GPS

The Global Positioning System (GPS) is a commonly used technology for locating positions on the earth.  Both recreational users and professional surveyors share the capability of measuring all three dimensions of a particular point but not always with equal precision.  Like the historic geodetic surveying methods that preceded it, GPS utilizes a network of satellites in space.  But the man-made constellations used by GPS receivers is different from nature’s stars in that they actually send electronic signals toward earth that reference their scheduled position based on a broadcast almanac and the precise time the signal is being transmitted.  The receiver of this signal uses that broadcast data to calculate the difference in time between the signal being sent to the time recorded on the handheld receiver in order to determine the local position.  Many GPS receivers are also data collectors that can store measured points to be shared with other applications. Common data sharing formats include GPX or the simple Comma Separated Value (CSV).  Here is a video on how to import GPX data into the ArcGIS software from Esri.

http://www.pocketgpsworld.com/howgpsworks.phpGPS receivers vary in the quality of their clocks which can affect the precision of their derived position.  Most devices are aware of their limitations and will be able to display the level of uncertainty in their measures.  However, only more advanced devices can attempt to go further in actually accounting for errors such as atmospheric disturbances of the signal or even filter data in order to exclude confusing reflected signals thereby producing a more accurate measure.  The quality of measures taken in the field are also a result of how directly the signal is received and how many different satellites can be compared effectively at a single moment.  The configuration of GPS satellites is constantly changing but can be described through various dilution of precision (DOP) measures.  It is best to schedule data collection whenever possible to ensure that the configuration is optimal over the geographic area you will be working. Many GPS units will include planning software to leverage the broadcast almanacs days ahead to determine the best time for data collection.  As more satellites are added to the orbiting network, acceptable configurations become more widely available.

Another common strategy to improve the quality of GPS data collection is to compare the results from your handheld (“autonomous”) device with a local base station in order to apply a series of calculations known as differential correction in either real-time or as part of a post-processing procedure.  Basically this process utilizes the random variations measured at a stationary point to remove a portion of the inherent error.

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