A draft survey measures the displacement of a vessel carrying bulk material (such as scrap metal which cannot easily be done) before and after loading/unloading. The difference between the surveys is the amount of material delivered to the client.
Normally, vessels have draft markings on both sides of the vessel at the bow, amidships and at the stern which must be read, usually from a small craft on the water to accurately read (avoiding parallax).
Calculations are extensive and variables of the vessel such as ballast tanks, fuel tanks, ships stores etc. must be measured and corrected for trim and list.