And they're just based on the available information, which is better than no information.
Measurements can be made and be useful at various points throughout the process. Obviously the end grade (3 or whatever point you've decided is the end) is the easiest to make since all the information would presumably be in at that point. But that doesn't make measurements at the starting point useless.
Again, I'm talking about the collective wisdom of professionals with expertise in the area, just just a random individual grade. That's the beginning measuring point.