A linear regression equation, η1=α(ε×h)+β, is obtained based on a depth h to a water zone of a predetermined water temperature, respective densities ρ1 and ρ2 of upper and lower layers above and below the water zone, and an absolute dynamic topography η1 obtained based on data measured by satellite (ε=(ρ2−ρ1)/ρ2). Further, h′ (depth of the water zone of the predetermined water temperature at each of a large number of spots) is determined according to this equation, η2=α(ε′×h′)+β, based on ε′ for each of the large number of spots calculated based on historical data, an absolute dynamic topography η2 at each of the large number of spots based on data measured by satellite, and the values α and β.