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When satellite is flying on the orbit for special task like solar flare observation, it requires knowing if the sub-satellite purview was in the ocean area. The relative position between sub-satellite point and the coastline is varying, so the observation condition need be judged in real time according to the current orbital elements. The problem is to solve the status of the relative position between the rectangle purview and the multi connected regions formed by the base data of coastline. Usually the Cohen-Sutherland algorithm is adopted to get the status. It divides the earth map to 9 sections by the four lines extended the rectangle sides. Then the coordinate of boundary points of the connected regions in which section should be confirmed. That method traverses all the boundary points for each judgement. In this paper, two algorithms are presented. The one is based on line-segments, another is based on dot-array. And the data preprocessing and judging procedure of the two methods are focused. The peculiarity of two methods is also analyzed. The method of line-segments treats the connected regions as a set of series line segments. In order to solve the problem, the terminals’ coordinates of the rectangle purview and the line segments at the same latitude are compared. The method of dot-array translates the whole map to a binary image, which can be equal to a dot array. The value set of the sequence pixels in the dot array is gained. The value of the pixels in the rectangle purview is judged to solve the problem. Those two algorithms consume lower soft resource, and reduce much more comparing times because both of them do not need traverse all the boundary points. The analysis indicates that the real-time performance and consumed resource of the two algorithms are similar for the simple coastline, but the method of dot-array is the choice when coastline is quite complicated.
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