地理知识是通过对地理科学领域独特的背景、过程和结果进行解释而得到的知识。如何有效表达和共享地理知识是地理学研究和应用中的重要问题。本文首先将地理知识划分为3个层次，事实型知识、规则及控制型知识和决策型知识，分析了不同阶段地理语言对地理知识表达与共享的支持能力。对比地图和GIS语言，发现新一代地理语言-虚拟地理环境VGEs（Virtual Geographic Environments）能够较为全面地支持3个层次的地理知识。针对VGEs，从多个角度分析了其服务于地理知识表达与共享的能力，包括虚实结合理论、地理模型库及管理系统、动态可视化表达和异地协同工作环境。结合面向珠江三角洲地区空气质量研究问题的虚拟地理环境平台，对上述各方面进行了阐述。最后从地理知识生成、管理与应用角度对未来研究提出展望。

Geographic knowledge, which is the higher level of geographic information, is acquired on the basis of geographic background, process, and results. The expression and sharing of geographic knowledge is important in many applications for urban and environmental planning and for any application in territorial intelligence. Expressing and sharing geographic knowledge is a significant issue in current geographic research. This paper is based on the evolution of geographic language from maps to geographic information system (GIS) and virtual geographic environments (VGEs). It aims to review the capability of VGEs to support geographic knowledge expression and sharing. To investigate the characteristics of different geographic languages and thusexpress and share geographic knowledge, we divided geographic knowledge into three levels:factual knowledge, rules and control knowledge, and decision-oriented knowledge. The evolution of geographic languages and their capability to support geographic knowledge expression and sharing were reviewed by discussing the relationship between geographic world, geographic language, and geographic knowledge. VGEs have more power to support geographic knowledge expression and sharing compared with maps and GIS. The outstanding features of such new generation of geographic language were reviewed considering their contributions to geographic knowledge expression and sharing. The features include interactive visualization, multi-channel perception, virtual reality technology, model base and model management, and spatially distributed collaboration. 1. Interactive visualization, multi-channel perception, and virtual reality technology enhance the expression of geographic world and geographic knowledge. The distance between geographic knowledge "writer" and "reader" is shortened. Thus, reaching a consensus on the expression and understanding of geographic knowledge becomes easy. Users in this virtual environment, which is the counterpart of the geographic world, can easily share geographic knowledge, unlike in traditional maps or dimension-reduced GIS. 2. Two cores of VGEs, data base and model base, are helpful in expressing and reusing geographic knowledge. Model base, which includes statistical models and dynamic geographic models, is the output from geographic facts and rules. Such geographic knowledge can be accumulated and reused by different users through an integrated model base. Users can analyze and simulate geographic phenomenon and process and acquire a high level of geographic knowledge to manage the geographic world. 3. Geo-spatial collaboration is developed in VGEs to support experts from multiple, spatially distributed disciplines and allow these experts to collaborate in conducting geographic simulation and analysis. In this way, they can share knowledge and obtain scientific cognition of the geographic world. Air quality problem is a crucial issue for the government of Hong Kong and the Pearl River Delta region. Thus, a VGE platform is developed for users from multiple disciplines to conduct air quality simulation and visual analysis by coupling meteorological and air quality models. In multiple perspectives, this case study analyzed the capability of VGEs in expressing and sharing geographic knowledge, with VGEs including virtual reality technology, geographic model base and management system, dynamic visualization, and geo-spatial collaboration technology. VGEs are under development and face key challenging issues. This paper is concluded with a discussion on future research concerning geographic knowledge creation, management, and application.