Researchers use the PlanetLab testbed for its ability to host experimental applications in realistic conditions over the public best-effort internet. Such applications form overlays whose performance is affected by the underlying topology and its evolution. While several topology information services have been proposed for PlanetLab, the TopHat system that we describe here fills a special niche. It is designed to support the entire lifecycle of an experiment: from setup, through run time, to retrospective analysis. TopHat does so in a new way, by drawing upon excellent, proven third party services, notably the Dimes and Etomic measurement infrastructures, for specialized measurements. TopHat has been developed as the active measurement component of PlanetLab Europe, the flagship testbed of the OneLab experimental facility. It is part of OneLab's larger effort to pioneer the federation of previously independent testbeds and measurement systems in order to provide a diverse global scale environment for Future Internet research.