English: GPS Survey Equipment at Weir Dyke Bridge. Photo taken at Weir Dyke bridge 172000. The gear belonged to a group checking water levels in nearby drains who had established a datum on the bridge using GPS (8 satellites) and this gave me an opportunity to check my Etrex, which uses 4 satellites, against their professional equipment. The difference was less than 1m - my Etrex was showing 20ft accuracy at the time. For another picture see 336893.