While handheld GPS units open up a wide range of recreational and functional applications, they're effectiveness when surveying varies. This is because not all handheld GPS devices are made equal, and the precision required by surveying equipment means very few devices qualify. Determining the suitability of a handheld GPS device for surveying depends almost entirely on the grade of its accuracy.
GPS Accuracy Grades
Not all GPS devices offer the same level of accuracy. Generally, these are broken down into three categories: civilian or commercial grade, mapping or differential grade, and survey grade. Accuracy varies between models and the supporting satellite system, but each category has a general accuracy threshold. Civilia-grade systems are accurate to within about 20 feet. Mapping-grade GPS devices are accurate to within about 3 feet. Survey-grade systems are the most precise, requiring accuracy within 1 centimeter.
Precision Requirement Reason
The reason survey-grade precision must be so high relates to the nature of survey work itself. Surveyors primarily need to map out private property, settle property-line disputes, map sewer and storm-water pipes, run ground control for aerial surveys and map accidents for police departments. In all of these fields, pinpoint accuracy is a must and as such the level of accuracy available in commercial handheld GPS systems is insufficient.
Equipment designed specifically for surveying exists. While survey-grade GPS usually includes a handheld device, this is not the only piece of equipment utilized. Often, survey GPS equipment will include a backpack with additional processing and receiving equipment, as well as a tripod for mounting the handheld device during measurement. This equipment can be precisely calibrated to deliver accurate information under a number of circumstances.
U.S. Geological Survey
The U.S. Geological Survey provides several guidelines for what qualifies as a survey-grade GPS receiver. Survey GPS devices must track at least eight satellites simultaneously on parallel channels, and be capable of recording the full-wavelength carrier phase and signal strength of L1 and L2 frequencies. They also must have sufficient memory capacity and battery power to record six hours of data at five-second intervals. Connected antennas also need stable phase centers and must be designed to minimize multipath interference.
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