Syntax :

ground {
[ meshsize size_value ]
boundary_line { vector [ vector [ .. ] ] }
[ fixed_points { vector [ vector [ .. ] ] } ]
[ ... ]
[ noise_only 1 ]
[ noise_height height ]
[ noise_level level ]
[ decimate percentage ]
[ direction vector ]
[ positioning ]
}

Tips :

meshsize
    defines size of the triangles. Best values are 64 or 128.
boundary_line
    These are polygon lines. MGS uses boundaries to compute the size and position of the grid. All grids are rectangular. You can use boundaries several times in a single ground statement. The generated brushes will follow the boundaries you have given.
fixed_points
    is optional, you can use it if you want to fix some points, without using a polygon line. You can use them to add holes in your grid.
direction
    is optional, it indicates the grid normal vector. Default is <0,0,1>. For example, if you are using the ground feature for a wall, you'd better use a direction in (xOy).

How it works :

Once you have given MGS your boundary conditions, MGS builds a grid, puts on your values, and smooths it. Technically, it means computing laplace(u)=0 on the non-fixed points. Finite differences method is used. It will get rather slow on big grids, but the triangle count will get too high anyway ...

The ground feature has seen lots of changes since previous version. The doc will be updated when text scripting interface for version 1.3 will be ready.