For some fields the land/sea mask is available. This devides the
grid points into land and sea grid points. Some datasets contain
fractional grid boxes, in which one grid box can contain a fraction land
and a fraction sea. In this case boxes with more than 50% sea are
classified as sea, and the rest as land. There are more strict options:
"only land" requires that the grid box is 100% land, "not land" that it is
0% land. (These options are hidden by default, click on "show/hide more"
to make them visible.) Generally the 50% criterion gives the best results.
The land/sea mask can be used to constrain interpolation and a few
analyses to land points or sea points only. In general the result will
indicate the kind of operation performed, "5lan" and "5sea" denote the
default 50% cut-off criterion.
The land/sea mask is considered a fixed field and does not take
into account the tides, sea-level rise, land reclaiming or erosion, or
continental drift. I try to include land-ice covered areas as land.