At Carrownderry in County Galway, a roughly circular cluster of trees, carefully enclosed and plotted on the first edition of the Ordnance Survey six-inch map, looked very much like a crannog, the kind of artificial island or lake dwelling built in Irish lakes and wetlands from the Bronze Age through to the early medieval period. The circular outline, the sense of deliberate enclosure, the presence on a map of considerable historical authority; all of it pointed toward something made by human hands.

When the site was inspected in 1990, the suspicion dissolved. What the cartographers had recorded, and what subsequent generations may well have assumed to be an archaeological feature, turned out to be a natural gorse-covered hillock, shaped by geology rather than by any islander hauling timber and stone into shallow water. The 1930 edition of the Ordnance Survey six-inch map gives the place a name, Cnocanroe, a small red hill, which in retrospect describes the landform rather more accurately than any designation borrowed from the archaeology of lake dwellings. The gap between the two map editions captures something of how place knowledge shifts: a feature that read as an enclosure in one cartographic moment had acquired a proper Irish name by the next, even if the underlying misidentification persisted until someone walked out to look.