This presentation will discuss the design of interactive raster masking capability for a web-based geographic information system (Web GIS) from the perspective of user-centered design. A case study will be presented by analyzing and discussing specific aspects of the user-developer dialog within the context of the software development life cycle. The presented case study will take the audience through the development of an open-source Web GIS software architecture (MapServer, PostGIS and OpenLayers) with particular focus on how the masking functionality was designed and implemented. Software engineering techniques informed by iterative cognitive walkthrough allowed for a dialog between Web GIS production and consumption that led to a more usable and useful cartographic interface. Key findings from this case study will be presented.