A geoid of the Izu Islands area.

Abstract

Two kinds of geoid map have been drawn for the area of the Izu Islands by employing the data of over sea leveling among the islands and of vertical deflection at each island. The over sea leveling has been made since 1967, and two closed leveling loops have been constructed among the islands and the Izu Peninsula, joining at Nii Sima to each other. Closure error of each loop is within 20-30 cm, when geoid undulation inside of each island and vertical deflection are taken into consideration. A geoid map thus drawn on the Tokyo Datum indicates a systematic shift of the Tokyo Datum. Another geoid map on a global datum, SAO-SE3, shows effects of the gravity low over the trench lying to the east of the mainland of Japan and the gravity high over the southern area of the Izu Islands. Extension of the geoid undulation from the mainland of Japan (Ganeko, 1976) to this area seems satisfactory. Key word田： geoid~over sea leveling.