If increased functionality creeps-in the schedule and timelines will increase as well risking success at early stages of the project. It is important to show early successes with basic functionality, and build on these in the future. Consider tackling the hardest requirements out in the project build cycle, and focus on requirements that are easiest to tackle to show early success. It is important to keep all requirements in mind for design sessions, but in order to get early feedback, requirements must be kept in scope. Collaborate between organizations and projects to allow for sharing limited resources towards a common goal.
Several organizations have the same basic requirements to serve mapping data.
Although time-consuming to set up and maintain working relationships across organizations, these relationships are very important over the life cycle of the project. Keep stakeholders involved in all cycles of the process. Getting end-user feedback early is invaluable. Let users critique screen shots before development begins, but prevent design by committee sessions from occurring in user feedback sessions.