http://www.ecmwf.int/en/faq/what-land-sea-mask-lsm
The model surface is logically divided into sea and land points by using a land-sea mask. A grid point is defined as a land point, if more than 50 per cent of the actual surface of the grid-box is land, for example with a TL511 resolution, islands like Corsica, Crete and Cyprus are represented by around five land grid points each.