Bare Rock or Soil

It is not possible to distinguish bare rock or soil from other spectrally similar categories including landfill, quarry, rocky shore and artificial rocky/hard shoreline in the supervised classification. However, by taking the initial set of areas with the same spectral signature, and subtracting known areas of landfills, quarries, rocky shores and artificial rocky/hard shorelines (delineated by existing topographic data) from this set, the bare rock and soil areas were defined. Further manual discrimination was also performed by identifying the highly modified areas of reclaimed land and construction sites (which both belong in the "Other" category) from the bare rock or soil habitat. The resulting product, i.e. the area classified as bare rock or soil, is therefore mapped with a high level of confidence.

Grassland

Once the initial, unsupervised classification was complete, aerial photos were examined, with the assistance of HKU specialists, to select areas of known grassland. In addition, preliminary site visits added additional known grassland areas to the knowledge base. These areas of known grassland showed a high homogeneity. The use of these data in the supervised classification resulted in a medium accuracy for grassland areas because it was difficult to separate spectrally grassland from shrubby grassland.

Shrubby Grassland

As expected, the unsupervised classification was not able to clearly distinguish shrubby grassland from other woody vegetation types. Therefore, preliminary site visits were conducted to determine the spectral signature of areas of known shrubby grassland. This information was then input to the initial mapping and the digital data reprocessed in a supervised classification. Although this improved the classification results, uncertainty remains for areas where the relative proportion of grass and woody plants are similar (eg potential intermingling with mixed shrubland habitat). The habitat is thus mapped with a medium level of confidence.

Mixed Shrubland

As described for shrubby grassland above, mixed shrubland required the input from preliminary site visits before the supervised classification could be performed. Mixed shrubland is mapped with low confidence, and uncertainties exist in distinguishing this class from shrubby grassland and lowland forest vegetation types.

Baeckia Shrubland

It is difficult to distinguish Baeckia shrubland from other woody vegetation types based solely on satellite imagery and aerial photos. Preliminary site visits provided significant assistance in the supervised classification process, but as Baeckia shrubs often mix profusely with other shrubs and tress, many areas and boundaries could not be mapped with confidence. Mapping of the category is therefore of low confidence.

Fung Shui Forest, Montane Forest and Lowland Forest

Fung shui forest, lowland forest and montane forest habitats were recognised in the unsupervised classification and refined in the supervised classification through the input of information from preliminary site visits. In addition, point locations of 115 identified fung shui forests, and boundaries for 17 of these habitats, were mapped onto the habitat map based on the information provided by Chu (1998) and thus differentiated from the remaining two forest types. The fung shui forests are considered to be mapped with medium confidence since the locations are known but the boundaries may require verification. The remaining forest habitat was delineated into montane and lowland forest based on altitude. As defined in TR1, the boundary between these two types of forests was considered to be 600 metres above sea level. Lowland forest habitats in many locations blended with mixed shrubland, Baeckia and plantation habitat types, and are therefore mapped with a low level of confidence. Montane forests are forest habitats located 600 metres above sea level and only few sites were classified into this category. As these habitat areas are less likely to be mingled with other shrubland habitats and highly unlikely to be blended with plantation, montane forest is regarded as having a medium level of mapping confidence.

Plantation or Plantation/Mixed Forest

This category was mapped through a visual identification of patterns in tree planting and through the input of information from preliminary site visits. As many of the "older" plantation areas have been invaded by other vegetation species, only a small number of plantation sites remained as recognised rows. Therefore, very few sites were classified into this category. While the confidence in the mapped plantation habitats is high, it is likely that more plantation type habitats can be identified through groundtruthing of the habitat map.