Due to the increased availability of geospatial data, web-based geographic information systems (WebGIS) have become more popular in recent years. However, the usability of these systems poses new challenges as user interactions are strongly affected by the map and are thus different from interactions with traditional user interface elements. In this paper, we propose a method for evaluating the usability of web-based geographic information systems by analyzing user intentions through map interaction patterns. We use a pattern mining algorithm to extract frequent interaction sequences from user sessions and label these with their interaction semantics that represent the users’ immediate intentions. To evaluate our approach, we conducted a user study with 60 participants in a WebGIS scenario and identified varying user strategies for a selection task based on two different geovisualizations. Our results indicate that the chosen approach can uncover the underlying intentions of users’ interaction patterns and facilitate insights into the usability of WebGIS.