The radial propagation of type-I ELMs in ASDEX Upgrade has been measured using a number of techniques. The most reliable technique uses a filament probe to measure the time difference between two separated probes and yields a mean radial velocity obtained of ~1.5 kms -1 . The radial velocity calculated using a time of flight technique suffers from uncertainties in the start time and yields radial velocities which are 3-4 times lower than those based on the filament probe. The filament probe data shows that all filaments do not leave the LCFS at the start of the ELM but rather leave over a period of up to 400 µs.
