# GIS Approach for Expressing Structural Landforms: Forms, Elements, and Relationships
## 2 GIS Approach for Expressing Structural Landforms
### Definition of Structural Landforms Based on Spatial Objects and Topological Rules  
Using the object-oriented method, the structural landform elements and their relationships are abstracted into spatial objects and their topological relationships. Based on the types and characteristics of the structural landform elements, the geometric types, attribute domains, and subtypes of the spatial objects of structural landforms are defined. Based on the morphological type of a structural landform, the names and rules of topological relationships between spatial objects are developed. Structural landform characteristics depend on the characteristics of each component of the structural landform and its occurrence, material composition, formation age, and other traits. (1) According to the geometric types, such as point, polyline, and polygon, the name of the element class and the geometric expression of the spatial object are designed. As shown in Figure 1, structural landform elements, such as fold cores, strata, rocks, and fault plates, are abstracted into planar geometric types. Fold axes, fault lines, contour lines, ridge lines, valley lines, and other elements are abstracted into linear geometric types. (2) The attribute field is used to describe the valid property field values of spatial objects. For example, for the occurrence of rock strata, the attribute domain is divided into three groups according to the dip angle attributes: 0\({}^{\circ}\), 0\({}^{\circ}\) to 90\({}^{\circ}\), and 90\({}^{\circ}\). For the attribute domains of rock material composition, rocks can be divided into different groups, including shale, conglomerate, limestone, marl, gneiss, and diorite. The attribute domain of the geomorphic formation age, according to the stratigraphic age, is divided into Quaternary, Tertiary, etc. (3) A subtype is a subset of objects with the same spatial properties as an object. An attribute field of a subtype must be associated with the corresponding property domain field. For example, in this study, the rocks were divided into horizontal, vertical, and inclined rock subtypes according to their occurrences. The stratigraphic elements were divided into Quaternary to Cambrian subtypes according to the stratigraphic chronology. The rocks were divided into soft, medium-hard, hard rocks, and other subtypes, according to the lithological characteristics, and each subtype was associated with the corresponding attribute field. The morphology of a structural landform is described by the relationships between its various components (structural landform elements). The relationship between elements is the topological relationship that has geometric meaning among the spatial objects of the structural landforms. In this study, the corresponding mechanism for describing element  
Figure 1: Design of the geometric types, attribute domains, and subtypes of spatial objects. relationships and topological rules was designed so that the structural landforms could be expressed through spatial objects and topological relationships. As shown in Table 3, the elements and their relationships for the nine structural landforms are expressed through spatial objects and their topological relationships, including the types, topological names, and topological rules of the spatial objects associated with each topological relationship. For example, anticlinal mountains are mountains formed by anticlinal structures. This structural form can be described by four elements (fold core, fold axis, stratum, and ridgeline) and their relationships. The structural form is expressed through the relative stratigraphic age of the fold core and the spatial relationship between the fold axis and ridge line. The spatial relationship can be expressed as the relationship between the fold core (polygon) and the stratigraphy (polygon), in which the stratigraphic subtype is the relative stratigraphic age. The axis of the fold (polyline) coincides with the ridgeline (polyline).