The context of mobility raises many issues for GIS applications. Mobile device limitations, including pen input whilst in motion, result in interfaces which are difficult to navigate and interact with. However, comparatively little research has been conducted to address the interface mobility problem for GIS. We are particularly concerned with the limited interaction techniques available to users of mobile GIS which play a primary role in contributing to the complexity of using such an application whilst mobile. Our research focuses on multimodal interfaces as a means to present users with a wider choice of modalities for interacting with GIS applications. The focus of this paper concerns a comprehensive user study which demonstrates the benefits, in terms of usability and efficiency, of a multimodal interface for the CoMPASS mobile GIS which we have developed.