How accurate are the GPS measurements that we get from the Global Positioning System? It is a question we are asked often, as you might imagine, in the normal course of our work. One of my graduate professors repeats a memorable mantra: “GPS is something that is very easy to do right. It is also very easy to do wrong.”
“GPS is something that is very easy to do right. It is also very easy to do wrong.”
I think of those words each time I set up my equipment, process field data, or instruct others in how to do so. As with many surveying technologies, GPS has numerous failure points. Some basic issues could severely deteriorate accuracy: for example, if one mis-measures the rod height or is working in close proximity to tall buildings or dense tree cover, your GPS survey results will be of poor quality.
Other problems can be invisible to the ordinary GPS user, and can even be undetected by professional surveyors. Earth’s atmosphere is incredibly important to GPS accuracy. The conditions in the troposphere, stratosphere, and ionosphere all impact the GPS signal in some way. Severe solar storms or local environmental calamities like volcanoes can both contribute to GPS error, at least in theory. Everyday events such as rain and thunderstorms can create erratic changes in the troposphere that render base station corrections less meaningful.
GPS Accuracy in the Field – Norwich, Connecticut
During a recent survey in the Yantic section of Norwich, Connecticut, our survey crews recovered and checked a Connecticut Department of Transportation (CTDOT) benchmark, in accordance with standard protocols. Our high-grade professional GPS equipment can receive signals from multiple constellations of satellites, on multiple radio channels simultaneously. Passersby or construction workers watch as the equipment is being set up and ask, “How accurate is that thing anyway?”.
In this case, we detected that an official registered elevation benchmark was erroneous by approximately 7 inches! Our crews took the time to do supplementary research, recover additional benchmarks in the neighborhood, and perform three-wire differential leveling before reporting our findings to CTDOT. After receiving word of the error, the Central Surveys office dispatched their own crews to check Franklin Surveys’ work. Our findings were confirmed; CTDOT corrected the benchmark to reflect the GPS results.
