Making ellipse keep the signed distance function

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Please, can someone help me: I want to use signed-distance function for ellipse. That means the contours must be equal distance from each other. But the distances are currently not equal. The matlab code is below. Thank you so much.

DomainWidth=2;
DomainHight=1;
ENPC=80;   
ENPR=160;
EW = DomainWidth / ENPR;       %  The width of each finite element.
EH = DomainHight / ENPC;        %   The hight of each finite element.
M = [ ENPC + 1 , ENPR + 1 ];
[ x,y ] = meshgrid( EW * [ -0.5 : ENPR + 0.5 ] , EH * [ -0.5 : ENPC + 0.5 ]);
[ FENd.x, FENd.y, FirstNdPCol] = MakeNodes(ENPR,ENPC,EW,EH);
LSgrid.x = x(:); LSgrid.y = y(:);       % The coordinates of Level Set grid 1

%%Generate ellipse

        cx = DomainWidth/2;
        cy= DomainHight/2;
          tmpPhi=   ( (LSgrid . x - cx)/cx  ) .^2 + (( LSgrid . y  - cy)/(cy)  ) .^2  - 1;
          LSgrid.Phi = -((tmpPhi.')).';
  FENd.Phi = griddata( LSgrid.x, LSgrid.y, LSgrid.Phi, FENd.x, FENd.y, 'cubic');
      figure(10)
    % %   subplot(2,1,1)
      contourf( reshape( FENd.x, M), reshape(FENd.y , M), reshape(FENd.Phi, M));
      axis equal;  grid on;colorbar;drawnow; 
    %   colorbar;
        figure(11)
        h3=surface(x, y, reshape(LSgrid.Phi , M + 1));  view([37.5  30]);  axis equal;  grid on; 
        set(h3,'FaceLighting','phong','FaceColor','interp', 'AmbientStrength',0.6); light('Position',[0 0 1],'Style','infinite'); colorbar;
  %%Code for creating nodes    
  function [NodesX, NodesY, FirstNdPCol] = MakeNodes(EleNumPerRow,EleNumPerCol,EleWidth,EleHight)
  [ x , y ]= meshgrid( EleWidth * [ 0 : EleNumPerRow ], EleHight * [0 : EleNumPerCol]);
  FirstNdPCol = find( y(:) == max(y(:)));
  NodesX = x(:); NodesY = y(:);
  end