Land use/cover classification and change detection
A modified version of the Anderson Scheme Level I (Anderson et al. 1976) was adopted to study the land use/cover change. Though the scheme was originally developed for the USA, it is the widely used land use/ cover classification system across the world (Mundia and Aniya 2006; Shalaby and Tateishi 2007; Yuan et al. 2005). The system proposed multilevel land use/cover classification of which level I classes can be mapped from Landsat data or from high-altitude airphoto/ imagery, whereas the extraction of information at levels II, III, and IV requires the use of high, medium, and low-altitude photographs, respectively. Six separable
land use/cover types have been identified in this study as water bodies, wetland/lowlands, built-up, cultivated land, vegetation and bare soil/landfill (Table 1). The 1960 land cover map has been developed from topographic maps which are created from aerial photographs taken in 1955 followed by extensive ground truthing in 1956. Two scanned topographic maps (sheet no. 79 I 5 and 6) were displayed on the computer screen. Using ArcGIS, the 1960 land cover map was digitized, edited, and leveled. Besides, a large scale map (1:20,000) of 1961 by Survey of Bangladesh (SOB) depicting the study area and its surroundings was employed to identify various land cover types in a GIS platform. The 1962 land use classification map with
seven categories by Khan and Islam (1964) was also useful to distinguish the land use/cover information on topographic maps. Finally, vector land cover data were rasterized to perform raster-based change analysis.
All the satellite data were thoroughly studied using spectral and spatial profiles to ascertain the digital numbers (DNs) of different land cover types prior to classification. Training samples were selected through reference data and ancillary information mentioned earlier.