How to Measure the Accuracy of Your GPS
Most GPS receivers will display their relative accuracy but here's a way to precisely measure how accurate it actually is.
First, find a USCGS "Classic Horizontal and Vertical Control" benchmark. (CHVC) They can be located using the NGS Data Explorer website:
https://geodesy.noaa.gov/NGSDataExplorer/
Enter your zip code in the search box and zoom in on the map to see benchmarks in the area. Select only those marked "Classic Horizontal and Vertical Control". These benchmarks have been surveyed to a high degree of accuracy using professional GPS equipment. Their accuracy is usually less than .5 CM or about 1/4 inch.
Once you have selected a benchmark, click on it and print out it's datasheet. The latitude & longitude are shown as degrees, minutes and seconds to 5 or more decimal places.
Next, locate the benchmark in the field. They are usually 3" diameter bronze discs set in a stable location like a bridge abutment, building foundation or other concrete structure. The CHVC benchmarks are always in open areas with a clear horizon and are usually well marked by local surveyors. The datasheet also describes it's location.
More information and sample pictures of Benchmarks can be found here:
https://libraryguides.usgs.gov/friendly.php?s=benchmarks/int...
Center your GPS on top of the benchmark and leave it undisturbed for a few minutes. For handheld GPS units, use position averaging. Record the coordinates displayed on the unit and compare with those on the datasheet. Position accuracy can then be calculated using this formula:
https://www.movable-type.co.uk/scripts/latlong.html
You can also use Google Earth to display the position error by plotting both the benchmark location and the GPS coordinates. The error can then be measured between the two locations using the Google Earth measuring tool. This method is slightly less accurate but it saves the effort of doing the math.
This method will determine the absolute best position accuracy your GPS is capable of producing. Keep in mind, these measurements are taken under ideal conditions and will vary with location.