To achieve a more accurate map, the spheroid is moved until the geoid at the area being mapped (arrow) corresponds best with the spheroid surface (fig.11.3) the combination of the spheroid and the location where its center is placed is called a datum. The purpose of the datum is to create the best possible fit between the actual geoid and the spheroid used for mapping. In this example, the center of the spheroid is slightly offset from the center of the geoid in order to achieve the best fit. Some datum have this offset; others do not. A datum in which these two centers coincide is called an earth-centered datum.