It is very possible. My dick has varying degrees of hardness and sometimes (especially if I'm alone/jerking off) it just won't get beyond a certain point. So the rigidity of it is really variable. It's not like I even have to measure to be able to tell - on a good day I will stick straight up, pulsing heavily and be very evidently at my fullest, and other times it'll be kind of half-assed, maybe slightly leaning, and not quite there. Comparing the two, you can totally eyeball the difference. If you've got a consistent erection day in, day out, you're actually quite lucky - my guess is that the people who are in good shape (such as yourself, Jovial) have the most 'stable' hard-ons.