The UTM Coordinate System

As a member of a search team, either in the field or command post, there are many ways to report or find your position.  You will all want to be on the same page, so to speak.  If you have purchased a GPS (Global Positioning System) unit and it is still in the default settings, your GPS is probably set to LAT/LON (Latitude/Longitude).  This is a great system for aviation and maritime use or areas of great flat plains.  Most large scale maps cover an area of 6 degrees of longitude, and LAT/LON coordinates are based on a scale of 1:1,000,000.  In SAR work, we would like a little more detail than that.  Most of the maps that we use in the field are USGS (United States Geological Survey) 7.5 minute quadrangle.  It represents a scale of 1:24,000.  Double check to see which Datum we will be using.  Older maps use NAD 27 (North American Datum 1927).  If the lines for UTM are there, then you just might be using NAD 83.  Today we will be discussing and learning to use the UTM ("Universal Transverse Mercator") system.  We will learn to find where we are as well as where we want to go using this method. This method is fairly simple to learn if you open your mind to it.  We will be using some compass techniques but for the most part, forget what you know (temporarily) and just remember you can teach old dogs new tricks.

What Is UTM?

UTM stands for Universal Transverse Mercator.  But who really cares what it stands for; the real question is where did it come from?  And why?  The UTM system was devised by the Military as the best way to draw the earth, which is round, on a flat piece of paper.  This is a very important note, as GRIDS (we will discuss later) are Perfectly Square. The system is widely used by the U.S. Forestry Department and the Military.  Five main characteristics of the UTM system are very important to remember.

• Each GRID on the map is square.

• There are no negative numbers.

• The East-West units of Measure are the same as North-South units.

• You will always read left to right and from bottom to top.

• It is decimal based. No messing with minutes and seconds.

• It is easy to abbreviate coordinates when working in a small area.

How Does It Work?

The GPS system is very easy to understand.  The handheld unit has a small satellite dish or antenna to receive signals from manmade satellites orbiting the earth at a known location to pinpoint your location.  The unit translates the signals into two long numbers.  Remember, as an expensive and sophisticated as the GPS is, YOU are the one that must know how to plot these numbers onto a topographical map to obtain your position, or you might as well have hiked with a rock for the last ten miles.  If you still don't seem understand how this works let's see if I can explain it  in easy SAR terminology.  Let's look at the handheld GPS unit as a compass (you remember those things that hang around your necks).  Now with our GPS unit we will triangulate (to find a position by taking a bearing using at least two fixed points) using satellites in outer space instead of mountain tops with water tanks.  The GPS unit receives a signal from multiple satellites orbiting the earth at known locations and then uses triangulation to pinpoint your position. 

What Do I Need To Do This?

You don't need an expensive GPS unit to use the UTM system; it just makes it a little easier.  As we all know, we can't afford to purchase a GPS unit for every member.  In fact, even with three or four GPS units, we may not be able to send all teams into the field with their own GPS unit. We would like the whole team to use the same system for ease of operations, the UTM coordinate system, and reporting their position.  So what do we need to use this system?

• A GPS unit or a compass (I will explain later).  

• A map of the area

• A plastic grid.
  
  

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The Map

Now for the BAD NEWS!  To prepare to read and plot UTM coordinates, you must erase all North and South lines and redraw grids using the blue tick marks.  Newer maps might have these drawn on them for you.  The blue tick marks can be found on the borders of the map.  We will be using the newly formed grid squares to find our location. Each GRID is a 10 unit by 10 unit square (remember the metric system?)

This is available from maptools.com in pdf format

Learning To Read

Go ahead and take out your map.  Some new maps printed after 1990 will have the GPS grid printed on them already.  Notice the numbers along the sides where the blue tick marks are; these are the UTM grid coordinate numbers we will be using.  If your map does not have the GPS lines on it, you will have to draw then in using either a pencil or permanent marker.