The very size of the dog is the thing that makes it unconvicing(and all the other zodiac bollocks). Now a smallish figure(Cerne Abbas giant, Uffington white horse etc) is clearly deliberate. You can see it, for a start. You can invent any figure you like using the lines on the ground, but only if you make a mile or more across. Mind you, the fact there is a plce called Wagg on the dog's tail is curious. Now, in Ashdown forest in Kent there are two ponds close together known as Robin Hood's Balls. As far as I know nobody has identified a ten mile long drawing of Robin Hood incorporating these features, but I'm sure a glance at a map will make this a fairly simple task, and should get you half a page in the Guardian and a one hour documentary on Channel 5(complete with re-enactments of forest life involving nerds with bows and arrows). I make a present of this idea to anyone who wants it.