I’m watching a talk about swiss-experiment.ch, which is a great project using a semantic wiki as a data infrastructure for a sensor network. The presenter started out by showing a graph of sensor data measuring temperature from the swiss alps in winter. The graph looked reasonable (i.e. it was cold). However, he pointed out that in fact the sensor was broken and the data was invalid. His quote (roughly):
Never trust someone else’s sensor without the metadata.
Fundamentally, you need provenance to enable the confident use of other people’s data.