77?g International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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The semantic description model is built in abstract level and 
concrete level, which is showed in Figure 2 in Unified 
Modeling Language (UML). It is an integrated model 
consisting of three parts: Data Profile, Data Content and Data 
Binding. Data Profile tells users what the data are about by 
describing the profile of the geographic information, such as 
the dataset name, data format, acquisition methods, involved 
processing, spatial extent, spatial resolution, provider, theme 
and so on. Data Profile belongs to abstract level description. It 
is biiilt by introducing metadata ontology. Data Content tells 
users what the data contain by describing the content semantics 
of geographic information data in concrete level. 
Conventionally, geometries of spatial data are explicitly 
represented. But spatial relationships are not explicitly or 
formally represented. In this paper, geometric ontology and 
spatial relation ontology are introduced into Data Content to 
explicitly describe spatial semantics of geographic data. In 
addition, fundamental geographic information ontology is 
introduced to describe non-spatial data semantics. Data 
Binding tells users where and how to access the data by 
providing URL, contact information and procedure to access 
data. 
1 
Figure 2. Ontology-based semantic description model 
4. DISCOVERY AND RETRIEVAL BASED ON 
ONTOLOGIES 
Conventionally, discovery and retrieval for geographic 
information is carried through based on keywords. However, 
keywords are not sufficient to find exactly suitable geographic 
information because lack of semantics and inference 
mechanism usually causes search results often to be too narrow 
or too large (Hartwig H. Hochmair, 2005). The emergence of 
ontology provides possibility to enhance discovery and 
retrieval. In this section, we discuss an ontology-based 
approach for discovery and retrieval of geographic information. 
4.1 Conceptual framework 
The key to improve discovery and retrieval is to solve 
problems of semantic heterogeneity between user’s search and 
description of geographic information in SDI. Thus, user 
ontology is needed to cooperate with ontologied introduced 
into the semantic description model proposed above. User 
ontology models concepts and relationships between concepts 
in user’s view during search. 
Figure 3. The hybrid ontology approach (Visser, U. & H. 
Stuckenschmidt, 2002) (modified)