Picture the map of Britain. Its strangely cadaverous shape, blobs of population and routes between them seem as familiar as our own faces; there is only one definitive map, surely? Not according to Joanne Parker, whose Britannia Obscura aims to tease out less corporeal cartography. Hers are not quite ‘maps of the mind’, for they exist as truly as a crisp new Ordnance Survey, but they are largely out of sight: above us, below us or otherwise in the shadows.