It is an international commercial custom that traded quantities of bulk cargo such as iron ore, coal, steel scrap, grain and some others  cargoes are determined by draft survey. During a draft survey, cargo quantities are calculated based on the change of ship's drafts between different stages in the loading or discharging process by using the scales and hydrostatic property tables on board the ship. This convenient method is universally adopted as the most efficient and reliable way of measuring the cargo quantity where accurate product properties such as density and stowage factor are not reliable.