Jasper Schaer, Hydro Survey Team, U.S. Army Corps of Engineers Galveston District,checks a GPS sounding tool prior to beginning a post dredge survey. Calibrating sonar instruments to local water quality is an important part of taking accurate measurements. Sounding measures the speed at which the single beam sonar waves pass through the water. Those sound waves are measured and recorded aboard the survey vessel, Tanner II. When a large ship passes, it disturbs sediment which slows down the sonar signal. With the heavy traffic in the Houston Shipping Channel, Schaer performs frequent sounding checks to ensure precise measurement of dredge material removed. Schaer and other survey crew members like him perform surveys before and after a dredging contract and after dredged sand is deposited offshore.