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Using a GPS and Coordinate Systems


Table of Contents

The GPS System
Using the GPS Constellation
GPS Coordinate Systems
Selecting a GPSr

The GPS System

Think of it. billions of dollars in military hardware, just so we can go play in the woods!

The GPS system consists of a constellation of a minimum of 24 satellites, each orbiting at roughly 12,500 miles above the earth. Each satellite weighs a bit over 1800 pounds, and is about the size of an SUV. The satellites contain four atomic clocks (timing is everything with GPS signals!), and NiCad batteries to provide power during eclipse periods. The satellites follow precise 12 hour orbits, and the positions of the satellites in the sky can be determined by your GPSr at any particular period of time or place.

The GPS system was originally created for military use

NAVSTAR 1 was launched in 1978 and soon there was a working constellation of 24 satellites. A system of selective availability prevented private citizens from attaining better than 100 meter accuracy. But it was clear early on that this system could provide a great benefit to the private sector, and in May 2000 selective availability was removed. The Department of Defense now utilizes technology that can deny GPS signals to selected areas if needed. GPS has since become an indispensable tool in navigation. It allows every square foot of the earth's surface to have a unique address. Which means you can find your car in the parking lot next Christmas!

How It Works


To triangulate, a GPS receiver measures distance using the travel time of radio signals. Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. To make the measurement we assume that both the satellite and our receiver are generating the same codes at exactly the same time (remember those atomic clocks?). By comparing how late the satellite's code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light to get distance.


So: 186,000mps * signal variation in seconds = distance in miles

Using the GPS Constellation

Acquiring Satellites

  1. Batteries should be fully charged
  2. Give the unit time to fully acquire the satellites. This may take several minutes depending on conditions
  3. Traveling more than a few hundred miles from your home area will put you beneath a different satellite configuration, and will require time for your GPSr to “re-learn” their positions

What affects reception?

  1. Tree canopy
  2. Heavy cloud cover
  3. Tall buildings, just like being in a canyon
  4. Rocks or canyons that obscure the horizon
  5. GPSr stuck inside your coat pocket

Satellites move and weather conditions change. If you are having a bad day, give it a minute and try again!

Navigating Under Tree Cover

eTrex models, with their small size patch-type antenna, have slightly less antenna sensitivity and may not perform quite as well as others under heavy tree cover. They will perform as well as any other unit in the open however.

Helix antennas, like those found in the Garmin 60 series, will have lower sensitivity if the antenna is placed close to a metal surface. This will be the case if it is stored in your pack beside metal objects. Patch type antennas work better if you want your GPSr to receive with the unit in your pocket. No GPSr will perform well unless its antenna has a clear view of the satellites. In many cases, sensitivity can be improved with the use of an external antenna.

Therefore, since you can’t cut down the trees:

  1. Look for clearings or a high point, where you can regain your signal and take a bearing.
  2. You may find the unit will re-acquire satellites faster if you turn it off, then power back on.
  3. Use your compass under heavy tree cover to check GPSr accuracy if the signal is weak.
  4. Consider using an external antenna to boost gain.
  5. Expect to rely more on your paper maps, and use the GPSr to verify your position when signal becomes available again.
  6. Plot a route on you map with a compass, based on your last known position.

What Is WAAS?

WAAS stands for Wide Area Augmentation System.

WAAS is a coordinated system of satellites and ground stations that provide GPS signal corrections, giving you better position accuracy. A WAAS-enabled GPSr can give you position accuracy of 9 feet, if conditions are ideal.
WAAS consists of about 25 ground reference stations placed across the United States. These monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and send out a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere.

The corrected message is then relayed back through one of two geostationary satellites,
with a fixed position over the equator (these are number 35 & 47 on your GPSr, and will require a clear southern view to receive).

Currently, WAAS satellite coverage is only available in North America.

GPS Coordinate Systems

We can define a point on a flat map using two lines that cross. One line that runs North/South and another that goes East/West. Where they cross is the location of the point you are interested in, a waypoint. Waypoints are presented as coordinates, a set of numbers for one line and another set of numbers for the second line.

The maps we use are flat, but the earth is a sphere. Trying to depict a round surface on a flat map distorts the true relationships a bit. This distortion complicates use of coordinate systems, and is one of the reasons that there are many, to meet the needs of the users. Out of these systems, we will focus on three that are most commonly used in GPS navigation.

Latitude and Longitude

Most maps have a grid marked on them. This is commonly the latitude and longitude system, marked in degrees.

The Earth's equator is the zero line, the base line for latitude, and the numbers increase both to the north and to the south from there to a maximum of 90 degrees, which is a single point at each geographic pole. The latitude lines all run east and west around the earth, and are referred to as the parallels because they do not cross each other.

The longitude lines, also known as the meridians, run north and south, and cross each other at the poles. The extension of a line is known by a different number on each side of the poles. The base line for longitude runs through a town in England called Greenwich and is called the prime meridian. The longitude lines are numbered east and west from the prime meridian. There are 360 degrees in a circle, but since we are measuring from a base line in both directions, the degrees east and west can only go up to 180 in each direction before meeting the base line on the opposite side of the Earth. Zero and 180 are neither east or west, until you begin to move away from either of them and you become either east or west longitude. The line exactly opposite the prime meridian on the globe is the international date line.

In the Bay Area

  • 1 degree Longitude equals just over 55 miles
  • 1 degree Latitude equals approximately 69 miles
  • 1 minute LON equals .917 miles. 1 minute LAT equals 1.15 miles
  • .001 minutes in LAT or LON equals about 5 feet


The most common system is one that shows coordinate as Degrees/Minutes/Seconds. You can only go up to 90 degrees north or south latitude, and just short of 180 degrees east or west longitude. At the equator a degree of latitude is over 69 miles wide , so smaller divisions are required for depicting waypoints accurately.

The system is best remembered like what is used on a clock. Think of a degree as an hour. Each degree is broken up into 60 minutes, each minute is broken up into 60 seconds. Longitude lines get closer together until they reach the poles, but latitude lines stay the same distance apart all the way to the pole.

A DMS coordinate looks like this -
N37° 19' 28.44" W121° 54' 42"

The N37 is the number of degrees of North Latitude (N = north). Remember that the degree numbering starts at zero, the Earth's equator. In this case the N37° line runs through the San Francisco Bay Area. The 19' is the number of minutes (' = minute) north of that. A minute is 1/60th of a degree. The 28.44" is the number of seconds (" = second) north of 19 minutes. A second, like on a clock, is 1/60th of a minute.

Degree Decimal Minutes

The system used most by geocachers is a decimal form of the system above. Each of these systems can be selected within the GPSr preferences, and the GPSr will convert data to display correctly.

A decimal coordinate looks like this -
N37° 19.474' W121° 54.703'

You'll notice the same degrees and minute, but the seconds are now presented in decimal form. This is figured from the DMS system by taking the seconds part of the coordinate and dividing by 60. For example, here is the DMS north coordinate used above: N37° 19' 28.44". Divide the 28.44 by 60, which gives you 0.474. So you get this: N37° 19.474’. This has been rounded to 3 decimal places as you will see in most applications.

To convert a decimal coordinate back to a DMS coordinate take the decimal part and multiply by 60. So .474 times 60 equals 28.44, and just make a DMS coordinate out of that: N37° 19' 28.44". This concept can be taken further by converting the whole latitude or longitude coordinate to a decimal degree. We already have the above N37° 19.474. Take the 19.474 and divide that by 60 to convert it to a decimal amount. You'll get 0.32457, so tack that after the N37 which will now read: N37.32457°. Each half coordinate is one long number, which can be shortened by the number of decimal places, depending on the accuracy you require.

Universal Transverse Mercator (UTM)

UTM coordinates are a military grid system used for quickly pin pointing map locations with reasonable accuracy.

The DMS coordinates above converted to UTM look like this:
10N 596420 4131434

The first number, 10N is the zone. Next comes the east/west number (the ‘easting’), and finally the north/south number (the ‘northing’).

The UTM coordinate system offers several benefits:

  1. A square grid. UTM Provides a constant distance relationship anywhere on the map. Compare this to DMS where Longitude lines converge closer to the poles.
  2. No negative numbers or East-West designators. Grid values increase from left to right and bottom to top. This is just like the X Y coordinate system you in math.
  3. Coordinates are decimal based, so no more minutes and seconds to convert.
  4. UTM coordinates are metric.
  5. The UTM grid lines are physically printed on most topo maps, while the DMS coordinates are only indicated along the map edge. Grid lines are 1000 meters apart.

Most GPS receivers will convert DMS coordinates into UTM coordinates, or from UTM to DMS or to many other coordinate systems. A Garmin or Magellan GPSr can be preset to display either DMS or UTM. Geocaching coordinates are typically presented as Degree Decimal Minutes.

Conversion between all the systems can also be done using a simple online tool at Coordinate Converter and Mapper

Selecting A GPSr

Features that make a GPSr good for outdoor orienteering or Geocaching may not be the same thing you would look for in a car-based system:

  1. A good size screen to display maps. Color and hi-resolution are desirable. How does it look in full sun? Does it need a back light turned on to see it in shade?
  2. Water resistance. You WILL drop it in a lake someday.
  3. Battery life. You will be far from a re-charger out there. Does it allow battery replacement in the field?
  4. Maps. Road and topo maps can be very helpful. Does it have a base map? What maps are compatible with it?
  5. How about enough memory to load additional maps? Does it have a removable memory card for upgrading? Topo maps use up a great deal of memory.
  6. Route and track storage. More routes and more track waypoints will allow you to save trails for future use.
  7. Waypoints. Most units hold 500 waypoints, some hold 1000. Geocachers may want to hold all the caches in their home area, all their finds, and other important waypoints. You will use up 1000 fast!
  8. Form factor. How will it be held while hiking? Are the controls easy to use? A rocker keypad makes one-handed operation simpler. Ease of data entry is important. See how the controls feel in your hand. Can it be used with gloves?
  9. 12 channel system. This is important for the best reception in difficult terrain and tree cover.
  10. External antenna jack. Hikers may safely store the receiver inside a pack with an antenna attached to their pack shoulder straps.
  11. Computer Interface. Can you hook it up to upload/download easily? Make sure it's NMEA compatible. USB compatibility is becoming more common in GPSr’s and will GREATLY speed up downloads of maps and waypoints.