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.