(I: Stiletto 120 @ W/L of 1.65 doing 270 deg. carving turns)
Eye, using objects of known size for reference. I don't trust my alti (Eureka: analog) nearly enough.
I'm not saying my eye is more exact than a good alti, but when you use the eye you constantly evaluate your distance to the ground, and if you were to start too low, you see it early and can abort the dive (going to toggles and landing in the intended direction).
I would *NEVER* recommend anyone to use their altimeter wrt. landings.