By performing a time-depth analysis (Van Eetvelde & K€ayhk€o,
2009), we delineated patches belonging to the same land cover
type on all three maps (1912, 1980 and 2009). For each land cover
type, we produced a new map consisting of the patches of continuity
over 97 years, which indicated the surfaces of historical
persistence of each land cover type. These new maps were created
by applying an overlay technique to the raster maps based on the
reclassification and codification of each land cover type. Specifically,
the new classes included information both on the land cover
type and on the map's year. Moreover, by using appropriate ArcGIS
tools, codes were chosen such that, by map overlay, this information
was not lost. The values in the new maps' attribute table were
used to conduct a quantitative analysis of landscape persistence. To
compute and compare the areas of historical persistence, we used a
class level numerical indicator that we call the persistence index. For
each land cover type l, the persistence index PEl is defined by the
formula: