Politicians, planners, and mapmakers have long used mapping to depict selected spaces, to document natural and humanmade changes within them, and to identify spaces where planning intervention is needed or can be helpful. Recent innovations involving big data, GIS-based research and digital datasets offer opportunities for maps and mapping that can lead to a better understanding of the interrelation of spatial, social, and cultural elements over time and to facilitate planning. A close analysis of the historic transformation of the built environment (such as land use, land ownership, infrastructures), the development of institutional structures (municipal boundaries) and the narrative that accompanies them (as embedded in maps and plans) through historical geo-spatial mapping can facilitate the identification of ‘gaps,’ where spatial, institutional, or cultural opportunities and challenges exist and where planning can be useful. Such an understanding can provide novel insights into the conditions and complexity of multiple transitions (energy, digital, technological) and provide a better foundation for future design. Our use of geo-spatial mapping to identify ‘gaps’ builds upon the work of Patrick Geddes and Jacqueline Tyrwhitt, who promoted the concept of survey-before-plan. As Tyrwhitt argued in 1950, the consistent overlaying of information can help us see patterns and outliers and derive meaning from huge, complex territories and large amounts of data (Tyrwhitt, 1950b). We can then better identify planning opportunities. Following an analysis of mapping as an analytical tool, we explore questions of sources, time, representation, and scale in the use of mapping at a time of increased availability of data. This article represents an initial effort to analyze the role of mapping as a tool of understanding, communicating, and ultimately planning through the lens of port city regions and their development over time. As a first step, it proposes conducting observations of historical geospatial mapping in port city regions in Europe: the Nieuwe Waterweg in the Netherlands, the Thames in the UK, and the Elbe in Germany. Probing the challenges and opportunities presented by historical sources, questions of representation and scale and data layers, the article concludes by proposing historical geo-spatial maps and mapping as a tool of display and comparative research and as a ‘gap finder.’