The only thing I can see in the graphical explanation is that the effect of gravity and thus the amount of drop has been assumed to be constant regardless of the launch angle.
It is a fact that the trajectory of a projectile changes with launch angle. In these illustrations the launch angle is determined by the LOS and can thus not be the same in both cases.
Facts to consider:
Cant has less effect the further away your zero is.
Cant has less effect the closer your LOS is to your bore line.
From what I have seen, the more unnatural your shooting position is to you the more likely you are to cant and vice versa.
It is much easier to cant without knowing at a higher magnification as you see less of the surroundings.
Shoot with a level when ever you can...
I said the effect of gravity. It is common knowledge that gravity is a constant but the steeper the launch angle of a projectile the greater the effect of gravity since more of the projectiles forward motion is used to overcome gravity. On the other hand if you launch a projectile at an angle below the horizontal the projectiles drop would be greater since it is aided by gravity. Therefore the amount of drop experienced would be different with a positive and negative launch angle. However this phenomenon would be close to unnoticeable in the case of a pellet at 50m but fairly exaggerated at 100m. In the attached diagram it is illustrated that the vertical component of the projectile's momentum either works with or against the gravitational acceleration force.
I'm aware that this is a moot point since nobody would shoot their rifle upside-down. So for the purpose of the discussion on the effect of canting of a higher or lower mounted scope on POI tilt should be concluded that it is equally bad in both cases and should be avoided at all cost.
What exactly are the rules regarding the use of scope levels?
Read the following articles by Jeroen Hogema
Scientist and shooter
- Air rifle cant experiment
EFFECT OF CANT ANGLE VARIATION ON POINT-OF-IMPACT IN AIR-RIFLE SHOOTING
Copyright Jeroen Hogema
Line-of-sight height and the cant angle effect in air-rifle shooting
LINE-OF-SIGHT HEIGHT AND THE CANT ANGLE EFFECT IN AIR-RIFLE SHOOTING
Copyright Jeroen Hogema, 5 September 1999
Shooting sport ballistics, ballistiek voor de sportschutter
Shooting sport ballistics
Copyright Jeroen Hogema
No, canting error is significant more or less everywhere. Mainly, it depends on the canting angle and the pellet drop, but the aiming method can modify, even neutralize it. Please read my article about the theoretical background and you'll see how this works. The point of this all is that you have to buy a bubble level and can forget canting errors, but some shooters (e.g. HFT) can't do this. The original question was that do high scopes increase canting errors and my answer was that generally not, in a few specific cases can increase but also decrease it.
Good point, I 've got a 50 mm high scope rail and I can use it as a reference plane for verticals. But of course the bubble level is the best way to keep the rifle vertical. I had tried an electronic level, too - but it wasn't so sensitive as the bubble and when the battery went flat I was in trouble... Bubble levels are cheap, sensitive and reliable.