This paper proposes an exploratory method for detecting qualitative change in trend curves that is robust against low quality data. First, a measure of qualitative similarity between trend curves is defined. Second, using this measure, a method of qualitative classification of trend curves and a method for detecting qualitative change in trend curves are developed. Last, these methods are applied to the analysis of land cover change in the Persian Gulf Area in 1982-1993, and advantages of these qualitative methods are shown in comparison with ordinary quantitative methods.