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Discovering GPS

RTI would like to recognize and thank Paul Pingrey for allowing us to use the content of his website( Digital Grove). The images and content found below stem directly from Digital Grove. Click on this link to check out Digital Grove in its entirety.


Unsure whether to purchase a GPS unit? See Discovering GPS, an article originally written for Paul Pingrey's friends at the Wisconsin Woodland Owners Association. It explains how GPS works and the differences between recreational grade units.

You might wonder, "Which is the right GPS unit for the job?" There are three general grades of GPS units: recreational grade, mapping or resource grade and survey grade. See the GPS Comparisons below to help choose the right tool for you.

Discovering GPS

A couple years ago I wanted something different as a gift for my wife, Karen. She enjoys exploring outdoors and keeps a journal with the locations of wild flowers she finds on her walks. I thought she might like a Global Positioning System (GPS) unit. With a GPS you can record your location to within a few feet. As it turns out, the Garmin GPS unit is one of her favorite gifts. Karen memorized the instruction book and within a day was exploring the backwoods of Devil’s Lake State Park in southern Wisconsin.
She especially liked the GPS unit because there are many places she wanted to go where there are no trails. Now when she starts a walk, she can click the unit and record where the car is parked. She can also enter coordinates for points she wants to travel to.

As she hikes in the woods, the GPS unit keeps track of where she is in relation to any of the points she’s interested in. The unit will tell her if:

  • she’s going in the right direction

  • how far away the point of interest is

  • how fast she is moving toward it

  • All that information is displayed on a screen that fits in the palm of her hand.

All that information is displayed on a screen that fits in the palm of her hand.


So how does GPS work? The US Air Force maintains a "constellation" of at least twenty-four GPS satellites that orbit the earth. The satellites are like the stars that people have navigated with for ages. The only difference is that the GPS "stars" send out radio transmissions that an antenna in your GPS unit picks up. If the GPS unit can get a signal from at least three satellites, it will calculate where it is by using an internal clock and simple geometry. Since the unit’s clock isn’t perfect and the satellites are moving (among other reasons), there is error in the location estimate. An error of one billionth of a second in the clock translates into about a one-foot error on the ground. The more satellites a GPS unit gets a signal from, the smaller that error is likely to be. A unit with a differential correction feature called "WAAS" (explained below) can be accurate to within about 10-16 feet on a fairly consistent basis.

To check her GPS unit’s accuracy, Karen switchesscreens (by clicking a button), and it shows her how many satellites it can "see" and what the estimated precision error is at that moment. The location of each satellite in relation to the others and their geometry relative to the GPS unit affects the Dilution of Precision (DOP). DOP values change through the day as the satellites move in and out of view as shown in the above sample chart for a specific day/location. The graph was produced by Trimble's GPS Planning Software described below. The lower the DOP value, the better your GPS location calculation is likely to be. Dense tree foliage, obstructions like hills and buildings or holding the GPS unit too close to your body can also block signals and reduce accuracy.

Although Karen's GPS seldom shows an estimated error greater than 60 feet, she is realistic about her unit’s accuracy. She wouldn’t pretend to be a surveyor with it. If she were looking for a section corner, den tree, deer stand, patch of flowers or our parked car, though, she can come relatively close.

The main difference between recreational grade GPS units is whether or not they display a base map. The less expensive units like Karen's basic eTrex costing about $100 have no built-in maps but are still very handy. They keep track of where you go with a dotted line on the screen. The screen will also show waypoints, which are ground locations that you click on as you travel or which you enter in advance as latitude/longitude coordinates. More advanced units costing $200 to $500 have built-in maps showing roads and streets. Some units (like my Magellan on the left) display topographic elevation lines that are downloaded from companion CDs such as Magellan's MapSend or Garmin's MapSource products.
If you are looking for a GPS unit, get one with the Wide Area Augmentation System (WAAS) feature. WAAS GPS units track a couple extra geostationary satellites that send out correction signals, enabling accuracy to around 10 to 16 feet under ideal conditions. (See more information below.) Garmin, Magellan and Lowrance are some of the more popular GPS brands featuring WAAS.
The Garmin GPS76S unit The Garmin GPS76S unit (purchased for about $360 from Amazon) has built-in maps and a larger display like the Magellan. It also includes an electronic compass (which works with the unit held horizontally) and a built-in barometric altimeter to improve the accuracy of elevation readings.
The GPS76S is a great unit, but the extras do shorten battery life. Karen's little Garmin eTrex is good for a couple days of hiking, rated at about 20 hours on two rechargeable AA batteries. My Magellan Meridian Gold unit is rated at 14 hours and hasn't had any trouble lasting an eight-hour field day. (The only complaint I have about the Magellan is that the power button sticks out too far, frequently causing the unit to be unintentionally turned off.) The Garmin GPS76S is rated at 10 hours with 2 AA batteries, however it might not last three hours on some cold winter days. All the bells and whistles cause a power drain, meaning you better have more batteries in your pocket.

If you don't really need the extra features like an electronic compass and altimeter, the longer battery life found in simpler units could more than outweigh the prestige of having a top-of-the-line unit. You would also be wise to carry a conventional magnetic compass and not rely entirely on your GPS unit in field situations.

If you want to get the most out of a GPS unit, use it with a computer-mapping program. USA PhotoMaps and OziExplorer for 2D maps or 3DEM for 3D views are available for free download from the Internet. Delorme's Topo USA, which costs about $100, is another excellent program to use with your GPS Unit. The programs communicate with the GPS unit via a cable that connects it to the computer. They download the tracks and waypoints from the GPS unit and draw them on a map.

Click for larger image
Click for larger image
The images above show the track of a hike at Wisconsin's Quincy Bluff, a 200-foot high sandstone ridge surrounded by flat lowlands. To help the ridge stand out, a 250% vertical exaggeration was applied in 3DEM.

GPS Comparisons

What are the limitations of a recreational grade GPS unit? Do you need more precision? The following table explains the basic differences between recreational grade and mapping grade GPS units.

Note that both recreational grade GPS (depending upon the features of a particular unit) and mapping grade GPS are capable of real-time differential corrections. Differential GPS (DGPS) significantly improves accuracy, potentially reducing real-time error to less than 3 to 5 meters (<10 to 16 feet). The basic concept is that coordinates provided by the NAVSTAR GPS satellites are compared to readings at fixed locations. The differences between the NAVSTAR GPS readings and the known locations are used to calculate corrections that are applied to your GPS unit's location display.

There are two types of real-time differential corrections available for recreational grade GPS:

  • Corrections based on signals from geostationary Wide Area Augmentation System (WAAS) satellites (broadcasting corrections from about twenty-five US North American ground stations ...more to be added in 2003). Currently, only two WAAS relay satellites are generally visible from the US. The satellites are positioned over the equator. You need a good view of the southern sky to receive the signals. Since the reception angle is extreme, tree cover, hills or other obstructions including buildings or even your body can block WAAS signals. Eventually there could be as many as 19 WAAS relay satellites, making WAAS a dependable DGPS system, but deployment will be slow in coming. For a WAAS tutorial and additional links to coverage maps, see (WAAS correction is not yet available outside North America but related European and Pacific systems are being developed.)

  • Corrections based on signals from US Coast Guard (USCG) Differential Beacon Receiver Transmitters located throughout the USA. You must be in range of a USCG transmitter to use the system. Maps of USCG transmitter coverage are available at the above link. (Other differential base station systems are also employed elsewhere in the world.)

Recreational grade units sold in the US typically use the WAAS system. Some recreational grade units can take advantage of USCG differential correction with an optional beacon receiver (e.g., the Garmin GBR 23 receiver, which can be configured to operate from a backpack). Mapping grade units might be designed to use WAAS, USCG or other reference station signals. They also save coordinate data in a format that can be used in post-processing correction programs, potentially resulting in sub-meter accuracy.

Survey grade GPS is not included in the table. Survey grade units are used where accuracy is crucial (as in cadastral surveys, highway construction and other engineering projects). They are capable of providing horizontal accuracy to within a centimeter. The costs (up to $40,000), training requirements, time involved to establish readings and other factors make survey grade instruments impractical for general resource management applications.

GPS Comparisons
Recreational Grade
Mapping/Resource Grade
Primary Uses (Including Forestry Applications)
  • General navigation (including real-time moving maps), outdoor sports, hiking, geocaching, etc. See other GPS uses here.
  • Collection of point data. Note: You will get better results if you pause 10-15 seconds or longer to assess GPS signal strength before recording waypoints. (Precision DGPS requires 10-15 minutes minimum at a static location for adequate satellite data acquisition and averaging.) Some receivers are 2-3 times more accurate in a vertical rather than horizontal orientation when collecting point data.
  • General forest reconnaissance using manual inventory methods
  • Estimating acreage of forest stands/areas, although potential errors for small areas or areas with complex boundaries may be unacceptable. See the European Commission "Note on the use of stand-alone GPS for parcel measurement" or "Estimate of Area Calculation Error Using Handheld GPS" for discussions on the topic.
  • Relocating individual trees, a group of plants, a nest, etc. if the spot is flagged in some way so it can be seen when you get close



  • Resource mapping and navigation
  • Collection of lines/roads and areas/stands in addition to point data. Associate GIS data collected on-site to the map objects.
  • Forest reconnaissance using digital data recorders
  • Measurement of areas to a higher level of precision (when used with differential correction) than possible with recreational grade units
  • Relocating individual trees, a group of plants, a nest or other local features that are not otherwise flagged

Note: You may need to plan GPS data collection to coincide with optimal NAVSTAR GPS, GLONASS (Russian GPS) or other satellite availability over your work area. Trimble offers free GPS planning software that determines the visibility of GPS satellites.

GPS staellite flight patterns
Horizontal Data Accuracy (Based on Manufactures' Literature)
  • Typically <15 meters (49 feet) with no differential correction. Error greater under conditions such as dense tree foliage, steep terrain or poor satellite orientation.
  • 3 to 5 m (or 10' to 16') with WAAS real-time correction. Recreational GPS units have no post processing correction capability.
  • Typically <15 meters (49 feet) with no differential correction. Error greater under conditions such as dense tree foliage, steep terrain or poor satellite orientation.
  • 1 to 5 m accuracy in real-time or <50 cm accuracy with post-processing correction using base station differential corrections
Vertical Data Accuracy
  • Not generally relied upon to collect vertical data, although more expensive units might include a barometric altimeter that manufactures claim to have ~10 foot accuracy.
  • Manufactures' literature warns that vertical positional error is likely to be two to three times greater than horizontal error (mainly because there are no satellites under you to use for triangulation)
Differential Correction Options
  • No post-processing capabilities

  • Many receivers offer real-time correction (WAAS or USCG)
  • Post-processing for accuracy potentially better than 50 cm horizontal location
  • Some receivers offer real-time subscription-based correction in addition to integrated WAAS or USCG systems
Data Points Stored in GPS Unit
  • Typically 500 waypoints. Tracklog point capacity will generally vary between 1,000 and 5,000 records.
For more data storage, you could transfer GPS data to a PDA. Freeware programs such as GPilotS are available for PalmOS handhelds or G7toCE for Pocket PCs that download and manage waypoint, track and route data. G7toWin manages waypoint, track and route records on a desktop PC.

Note: If you need an inexpensive serial cable to connect a GPS unit to the current line of iPAQ PDAs ? including the iPAQ h1910, 3800 and 3900 models ? you can get them at SupplyNet or eXpansys.

  • Up to 50,000+ records, depending on model

Use G7toWin to capture GPS unit screens or to store GPS data from Garmin, Magellan or Lowrance receivers.
Ease of Use
  • Ease of Use
  • Requires training
External Antennas
To get good GPS signals with a handheld unit, the receiver's antenna must be oriented correctly. Units like the Garmin eTrex and Geko with a patch antenna work best held horizontally so the face of the unit looks up at the sky. Others with a quad-helix type antenna (like the Garmin 72/76 series or the Magellan Meridian) work best with the top edge pointed skyward.

Trying to maintain an ideal antenna orientation can be annoying if you are carrying a GPS, especially while hiking or doing other field work. An external antenna is a perfect solution if your unit accepts one. Here are a few options:

  • Small, light antennas like a low-profile Garmin 27C (left, a thin black box small as half a deck of cards, costing about $70) can be attached to your hat or to the top of a pole on a backpack or vest. Such units draw very little power from a GPS unit but can boost signal strength. With an external antenna attached, you can put the GPS unit out of the way inside your coat or in a pocket (a convenient way to maintain battery performance in very cold weather).
  • Larger dome antennas can be mounted on a pole-equipped backpack. The Garmin 17N has the GPS receiver built into the dome and is popular for use with field data recorders or Windows CE devices to collect information in the field.
  • Simply mount your handheld GPS in the proper orientation on a pole attached to a backpack or vest. (The lightweight, waterproof Geko 201 works well as an external antenna for field work.) Connect the GPS with a cable to a Palm or Pocket PC and use software on the PDA to view maps and record locations. Bluetooth GPS units that communicate with PDAs without the use of any cables are also available.
External Power Supply
Any GPS unit or other electronic device with a power supply cable that connects to a 12-volt cigarette lighter outlet can be operated with a portable battery in the field. 12-volt power supplies are relatively inexpensive ($30 to $80) but heavy (three to seven pounds). They are often sold in carrying cases, complete with AC and DC recharging systems. If you are running multiple tools (such as external antennas, data collectors and cellular phones), you might want a power supply with multiple ports. CycoActive also offers a light-weight C-cell battery pack for eTrex and Geko GPS units.
  • Recreational grade units are a good companion alongside a clipboard, paper and pencil for field work. Write down waypoint numbers for manual data plots and review your GPS track as you proceed or later in the office.
  • Transfer waypoints or tracks (based on property corners or boundary lines) from an aerial photo to the GPS unit to use as references when traversing a parcel of land.
  • Recreational grade GPS units work well with a variety of field data recorders, including relatively inexpensive Pocket PC devices like the iPAQ or ruggedized units like the Juniper Allegro®.
More Information


Does the new GPS technology mean that resource managers are likely to abandon their compasses and stop pacing to measure the distance they've covered? Maybe.

Many will find that the most efficient way to move quickly across the countryside is still by sighting through a compass notch and counting steps. If you've done much fieldwork, though, you know that after making a few offsets to get around bramble patches, cliffs or sinkholes and swatting mosquitoes (losing your pace count), your precise location can get ...well, a little hazy. In those situations, a GPS location fix can help you get back or stay on course.

For other resource managers, GPS combined with the use of handheld data recorders will revolutionize field inventory procedures. The new tools allow foresters, for example, to see real time maps of where they are in the woods. Sampling grids can be overlain on maps, with GPS guiding foresters to each sample plot location. The resulting data can be more accurate and processed in record time compared to traditional navigation and manual data entry.

The grade of GPS unit you select depends on the type of field work or data collection involved. In many instances (including most forest inventory tasks), a recreational grade unit may be entirely adequate. If a forester were setting up a timber harvest, however, that's a different situation. When a couple dozen trees get cut across the neighbor's property line because of a mere 66-foot GPS error, the neighbor is likely to be hopping mad! Better use a resource/mapping grade GPS if you are relying on the instrument for boundary location or research where location and area calculations are critical.


School of Environmental and Forest Sciences
USDA Forest Service State & Private Forestry
WSU Cooperative Extension
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Last Updated 10/13/2022 12:33:48 PM