Jasper Schaer, Hydro Survey Team and technical lead, left, and James Hiroms, boat captain, right, both from the U.S. Army Corps of Engineers Galveston District, check an Aid to Navigation marker for a tide marker. Aids to Navigation sometimes get displaced by large ship traffic. It’s important for surveyors to have an accurate measure of water depth to precisely calculate how much material a dredge removes to complete a contract.