This paper describes the usability evaluation of a WebGIS. The system under evaluation has been developed for a real-world community in the Swiss canton de Vaud, namely for two people who are involved in wine making. The system's usability is tested with users from this community. The goal of the study is to validate hypotheses and answer research questions focusing on users'performance and specific factors that influence user's performance, such as experience and training or particular interface features. To analyze these hypotheses and research questions, a methodology has been developed to capture the users'interaction during the hands-on evaluation and to analyze the captured data. We found that these differences in performance are related to the users'experience and expertise in the use of similar systems, but also to specific interface features. The findings presented in this paper are useful for developers and designers of WebGIS and can help to improve these systems.