Normally, the baseline from which the territorial sea is measured is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state. This is either the low-water mark closest to the shore, or alternatively it may be an unlimited distance from permanently exposed land, provided that some portion of elevations exposed at low tide but covered at high tide (like mud flats) is within 12 nautical miles (22 km; 14 mi) of permanently exposed land.
Nauru's Territorial Sea Baseline was defined using the 1993 topographic map produce by Survey Graphics Pty Ltd of Perth, Western Australia, under the direction of the Ministry of Island Development and Industry. The map is based on UTM Grid Zone 58, which is referred to the World Geodetic System 1972 (WGS 72). The WGS 72 coordinate system was then converted to WGS 84 using transformation parameters tabled on Nauru Maritime Boundaries Technical Report (TR 399).