How to perform regridding (1.0d to 0.5d) using bilinear interpolation for an irregular area?
Try something like this:

[x_grid,y_grid] = meshgrid(1:100,1:100);
z_grid=griddata(x_data,y_data,z_data,x_grid(:),y_grid(:),'v4');


You can change the method of interpolation if you prefer something else.

See:
http://www.mathworks.com/help/matlab/ref/griddata.html