Box 14.1 describes an application example of cumulative watershed. Which operation, counting or Boolean, did the study use for compiling the cumulative watershed?

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Box 14.1 An Application Example of Cumulative Viewshed Wh hen two or more viewpoints are used in an analysis, the interpretation of viewshed analysis can differ significantly depending on if the interpretation is based on simple or cumulative viewshed. This dif- ference is demonstrated in a study by Mouflis et al. (2008), which uses viewshed analysis to assess the vi- sual impact of marble quarries on the island of Thasos in northeastern Greece. There are 28 quarries with 31 hectares (ha) in 1984 and 36 with 180 ha in 2000. The input data for viewshed analysis consist of a 30-meter DEM and a line raster showing the quarry perimeter cells from each year. Their results show that the visible area of the quarries increases from 4700 ha (12.29 percent of the island) in 1984 to 5180 ha (13.54 percent of the island) in 2000. The increase of the viewshed in binary format (vis- ible or not visible area) is relatively small (+1.25 percent of the island). However, when the cumulative viewshed is considered, the total visible area is increased by a factor of 2.52 and the range of visible quarry perimeter cells is changed from 0-201 to 0-542.