There is loads of articles and information about Alexander Thom on the internet - which I'm sure is what thesweetcheet meant. Are stone circles a true circle or an ellipse - I'm seem to recall its been discussed here before - or maybe it was the long since defunct Avebury Forum. Avebury for example is not a true circle.

I was lucky enough to meet Margaret Curtis when visiting Lewis - she took us into her garage and did this thing with a piece of string, a stick and a bucket of sand.