Oil is complex to measure. Each delivery usually comes in tens of thousands of barrels. To add a bit of complexity, oil changes its characteristics based on temperature. The hotter the oil, the more volume it will occupy. If the storage vessel contains water, the oil will float on top, reducing the intended capacity of the vessel. These dynamic characteristics can lead to differences of opinion on the actual volume of oil contained in the vessel. This is why independent inspectors are employed to provide non-biased measurement of all storage vessels before and after any oil transfer at an oil terminal. These are called barge or ship measurements.
Once the starting volume is recorded, the oil is transferred to other storage tanks via a meter. This meter also provides an unbiased measurement of the oil volume once ashore. These are called shore measurements.