I’ve downloaded the GRUMP 30 arc-second area grid (ASCII format), and am confused by the units. The meta data says that the units are km2 of land area, but when I unzip the file (using gunzip), producing gluareag_alpha1.asc, then the land cells have values of 56 (or less, if not completely land). Also, the missing value should be -9999 but the data file has 0 instead.
The units for the land area grids are in ha rather than square km (this
allows us to distribute the grid as Integer rather than floating point).
This is only for the global grids, the country and continent grids are
in square kilometers.
We set ocean, inland water, and permanent ice areas to zero in the land
area grid (similar for the population grids), thus the lack of -9999
values in the grid.