after the merging of fragments by the scans in the four directions segmentation mentioned in the b step 419 is shown schematically in fig5 however it has the following problem even though some region should remain as a hole with no elevation the unwanted elevation is recovered in the c step 420 since some region becomes smaller than the threshold of the c step 420 due to the segmentation in the third process 103 of the noise remove segmentation scheme segment the interpolated region after the first process 102 of cog eci elimination and decide whether the segment should be removed based upon the size of the segment in the a step 517 of the third process see fig7 search neighboring grid cells when the current grid cell has an elevation and assign 501 506 a segment id to the current cell with the similar method to that in the a step 418 of the second process check if the current cell has an elevation 502 here 4 connectivity scheme is used to connect the cells and an island connected to the land with 8 connectivity scheme is regarded separate from the land see 503 if the neighboring cells have a segment identification number id assign 504 the minimum value of segment ids to the present cell if the neighboring cells have any id it is not necessary to choose the minimum value and it is possible to choose a maximum value as long as this value is applied to the whole image if no neighbor cells have an id assign a new id u2014 the largest id so far plus one 505 repeat the above processes from the top left to the bottom right and complete.