Because the public is becoming more aware of GPS, in part due to its use in military weaponry, in mapping Space Shuttle debris and GeoCaching, we thought it only proper to provide you with a brief and accurate overview of the Global Positioning System.
For information about the origin of the GPS constellation and how it works, we offer two Web sites that will answer most of your questions. Visit them at your leisure.
Here, we wish to provide you with some basic information to help you understand what GPS is used for and to define the three basic accuracy categories.
The reason we talk about GPS in terms of accuracy, is because it's a good idea to use the right tool for the right job. And just because you say, "I GPS'd it, so I know it's accurate," couldn't be further from the truth (ground truth, that is).
The categories are described as: Recreational, Resource, and Survey.
Recreational GPS receivers are designed to do exactly what the name implies: to assist you in finding out where you are, or where a "waypoint" is, while hiking, backpacking, fishing, hunting, skiing, etc.
The Department of Defense (DoD) says that civilian users can expect to get 15 meters (49.2 feet) of horizontal accuracy, or better, 95% of the time*. Does that mean the error could be worse than 49 feet? You bet it does. Since the accuracy of your position depends on so many factors (number of satellites, satellite geometry, signal strength, atmospheric conditions, occupation time on the point, etc.), you can never be sure exactly how accurate your position is. Having said that, typical horizontal accuracy is about 15-25 feet. Not bad if you're looking for the ski lodge, your vehicle or your campsite.
Some recreational systems have built-in WAAS (Wide Area Augmentation System) receivers that pick up correction signals, further reducing position error to a couple of meters.
Recreational receivers sell from $79. to $1,000.
Resource GPS (sometimes called "Mapping GPS") provides 10 cm (4 inches) to 5 meters (16.4 feet) of horizontal accuracy. These are typically used in professional mapping and GIS data collection applications. The city or county in which you reside may be using these systems to obtain accurate locations on storm drains, road signs, utility poles, etc. Systems that provide 50 cm accuracy are used to map public utilities in urban areas, and often the 5 meter systems are used to map logging roads, wildlife habitat and hiking trails; all applications that may not demand higher accuracy.
In addition, these mapping systems are sophisticated enough to allow the user to control every aspect of the GPS signal, which, in turn, controls the quality of the position in terms of precision and accuracy. They also provide the user with powerful data capture tools, such as using a form containing a list of features, attributes and attribute values which can be used to describe the features in great detail. This information is brought back to the office and sent to a GIS software package capable of managing multiple and complex layers of data.
The stated accuracy can be obtained in real-time by utilizing the Coast Guard Beacon, Omnistar, Racal, or WAAS signals. Better accuracy can be obtained by post processing field data in an office software program.
Mapping systems sell from $650. to $10,000.
Survey GPS produces 1-2 cm of horizontal accuracy (3/8" - 3/4"). It is used for property surveys, cadastral surveys, as-built surveys, construction stake-out, and other applications where very high accuracy is required. These systems require a more "open sky" to produce results quickly, so they are not often used in heavily wooded areas or other locations where obstructions are prevalent.
They allow the surveyor to produce highly accurate results, over sometimes great distances in a very short period of time. Current technology allows the user to get 1 cm of horizontal accuracy with an observation time of only 5 seconds, remaining stationary on a point.
Accuracy can be obtained in real-time using a portable base station or by post processing field data in an office software program.
Survey systems sell from $9,000. to $50,000. depending on how many receivers are purchased. More receivers result in greater productivity.
So, remember, when someone tells you they used GPS to locate something, it only tells you they used satellites to take the measurements necessary to calculate a position. The quality and category or "grade" of receiver is what determines the accuracy of that location.
We do not sell Recreational GPS. We sell Mapping systems and can refer you to those who sell Survey systems.
If you have any questions about GPS precision, accuracy, or cost, please call or email Jim Lahm at (503) 624-6133.
*this applies to all GPS receiver categories assuming no corrections are applied.
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