Geophysical Survey at Land at St Mary Cray, East, London Borough of Bromley

A temporary grid system was established over the site and marked out using canes. The location of the grid will was set out using an RTK GPS system theoretically accurate to some 0.01m and referenced to OS co-ordinates. Hand Held: Data was collected using a Bartington Grad 601-2. The instrument consists of two paired sensors (see below) and readings are logged at 0.25m centres along traverses 1.0m apart across 30m grids. Two sensors mounted 1m horizontally apart and very accurately aligned to nullify the effects of the earth's magnetic field. Cart: Data was also collected using a cart carrying four paired Bartington magnetic sensors. Four sensors mounted 1m horizontally apart and very accurately aligned to nullify the effects of the earth's magnetic field. Each data point is geographically referenced using an on-board Trimble RTK survey grade GPS system. Readings will be taken at 0.125m centres along traverses 1.0m apart. The collection of data provides an appropriate methodology balancing cost and time with resolution as per Historic England guidelines. Readings relate to the difference in localised magnetic anomalies compared with the general magnetic background. The magnetometer survey has not recorded any magnetic responses that could be interpreted as being of definite archaeological interest. A few discrete anomalies and trends of uncertain origin have been marked which have probably been caused by variations in the underlying geology or agricultural processes. Two former field boundaries and a service pipe have been identified in Area 3 along with bands of increased response which have a natural origin. Raised levels of 'background noise' has been recorded across the magnetic data which has been caused by the spreading of green waste.