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landscapes. Crisp classification is particularly suited to 
construct objects of different object classes. 
In natural landscapes a continuous type of 
classification has to be applied to accommodate the 
quantification of ecological gradients. À proper 
representation of gradients is of ecological importance, 
because these situations often posses high natural 
values. The notion of continuous classes can be 
expressed with the mathematical concept of fuzzy 
classification (Wood and Foody, 1989; Blonda et al. 
1991). 
The process of image interpretation holds a number of 
hierarchically ordered classification steps following 
from a land-cover hierarchy. An example hierarchy is 
depicted in figure 3. On the first level vegetated and 
non-vegetated areas are distinguished. Because the 
cover types road and water can be easily derived from 
topographic maps, the only non-vegetated class to be 
isolated in the image is 'blond sand' Hence, the 
isolation of blond sand is subject of the first 
interpretation step. The second step involves the 
subdivision of the vegetated area in woody and 
herbaceous vegetation. The third phase deals with the 
subclassification of wood into several species or species 
groups. Finally, the herbaceous vegetation is 
subdivided in structural types in the fourth phase. 
Given the spectral and geometric resolution of the 
applied image and the appearance of the land cover 
types or classes in the terrain, one has to determine 
whether the class has a crisp or fuzzy character. 
Consider for example false colour images with a 
resolution of 0.5m, than the classes wood, herbaceous 
and sand are presumed to form discrete objects. Hence, 
the crisp classification of the image to the three classes 
deals with the estimation of a triple of membership 
values (MV ^, My ***5 My?"5 for each image pixel, 
where only the values 0 and 1 are allowed for no 
membership and full membership respectively. For 
instance the vector (0,0,1) means that the pixel is 
covered with sand. The subclassification of wood to 
species types is performed in a crisp classification 
applying texture features. 
Subsequently, we want to quantify the continuous 
internal variation of the herbaceous cover class through 
fuzzy techniques. The variation in herbaceous 
vegetation structure is categorised in five classes hs ¢ 
ths, hs, -.hsj, which were qualitatively described as 
prototypes (table 1). In the terrain some sites will show 
a high resemblance with only one class, while others 
have properties belonging to two or more of these 
classes. Typically, these partial memberships can be 
Quantified in a fuzzy classification, where the 
resemblance of a site with a class is indicated by a 
membership value MV, Ve'tl01) Hence Ya site) is 
characterised by a vector of five membership values 
MM Mau Mes Mn) For instance the 
vector (0, 0.6, 0.4, 0, 0) indicates that the site has nearly 
equal resemblance with hs, and hs, and no similarity 
with hs, hs, and hs, The problem of fuzzy image 
interpretation is now to estimate a vector of 
membership values for each image pixel covered with 
herbaceous vegetation. A detailed description of these 
estimation methods captured by a the supervised 
hierarchical classification system is reported by Droesen 
et al. (1995). 
Tab. 1. Description of the herbaceous structural 
subclasses (hs) (after Assendorp and van der Meulen, 
1994). 
  
hs1: Thin grass/herb cover with blond sand 
Blond sand, i.e. sand with negligible amount of organic 
matter, has, by far, the largest contribution in this 
coverage type. It is however accompanied by pioneer 
plant types. Herbs are annual as well as biennial. Grass 
types are mainly solitary and clonal which react more or 
less positive to wind activity. Tussock forming grass 
types can be present. 
hs2: Intermediate herb/moss cover with grey sand 
Largest contribution to the overall coverage is with 
mosses who react more or less positive to or can sustain 
some geomorphological activity. Bare grey sand, i.e. 
sand with organic matter content, has a substantial 
contribution to the overall coverage. Herbal plant types 
are annual and biennial with locally some perennials. 
Some woody plants at the sub-pixel level can occur, 
grasses are solitary and tussock forming. 
hs3: High moss cover 
A total coverage of the soil with mosses and lichens, 
very locally with annual and biennial herbs. Grasses are 
nearly absent. 
hs4: High moss and low grass cover 
The soil is totally covered with mosses combined with a 
low herbaceous vegetation. Herbs and grasses are 
mainly small though larger woody plants at the sub- 
resolution level can occur. 
hs5: High grass/herb cover with litter 
Mainly grasses and perennial or clonal herbs cover the 
soil completely. The herbs are partly woody plants at 
the sub pixel level. Dead ectorganic matter determines 
partly the nature of this type. 
  
217 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996