## 2 Basic Ideas  
The concept of geographic scene understanding of remote sensing images includes two aspects, namely, \"remote sensing image understanding\" and the \"geographic scene\". Remote sensing image understanding is a cognitive process to realize the objective things and their laws reflected by remote sensing images through observing, distinguishing, identifying and reasoning remote sensing images and interpreting the content of remote sensing images semantically. The geographic scene is a regional complex with a specific structure and function, which is composed of various natural and human factors in a certain region [10]. In remote sensing images, the geographic scene is a closed region composed of different ground objects. The geographic scene generally involves three aspects: (1) the constituent elements of the scene structure, (2) the relationship between these elements and (3) the function of the set of these elements. Therefore, the research object of geographic scene understanding of remote sensing images is the regional complex composed of ground objects with certain spatial distribution patterns. The research objective is to interpret the research object as a series of meaningful and understandable semantic information. HSR remote sensing images can reflect the more detailed composition and spatial distribution of the ground objects, which is a microcosm of the real geographic scene. Owing to increased spatial resolution and unique imaging methods, geographic scenes in HSR remote sensing images have the following characteristics:  
1. The categories of ground objects in the geographic scene are diverse. The same category of geographic scene can contain different ground objects, and different geographic scenes can also contain the same ground objects. Different objects also have different characteristics in terms of spectrum, texture and structure [11]. 2. The categories of ground objects in the geographic scene have variability. The change in the categories of some ground objects in geographic scenes does not necessarily lead to a change in the whole semantic information of geographic scenes [12]. 3. The spatial relationship between ground objects in geographic scenes is complex. Different distribution forms between ground objects lead to different semantic information of geographic scenes. Other relevant characteristics are shown in Figure 1.  
The characteristics of HSR remote sensing images also make the following special features in understanding geographic scenes:  
(1) The semantic information of geographic scenes in HSR remote sensing images is hierarchical. The content description of HSR remote sensing images has the hierarchical inclusion relation of \"Pixel-Region-Target-Scene\". Different levels of image content reflect the semantic information with different levels of abstraction, which can be divided into the visual layer, object layer and concept layer (Figure 2). The visual layer is the description of pixel-level image content, including color, texture, shape and other original visual characteristics. The semantic information of the visual layer can be obtained directly from image processing without any external knowledge and experience [13]. The object layer is the description of region-level and target-level image content, including the individual features of objects and the local features of spatial relations among objects [14]. The semantic information of the object layer needs to be obtained through simple reasoning, and it is necessary to use external knowledge and experience to assist this reasoning. The concept layer is a description of the scene-level image content, including the abstract attributes of the image. The semantic information of scene level involves the semantic features of scene representation or higher-level behavior or emotion analysis, and it needs to link image content with abstract concepts through complex reasoning and subjective judgment [15].