MATLAB: How to measure surface area layer by layer using Matlab

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i am using following code to find area of layers. I am trying but masking somewhere some mistake. Any help from community will be highly appreciated.
Attached file are the xyz-coordinates. I measure the surface area of one layer using bellow code. This coding is combining all layers. I want to find area of each layer individually. Regards to all for their cooperation and guidance. 
vertices = load(' attached file ')
vertices = round(sortrows(vertices,2));
Vx = round(vertices(:,1));
Vy = round(vertices(:,2));
Vz = round(vertices(:,3));
G_surface_area =0
layer = []
surface_area = 0
for i = min(Vy):10:max(Vy) % This for loop, i am using to measure by 10 spacing. 
   minVy =  i
        for j = i:i + 9   % This measure is for layer to find its area. 
           
            layer_new = vertices(any(Vy== j,2),:);      
            layer = [layer; {layer_new}];
        end 
             
               layer = cell2mat(layer)                   
                Lx = layer(:,1);
                Ly = layer(:,2);
                Lz = layer(:,3);
                surface_area = polyarea(Lx,Lz)
       
           G_surface_area = G_surface_area + surface_area
            
end

Best Answer

data = importdata("Cone_20000_points.txt") ; 
data = round(data) ; 
x = data(:,1) ; y = data(:,2) ; z = data(:,3) ; 
zi = unique(z) ;   
N = length(zi) ; 
A = zeros(N,1) ; 
tol = 10^-5 ; 
for i = 1:N
  idx = abs(z-zi(i))<=tol ; 
  % layer coordinates 
  xl = x(idx) ; yl = y(idx) ; zl = z(idx) ; 
  Layer = [x(idx) y(idx) z(idx)] ;   % these are the Layer coordinates 
  % get boundary of layer 
  id = boundary(xl,yl) ; 
  Boundary = [xl(id) yl(id) zl(id)] ;   % Boundary coordinates 
  A(i) = polyarea(xl(id),yl(id)) ; 
end
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