A cousin of the topographic survey, a hydrographic/bathymetric survey depicts the lay of the land under the surface of the water. In this part of Virginia, this type of survey is typically used for a dredging operation. For example, if a person owns a dock, an accumulation of silt can make access to the waterway impassible. A hydrographic survey is needed to determine the amount of material to be dredged that lies beneath the water. If this occurs on a navigable waterway, the U.S. Army Corps of Engineers and the Department of Environmental Quality (DEQ) are involved in the regulation of dredging permits, and must be contacted to assess the need for such an operation. A pre-dredge hydrographic/bathymetric survey is required to determine how much silt will be allowed for removal and a post-dredge survey is required to verify that the proper amount was removed.