A DRAFT SURVEY is a calculation of the weight of cargo loaded or unloaded to or from a ship from measurements of changes in its displacement. The method is based on Archimedes’ principle. The procedure is standardized by the UNECE for the measurement of coal cargoes. The draft survey is performed by reading the ship’s draft on the draft markings at six standard points on the hull: forward, midships and aft on both port and starboard sides. Corrections for factors such as trim, water density and non-cargo weight changes are made before calculating the cargo weight change. Draft surveys to determine the weight of cargo loaded and discharged to provide the quantity that determines how much both the buyer and seller have to pay each other in addition to the correct amount of freight charged by shipping lines, which is based on tonnage carried.