1.create a coverage,i.e www
2.command:polygrid www <out-grid> type
                 cell size:0.1
                 grid origin(x,y):0 0
                 grid size(nrow,ncol):100 100
3.command:gridascii <in-grid> www.txt
4.www.txt is a text file which can be used by other programs.
5.command:asciigrid
6.command:gridpoly
7.so we can obtain a new coverage, using grids to represent old 