This directory contains a variety of land-water masks.
The masks are stored in 8-bit (greyscale) PNG files.
In the masks, land is indicated by 0 and water is indicated by 255.
For some masks, a value in-between indicates that a pixel is close to the
coast (it does NOT mean that it's partly land, partly sea; see the
documentation for blur_lsm for details).
The lat/lon grids are defined in NetCDF-files associated with each mask.

HOW TO MAKE A MASK
==================

Step 1: create the NetCDF mask with GMT

Example:

$ grdlandmask -R0/360/-90/90 -Df -I1m -N1/0 -Glevels.grd -V

For documentation, see ``man grdlandmask''.

Step 2: convert the NetCDF to PNG

This NetCDF mask is much too large. We can compress it up to a factor 1000
by converting it to PNG. This task is performed by lwm_netcdf2png:

$ ./lwm_netcdf2png levels.grd land_water_mask_1min.png land_water_mask_1min_latlon.nc

This will read the NetCDF from levels.grd, write the actual mask to
land_water_mask_1min.png and write the lat/lon-values ta
land_water_mask_1min_latlon.nc.

Step 3: (to be implemented) You might want to add interfaces in Atmlab to
read the land-sea-mask, although for large masks, it may be more efficient
to read only the parts of the mask that are needed (I don't know if that's
possible with Matlab's NetCDF routines though).

THE MASKS
=========

* 1-minute mask
    - mask:             land_water_mask_1min.png
    - lat/lon-values:   land_water_mask_1min_latlon.nc

* 5-minute mask
    - mask:             land_water_mask_5min.png
    - lat/lon-values:   land_water_mask_5min_latlon.nc

* 1-degree mask:
    - mask:             land_water_mask_1deg.png
    - lat/lon-values:   land_water_mask_1deg_latlon.nc

Further reading:

* grdlandmask(1gmt) man page
* http://en.wikipedia.org/wiki/Portable_Network_Graphics