How To Use A GPS: Coordinate Systems and Datums

If you haven't read the first article in this series, you might want to do that now to have a better understanding of how the Global Positioning System and your handheld unit work, how to obtain a good position fix, and common GPS problems. See:


How To Use A GPS: The Basics And Background

So, you've decided to purchase a GPS for some reason. You're a backcountry traveler perhaps, who wants another form of navigation. Or maybe you're...

What are these?

W 110* 15' 30"
N 33* 07" 30"

12S 0342340
38881280

Understand coordinate systems to use a GPS.Location information on a GPS is based on two main parameters--a coordinate system and a datum. It's necessary to have a good understanding of both of these concepts in order to effectively use a GPS.

I'll start with coordinate systems, including what they are and then how they come into play with your GPS.

A coordinate system provides a means of defining a location by measuring horizontal and vertical distance and direction on a map. Distance and direction from what? Well, that depends on which coordinate system is being used.

There are two main types of coordinate systems--angular and rectangular. Angular coordinate systems -- like Latitude-Longitude -- are designed for a three-dimensional object like a globe. This isn't a grid system, because the lines don't meet at 90-degree angles, except along the Equator.

A rectangular coordinate system on the other hand -- like UTM (which stands for Universal Transverse Mercator) -- is a grid. This type of system is designed for a 2-dimensional object like a map, and lines meet at 90-degree angles. The Township-Range-Section lines used in the U.S. also form a grid.

Some maps, like U.S.G.S. topographical maps, have both angular and rectangular coordinate systems on them.

One coordinate system is no more or less accurate than another. They are simply different methods of positioning a point.

Latitude-Longitude: Most people, I'm assuming, are familiar with "Lat-Long" at least in name and can point to the latitude and longitude lines on a globe. Many of us learn a little about this system in school. But as a refresher, here are some basic points about this angular coordinate system:

• Latitude-longitude is expressed in three basic formats
  --degrees/minutes/seconds
  --degrees/minutes/decimal minutes
  --degrees/decimal degrees.


• Latitude measures north-south angular distance beginning at the Equator, which is the 0 line. These are the horizontal lines on a globe, spaced at an equal distance and never coming together. Latitude should be expressed as being north or south of the Equator.


• Latitude lines are 15 degrees apart, which is equal to a constant 1,035 miles.


• Longitude measures east-west angular distance, beginning at the Prime Meridian in Greenwich, England. This is the 0 Longitude line. Longitude should be expressed as being east or west of the Prime Meridian.


• Longitude extends out to 180 degrees at the International Date Line.


• Though Longitude lines are 15 degrees apart, the actual distance between them becomes smaller as the lines come closer together as they approach the poles. This means that a degree of longitude is not a constant.

The above image and the latitude/longitude images below are in the public domain.

UTM: The Universal Transverse Mercator system or "UTM" may be less familiar than Latitude-Longitude, but, to many people, it's easier to use and understand than Lat-Long once they get the hang of it. This is because it's a grid system specifically designed for two-dimensional maps.

With the UTM system, the earth is divided into rectangular-shaped zones. Imagine a map of the world spread out on the table. On that map, there are horizontal and vertical lines, all intersecting at 90-degree angles. These intersecting lines create a grid--20 rows of 60 boxes. The columns are labeled by number (1 through 60) and the rows are labeled C through X, omitting O and I, so they aren't confused with the numbers zero and one. Each UTM "zone" is therefore expressed by a number and letter. For example, I live in Flagstaff, Arizona, which is in zone 12S. (The "S" should not be confused with the word "south.") Someone in West Palm Beach, Florida would be in zone 17R.

Each zone is centered on longitude line. The lettered rows are based on latitude bands running every 8 degrees.

What a UTM coordinate looks like and what it means:

Here is a coordinate for a location in Arizona....

12S 0437925
      3892246

"12" is the east-west zone
"S" is the latitude band number
"0437925" is the "Easting," which corresponds to the number of meters a location is from the west side of the zone. (Eastings are found along the top and bottom of topographic maps.)
"3892246" is the "Northing," which is the distance in meters from the Equator in the northern hemisphere. (Northings are found along the sides of topographic maps.)

There are no negative UTM numbers.

When looking for a UTM coordinate on a map, which you can then enter into your GPS, you'll see that the numbers along the map edges show just the first four digits--the millions, hundreds of thousands, tens of thousands and thousands of meters. (For example, if the Easting is 0437925, you'll actually see just 437 and not the zero in front of it. If the Northing is 3892246, you'll see just the 3892.) You need to measure and add the last three numbers to get the hundreds, tens and single digits.

You can make this measurement using plastic grid readers, which you can buy from the U.S.G.S. or from many commercial vendors. Or you can make a UTM grid reader with a small piece of paper, marking both edges out from the corner in 100-meter increments. You can use the map's bar scale as a guide.

These are some points to keep in mind when using the UTM coordinate system on your GPS:

• Use all seven digits when inputting the Easting and Northing numbers. This means you may need to put a zero in front of the Easting. Your GPS won't take shortened numbers.


• Don't round off or replace the last two or three digits with zeroes if entering a UTM coordinate or reporting it to someone else. This could result in differences of hundreds of meters, which can be an issue in extreme terrain, bad weather, at night or when looking for something small.


• If using the Latitude-Longitude coordinate system, be sure you know which format you're selecting--degrees, minutes, seconds (DD MM SS); degrees, minutes, decimal minutes (DD MM.MMM); or decimal degrees (DD.DDD). If giving Lat-Long coordinates to someone else, you need to read the information correctly, or they may think, for example, that you're giving them degrees, minutes, seconds when you're actually reading off degrees, minutes and decimal minutes.
  

Which coordinate system your GPS is set on is just one part of the equation. The next question is which datum it (and your map) will be using with that coordinate system.

So what is a datum?

A Datum is a mathematical model that calculates the size and shape of the earth.

Datums are necessary for map-making, where the three-dimensional landscape has to be converted to a two-dimensional surface. But there are many different datums, and not all maps use the same one.

A list of just a handful of the datums that exist looks like this:

NAD27
NAD83
WGS84
ITRF00

Where to find the datum used to make a particular map:

Maps have an information block, usually in the bottom-left corner. This where you can find out things like who made the map, when it was made, when it was edited, and so forth. Included in this block of information, you may see the words, "1927 North American Datum" (aka "NAD27) or "North American Datum 1983" (NAD83) to name just two that are common in the U.S.

U.S. Geological Survey topographical maps are made using the NAD27 datum as are Forest Service maps. Aeronautical maps like those used by helicopters are made using the global WGS84 datum.

"If you are comparing GPS coordinates to a chart or map, the map datum in the GPS unit must be set to match the chart or map's datum for accurate comparison."

www8.Garmin.comDatums are also part of the equation on a GPS. When you take a new GPS out of the box, it will most likely be pre-set to WGS84. You can change the datum at any time if necessary.

It's also important to note that some units, such as the Garmin Extrex receivers, will revert to the WGS84 datum whenever you change coordinate formats (ie. from Lat/Long to UTM). So if you're intending to use a datum other than WGS84, you'll have to reset it.

You MUST set the datum on your GPS to the same datum as the map you're using in order to get accurate position information for using on that map.

Also, if you're exchanging information with another GPS user, you both need to be using the same datum -- or at least know what datum the other person's GPS is set on and convert the information they give you or vice versa. Otherwise, you'll essentially be speaking two different languages and there will be miscommunication.

A difference in datums can result in a difference of as much as 200 meters (about 600 feet). This deviance may not be a big deal in, say, a big meadow or areas where you can easily see your target from several hundred feet away, but imagine what the consequences might be in the Grand Canyon for example, where two points that are 500 feet apart horizontally can be several thousand feet apart vertically. This can also be a big issue in bad weather.

When datum doesn't really matter:

If you are NOT using a map in conjunction with your GPS or exchanging information with another GPS user but are, instead, only using your GPS to mark points to return to them later, then it doesn't matter which datum you're using.

The REI Outdoor School offers navigation and GPS classes at store locations throughout the U.S. To see if there's a location and class near you, visit www.REI.com/OutdoorSchool and click on your region to the right. Then scroll down to the "Navigation" section.

Also, the U.S. Geological Survey offers a free GPS, map and compass class each month at the Denver Federal Center. Arrangements can be made for instructors to travel to other areas for large group presentations. If you live outside the Denver area, all instructional materials are available for download. Call 303-202-4689 or e-mail gpsworkshops@usgs.gov for information or reservations.

And check out the Garmin online GPS "Academy" and learn a little from Miss GPS. (To begin, click on the stack of books and then GPS 101 on the blackboard.)

 
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