Having just finished up with our local cross country season, I just want to add my 2 cents on accuracy in cross country measuring. I've measured 2 courses here, one in Fort Dupont Park in SE DC, the other in Colmar Manor Park in neighboring PG county. I am confident that the course distance has remained correct but this comes with a price: namely, I have set them up myself every time they are used! With help of course-- however I do think that getting a good and (semi-)permanent measurement is possible in some cases. A helpful feature of these 2 courses is that there are lots of pretty well-defined trails to incorporate in the route, and for "open" areas you can establish a few key points that runners must go around.
Each of these courses, by the way, has 3 variations so that there is a different distance for high school, middle school, and elementary. The high school course is 5 km-- I get that right and then build the variations so that elementary course is around 3200 meters and the middle school is around 4000 meters.
But-- how confident am I that I could write the specifications well enough that I could turn over the course setup to others? I need to ponder that one.
One reason for going to this trouble is that I think you can legitimately talk about course records if you keep the course consistent and correct.