A surveying system 10 includes a data storage system 12 containing survey data defining a terrain model 14 of a region 16. A processor module 18 is configured to interrogate the data storage system 12 automatically to assess characteristics of the survey data to determine whether or not the survey data require updating and to provide instructions automatically to scanning equipment 26, 28 to scan the region 16 to provide updated survey data to the processor module 18 to enable the processor module 18 to update the terrain model 14.