Strategic decision-making is often based on multiple decision criteria and on the decision-makers’ preferences regarding relative criterion importance. We present version 2 of a map overlay and opacity tool (moot2), which supports the visual exploration of decision-making scenarios. The tool uses a basic function of geographic information systems (GIS): visual map overlay. In this approach, visual overlay serves as a representation of weighted linear combination in multi-criteria decision analysis (MCDA). Using the tool, we model a global human influence index for the extent of the province of Ontario, Canada. The index is a weighted overlay of six indicators that represent human influence on the land. The semi-transparent overlay of the six indicator layers occurs online in a web browser, making it accessible to distant collaborators in low-technology settings. The software architecture of moot2 includes the OpenLayers JavaScript library to display thematic Web Map Service (WMS) layers, such as the human influence indicators, on top of popular base map services, such as OpenStreetMap or Google Maps. The tool also uses jQuery, a JavaScript library that enables interactions between the user and the map environment. Layer opacity is manipulated by the jQuery Slider, permitting the user to generate on-the-fly weighting schemes for the combination of the human influence indicators. By removing the analytical engine (i.e., GIS), decision-makers can perform rapid MCDA without recalculating composite evaluation scores. The case study demonstrates how visual-spatial MCDA supports the refinement of decision-making parameters such as criterion weights.