Preston Engineering Survey were called in by a contractor working on Brownsea Island near Poole in Dorset to check the Ordnance Survey benchmark on the island. There had been some confusion over how the level of the benchmark related to mean sea level. Upon inspection, it was clear that the benchmark had been moved during construction work in recent years and therefore it was essential to check the level against OS datum.
A static GPS receiver was set up on the island and 3 GPS vectors were observed in order to minimise any possibility of error or mistake; the first vector was taken to the OS active GPS station in Poole, the second to the nearest OS passive station at Hengistbury Head and a third vector to a traditional OS benchmark near the Brownsea Island ferry terminal. All three vectors were within acceptable tolerances and so a new value for the Brownsea Island benchmark was established.