% You may wanna change the values below according to your
% particular problem
file_name = 'data_2d.out'; x_min = 0; x_max = 1; y_min = 0; y_max = 1;

% read the file 
data = textread(file_name);
% find the size
n = sqrt(size(data,1));
% reshape the data
data = reshape(data,n,n)';
% construct the vectors of x,y coordinates
x = x_min : (x_max-x_min)/(n-1) : x_max;
y = y_min : (y_max-y_min)/(n-1) : y_max;
% plot the data
surf(x,y,data);