I found this old topic whilst looking for something else...
Lyle's blog post is
here.
Essentially, this is an application of geometry (
similar triangles) to measure out an object's height on the ground.
The technique, as described by Lyle (using a 300mm stick at arm's length), would result in a 'safe distance' of about twice the building height, because "arm's length" is about 600mm. So, aligning the 300mm stick with the building height at a distance of 600mm would result in a similar triangle where the base length is twice the height; i.e. the safe distance is twice the height.
This puzzled me for a while, but I thought it might be a prudent safety measure to stay twice the building height away to give protection from flying debris in the event that the building collapses; possibly a useful 'rule of thumb'.
Then I read the blog, and saw Andy Harris' post. In this, he explains the lumberjacking origin of the technique (I've seen it used to measure the height of trees). Andy's technique takes a stick that is likely to be longer than your arm (1m), and then adjusts the grasp on the stick until one end just touches the shoulder. The reason for doing this is to ensure that the height of stick protruding from your hand is the same as the distance from your eye to the stick. Aligning the 'visible' (i.e. above the hand) part of the stick with the top and bottom of the building/tree result in a similar triangle of 1:1, i.e. the distance from eye to tree is the same as the height of the tree. This should ensure that you're
just outside the fall range of a felled tree. You can measure the height of the tree either by holding the stick sideways as described, and getting someone to stand where the tip of the stick appears, or you can simply walk towards the tree, counting paces.
For completeness, you need to add the height of your hand from the ground to any measurement...
