it?

For the five people that read this blog I’m going to cut to the chase and lay out exactly what the grid is in one paragraph without getting into the details.

The Universal Transverse Mercator (UTM) grid divides the Earth into 60 vertical zones that are each 6° of longitude wide and are centered over a longitude line.  The zones are numbered 1 to 60 with zone one starting at 180° W longitude and zone numbering increasing to the east.  North America is covered by zones 10-19.  The zones are separated into 20 latitude bands lettered from C to X (south to north) with “I” and “O” omitted (I’ll say why in the detailed discussion).  Each zone is 1,000,000 meters wide with 500,000 in the center.  If you are walking in a UTM zone the northing will increase as you walk north and the easting will increase as you walk east.  The northing is your distance, in meters, from the equator an the easting is unique to the zone.  That’s the quick and dirty description of the UTM grid.  Now for the details.

Full UTM Grid.gif

Can you guess who designed the UTM grid?  That’s right, the military.  It was designed by the Army Corps of Engineers in the 40s to better calculate distance between two points.  Calculating distance between points on a latitude-longitude grid required complicated trigonometric formulas (remember, no calculators in the 40s).  The UTM grid required only the Pythagorean theorem (a2+b2=c2) to calculate distance, which can be done on a piece of scratch paper.  I’ve done it many times to calculate distance when I didn’t have a GPS in front of me.

“Mercator” in Universal Transverse Mercator refers to the Mercator projection of the Earth developed in the 16th century.  It was designed to represent the Earth on a two-dimensional piece of paper while preserving angles and approximate shapes but distorts distance and area.  The transverse Mercator is similar but uses non-linear scaling to preserve distance and area.

As I stated above, the grid is divided into 60 zones which are each divided into 20 sections or “Latitude Bands”.  The bands run from 80°S to 84°N with the first band starting with “C” in the south and ending with “X” in the north.  The letters “I” and “O” are omitted because the military always eliminates them due to their similarity to numbers.  Most of the bands are 8° high with band “X” extended to 12° to cover all of the land on the earth.  The remaining letters, “A”, “B”, “Y”, and “Z” are also used.  They cover the western and eastern sides of the Antarctic and the Arctic regions respectively.

US UTM Zones.png

A typical coordinate in the UTM system needs to have four parts.  They are the zone, band, easting, and northing.  A coordinate in Nevada could be 11T 587563 4375648.  That translates to zone 11, band T, with an easting of 587563 and a northing of 4375648.  The easting indicates that the coordinate is just east of the center of grid zone 11T.  The northing translates to 4,375,648 meters north of the equator.  If you are in the southern hemisphere the northing still refers to a distance to the equator.  The grid was designed so that you never have a negative number.

We typically walk on a particular northing or easting when we are doing survey.  Whenever I’m walking an easting line (maintaining a constant northing) I like to think about the fact that I am walking parallel to the equator.  It’s a little amazing that this system is set up that way and that I can parallel a line that is over four million meters away.  Geometry is fun!

Hopefully this clears up a little confusion for some people.  There is a lot more information on map projections, datums, and the UTM grid online.  Take a minute to learn more about the tools that we use every day.  It will make us all better scientists.

See you in the field!

Written in Eureka, Nevada on Independence Day Eve.

(Independence Day festivities begin with cannon fire at 5:22 am, sunrise! Wish we didn't have to work during the parade and games)