I contend that most any XC course measured with our certification methods will at least potentially be much closer to the "actual" distance (where a steel tape measurement is the standard) than the measuring wheel commonly used to set up high school and college XC courses.
First, as Mark Neal has shown, measuring wheels, even when carefully guided along course tangents, is insufficiently accurate to the advertised distance.
Secondly, I have learned that visualizing and riding/guiding along course tangents is less intuitive than I might have expected. It seems to me that some people with lots of experience directing races have little or no ability to see the SPR on any given course, with or without a measuring wheel.
Finally, if the cal course for an off-road course measurement is set up on the same surface as the course, and the course is measured as a series of segments with each locus clearly identified with respect to nearby fixed objects, I submit that such a course, as measured by an experienced certifier, would stand up to a steel-tape verification.
One problem is the "nearby fixed objects" if the path changes direction through an open field with no such objects around. Oscar and Mark have shown that our civilian-grade GPS apps seem to have too much variance for GPS points to be relied upon.
However, when a course to be measured has plenty of such objects, I maintain that a valid course can be measured and mapped. Such a course will be more accurate than anything other than a steel-taped course, IMHO.
A friend recently bragged about his young grandson running 14:10 for a 5K XC event. After congratulating him, I asked what this time indicates, since "5K" XC courses are not certified, may or may not be accurately measured, and tend to differ in length by non-trivial amounts. "Of course", he didn't respond.
My running club, MCRRC, advertises and conducts a "5K" XC on a course that I conclude is barely 3.0 miles. No one cares. I'm willing to bet that, if this course was "in reality" closer to 3.3 miles, most everyone would then care.