Moored instruments are anchored to the ocean floor. This prevents the instruments from drifting in currents, and enables them to take measurements repeatedly at the same location. This type of sampling is termed Eulerian. After they have been deployed for several months, they can be retrieved and serviced. These units can be equipped with several different instruments depending on what oceanographers are interested in investigating. Most measure temperature and salinity (CTD). Often they are outfitted with a current meter to measure the velocity of water in the area.