The plan seems to be use a combination of GPS and digital photography to map out a geoglyph, a large drawing on the ground, using geeks around the UK to make up the points in a kind of join-the-dots exercise on a massive scale.
The image that people will create, basically by dropping stones on the ground and pins in a Google Map with images attached, is of John Harrison‘s Chronometer H5, the 18th century technological marvel that gave us an accurate way to measure Longitude.
The project seems to be in part to promote Shozu, a mobile software application which, as Lloyd Davies pointed out in a comment yesterday, can be set to automatically attach a geotag (location reference) to photos that you upload to Flickr, and possibly elsewhere.
I grew up partly in the Vale of the White Horse, the white horse being one of the oldest geoglyphs in the UK. I recall that people said it used to be a serpent until King Alfred‘s soldiers carved legs on it to celebrate a major battle – but I can’t find any trace of that story on Wikipedia or other websites.