Mapping 3d data on a 2d color map

Gnuplot has a very easy and straightforward way to convert a x-y-z data into a 3d plot or a 2d color map. Let's write the data in the form;
1 1     0.0
1 2     0.4451
1 3     0.4249

2 1     0.13142
2 2     0.0
2 3     0.983

3 1     xxx
3 2     xxx
3 3

A space between chunks of data is important. Using commands below you can get a 2d plot;

set size square
set pm3d map    # Remove the word 'map' to get a 3d plot
set xrange [1:261]  # Change the range to suit your needs
set yrange [1:261] 
splot 'all.dat'