Participatory geographic information systems (PGIS) have been touted as a technology to democratize local decision making. The promise of PGIS has been largely pinned on the assumption that maps as tools of communication are rich in informational content, and capable of fostering shared understanding and analytical thinking. Whether maps alone can indeed be the medium for supporting shared understanding and analytical thinking in participatory decision making is still an open question. Much of our knowledge in this area is still anecdotal, and there is a need for empirical work to supply the evidence. In an attempt to test the applicability of maps, decision models, and Internet-based communication tools for PGIS, software called spatial understanding and decision support system (SUDSS) was built using asynchronous spatiotemporal environment, and tested in a realistic setting representing public land-use planning debate. This chapter provides a description of the software and the experiment, followed by the discussion of its results and their significance for the future development of similar public participation spatial-decision support tools.