The visualization of georeferenced thematic data by virtual environments represents a key functional requirement in a large number of geo-information applications and systems. The perception of large, detailed thematic data sets, however, can be inhibited by visual clutter and information overload, especially in case of three-dimensional data. Level-of-detail visualization techniques can mitigate these issues and guide the viewers in their analyses. While the commonly used geometric level-of-detail techniques can reduce the complexity of the data's reference geometry, they generally do not take into account thematic data. This article presents two spatial aggregation techniques for generating level-of-detail representations for thematic data: The scene-based technique aggregates data solely based on spatial locations, thus supporting visual analysis of data with arbitrary reference geometry. The object-based technique performs aggregation based on scene-specific objects and their hierarchy to facilitate per-object analysis. Both techniques operate in real-time during rendering to support on-the-fly and on-demand level-of-detail generation for the visualization of spatiotemporal data sets. We demonstrate the application of both techniques using real-world data sets, including solar potential analyses and the propagation of pressure waves in a virtual city model.