If I had realised how much fun Pythagoras's Theorum could be I would have paid more attention at school.

With two giant tape measures we made triangles to find the corners of 20m squares all the way up the track and across the hill fort. We then hammered in stakes so that next weekend the real fun can begin.

I now know, and will forever remember, that the diagonal measurement across a 20m square is 28m 28cm.

I also know that to find your mark you sometimes have to crawl through trees, wade through brambles or stamp through nettles. I learnt that "spot on" is a more reassuring phrase than "that'll do" and that Wincobank Hill has far more holes in it than I had ever noticed before.

There were some new members of the team today in addition to Roger, Mick and myself - Byron and David who both played on the hill as children, also Hilary and Ken who come from the far side of Sheffield.

The site is all marked out now and Roger has captured the points by GPS. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites orbiting the Earth. How extraordinarily amazing it is that this process of measuring with tapes and hammering stakes into a hillside in Yorkshire connects our prehistoric Celtic past, a mathematician from Ancient Greece and American space scientists. It makes me feel part of something very special.