Well I've tried estimating heights, and I just can't do it.
So how about this?
- Spot the suspect aircraft
- Hold out your arm (length .62m in my case) = R1
- Measure the span of the aircraft against your index finger = S1
- Identify the aircraft (e.g. PA28, span=9.8m) =S2
- 500 ft rule distance (152.4m) = R2
Then S1/R1=S2/R2
=> S1=R1xS2/R2
As S1=500 feet (152.4m), then S1=4cm, as near as dammit
So if the span at arm's length is more than 4cms, then the PA28 is closer than 500 feet.
All I need to do now is calibrate my arm and finger against a PA28 at circuit height.
SD