GIS 101
What is GPS?
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.
So how does it work?
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.
Source: www.garmin.com Copyright 1996-2004 Garmin Ltd. or its subsidiaries
Do you achieve different accuracy levels with different receivers?
Yes, there are three different grades of GPS receivers commonly referred to as survey grade, mapping grade, and recreational grade.
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Survey Grade are the most accurate and most commonly used in situations where accuracy is essential (engineering applications, property boundary determinations, etc.). They typically provide true positional accuracy within a centimeter in the horizontal direction and elevation accuracies within 10 centimeters.
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Mapping Grade receivers must be differentially corrected GPS to reduce positional errors. Differential correction is the process of improving fixed positions utilizing data from a base station. With differential correction, horizontal accuracies from one to two meters can be achieved, while vertical accuracy is around 3 meters. These receivers are most commonly used by GIS professionals for gathering data for inventories, resource mapping, environmental management and infrastructure management.
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Recreational Grade are the least accurate units. Typically used for outdoor recreational activities, these receivers can have up to 20 meters in positional error.
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