Bedmap2: improved ice bed, surface and thickness datasets for Antarctica - Fretwell, P; Pritchard, HD; Vaughan, DG; Bamber, JL; Barrand, NE; Bell, R; Bianchi, C; Bingham, RG; Blankenship, DD; Casassa, G; Catania, G; Callens, D; et al. (CRYOSPHERE, 2013)