GPS devices are great tools and could be the most accurate measuring device for a runner training other then the accepted measuring tools (Calibrated bike/Jones Counter, Steel Tape, Measuring wheel). But it's not accurate enough for USATF or World Athletics Certification.
I have a very good one - Garmin 62c ( old model ) that is mounted on my handlebars to assist in informing me when a mile/km mark is approaching when I'm measuring.
Here is a statement by the USATF Road Running Technical Council in regards to GPS devices:
"GPS devices work by receiving signals from satellites. The quality of different GPS units can vary, but all of them can be affected by conditions such as buildings in urban environments or heavy overhead tree cover that interfere with reception of the satellite signals and can cause them to be inaccurate.
Race courses Certified by USATF are measured by a proven method that incorporates the calibration of measuring devices against a steel tape and are verified by multiple measurements."
Race courses are measured along a well-defined path called the “SPR”—the Shortest Possible Route that a runner can possibly run. Most runners don’t actually run the SPR, so the distance recorded by their GPS device will usually be longer than the certified length of the course, even though the course was properly measured along the SPR according to USATF rules.
But my best tool is Google Earth Pro. For measurers it's the greatest thing since sliced bread. I probably spend at least an hour or two each day configuring courses for upcoming events with it. And carefully used I can come up withing 10-20 meters compared to the calibrated bicycle method if the course is exposed. Often there may be trees or buildings in the way but at least with trees there is the option to use the "Historical Imagery" options to view the area in the fall when there may not be leaves on the trees. When used carefully, I can identify the approximate location of split points which helps in reducing major errors in the measurement process (calculation error or broken counter).
If someone wants to send me a detailed map of the cross country course or a kmz file I would do a basic measurement.