Looks like a correct installation to me. Like the others said, it's the elevation of the vent opening that is critical. The lower it is, the lower the pond's water level will be when the siphon action stops. Regardless of the elevation of the vent opening, the siphon action will only start when the pond's water level reaches the same elevation as the high point of your siphon pipe, and the end of the vent pipe is submerged. Also, make sure that the threaded plug in the vent cleanout is not leaking. A leak anywhere in the vent would suck in air and break the siphon effect.
