Even so a GPS is not a measuring approved tool to certify courses I always used as a back-up.
Recently I was doing a 10k laydown course with the splits starting from a known point, Finish line. We started to measure being myself the first rider laying down and marking the course and the second rider to chek on the marks. The 6mile mark went well but we did push the bikes one kilometer further by my GPS to find the 9 km mark. Wrong the 9 km should have been around 640 meters away from the 6 mile mark. We turn back to the start point and I double check on the Excel page and noted a bad formula on the numbers, so we were wrong.
If I had just trusted my calculations with the use of the Jones counter I would it find out at the end of the course that only the 6 mile was right and all other marks were wrong.
A GPS SAVE ME