GPS data have been filtered with a two-pole, low-pass Butterworth filter with a 30-minute cutoff period. This 30-minute window was chosen based on a worst-case horizontal positional uncertainty of 0.035 m and a base ice velocity of ~650 m a-1, following from which assumptions the period over which velocities can be resolved is ~0.5 hours. Uncertainty was calculated based on a conservative estimate of the positional uncertainty of ±1 cm propagated through the velocity calculation.

Seismometer data were decimated to 100 sps and a 2-pole, zero-phase bandpass filter (10-50 Hz) was applied to eliminate instrument and high-frequency noise.

We assign UAV-derived orthophoto and DEM data uncertainties of ±0.12 m horizontally (~1.1x the ground sampling distance) and ±0.14 m vertically (~1.3x the ground sampling distance), following Chudley et al. (Cryosphere, 13, 955-968, 2019). Uncertainties in the velocity field were calculated based upon a displacement uncertainty of 0.17 m.

A detailed description of the data collection, quality control, processing and analysis is given in:

Chudley, T.R., Christoffersen, P., Doyle, S. H., Bougamont, M., Schoonman, C. M., Hubbard, B., and James, M. (2019) Supraglacial lake drainage at a fast-flowing Greenlandic outlet glacier. doi:10.1073/pnas.1913685116