And if you defined the point by fixed measurements from three fixed objects there would be no ambiguity whatsoever, as there would be only one point that met that description.
I'm also wondering how many decimal places one would have to go to get accurate enough to locate the ends of a calibration course with GPS coordinates. When I was doing the groundwork for the measurement of the cross country course in Peoria, I visited in November, a week or so after the state meet. The IHSA had set permanent metal markers in the ground at the ends and center of the starting line, both ends of the finish line, and the five half mile splits along the course, and after a thousand kids had run the course, and when the grass was dormant for the winter they were easy to find. So I took my GPS device, laid it on each marker, and recorded the readings. When we got back there in July, the grass was high and the GPS was accurate only within the 5-10 meters that it usually is. We needed a metal detector to find most of the IHSA's marks.
Based on that experience, the only thing I would use GPS coordinates for is split points that I couldn't measure to anything else. I certainly wouldn't use them to describe anything that requires the accuracy that the end of a calibration course does.