I am creating annual water percentage image over a region. There is lof of missing data as far as water/land mask is concern as there are clouds or nodata values for the landsat 7 gaps. How can I show percentage of water over an year with such missing data values. I don't think that my denominator should be the same in calculating the percentage. Also, taking zero for missing data is also wrong. What is the industry standard when doing this. Help!