# GIS Approach for Expressing Structural Landforms: Forms, Elements, and Relationships
## 2 GIS Approach for Expressing Structural Landforms
### SLML Method for Expressing \"Forms, Elements, and Relationships\"
#### 2.3.1 SLML \"Object-Attribute-Relation\" Model  
The spatial forms of structural landforms are abstracted into spatial objects and their relationships, and based on extensible markup language (XML), the spatial form of regional geological structures is described uniformly to construct a structural method for the spatial forms of structural landforms.  
Based on XML, the SLML was designed.  
In the SLML \"object-attribute-relationship\" model, the types of objects include semantics, locations, geometries, attributes, relationships, and spatial references. SLML object types are represented using a series of XML schema elements and types. Based on the object/relation pattern, the semantic identification, coordinate parameter type, geometric symbol type, attribute type, and relationship type of spatial forms of structural landforms are constructed. Specifically, the model includes semantic schema, location schema, shape schema, attribute schema, and relationship schema. The coordinate reference system for the schema is defined and confirmed by the <include> element.  
The SLML \"object-attribute-relation\" model should be able to fully describe the semantics, geometric positions, spatial forms, attribute characteristics, and relationships among elements of structural landforms. The UML design of this method is shown in Figure 2.  
Structural landforms are composed of features including semantic terms, positional accuracy, geometric shapes, relationship types, and attribute types; these terms define the landform spatial references. (1) Semantic descriptions can be divided into square mountain semantics, anticlinal mountain semantics, synclinal mountain semantics, anticlinal valley semantics, synclinal valley semantics, single mountain semantics, hogback mountain semantics, fault block mountain semantics, fault valley semantics, and descriptive semantics, which are used to express different surface forms of structural landforms; the type of structure is a code list. (2) Spatial positions are expressed by structural landform positions; the structural type is data. The spatial position of structural landform can be composed of spatial coordinates in GIS. (3) Geometric forms are expressed by structural geometric elements, which can be composed of geometric elements (points, lines, and polygons); geometric elements are composed of positional elements. (4) Attribute characteristics are expressed by abstract attribute elements, which have four subcategories: geomorphic age, rock (material composition), rock occurrence, and compound attributes. (5) The relationship between elements is expressed by the relationship between structural elements and the topological relationship between geometric elements. The correlation and topological relationships are used to express the nonspatial/spatial relationship between the structural and geomorphic elements, respectively. (6) A spatial reference is a set of parameters that defines coordinate systems and other spatial properties for constructing geomorphic spatial objects. These parameters include the coordinate system, coordinate resolution, processing tolerance, and spatial domain. The coordinate system provides the framework for defining the spatial positions of structural and geomorphic elements. Coordinate resolution is the level of detail used to record the spatial locations and the shapes of structural and geomorphic elements. Processing tolerance is used to determine the processing accuracy of the topological treatment of structural and geomorphic elements. The spatial domain is used to establish the spatial range covered by structural and geomorphic elements.