Contour lines do not agree with data points

Question

Mark Larson 2013-7-1

0
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https://ww2.mathworks.cn/matlabcentral/answers/80806-contour-lines-do-not-agree-with-data-points

Hi all,

I am having some funky issues with contourf (and other contour functions).

See image for best description, but basically I am trying to contour very poorly distributed points (i.e. clustered in two places, and randomly distributed elsewhere). I am having trouble getting the contour intervals to make sense with the data. As the image shows, (the size of the squares indicates the magnitude of the Z value) the contours don't really fit the data around the clustered area, and for whatever reason it creates two bulls-eyes in the middle of "nowhere."

I am using M_Map http://www.eos.ubc.ca/~rich/map.html , a set of routines that assist Geoscientists in plotting maps of the earth. But I don't believe that this is the cause of error.

Attempts at fixing : -re gridded with many values. -tried different contour functions (some worked some did not) -tried different contour methods

-----start code

[X1,Y1]=m_ll2xy(lonmin,latmin);
[X2,Y2]=m_ll2xy(lonmax,latmax);
[xq,yq] = meshgrid(X1:.001:X2, Y1:.001:Y2);
for i=1:3:length(D2) 
  [X,Y]=m_ll2xy(lon(i),lat(i));
  xx(i)=X;
  yy(i)=Y;
end
AmplMatr = griddata(xx,yy,maxtmpZ,xq,yq,'cubic'); %construct a Matrix of the Amplitudes of the seismic waves
AmplMatr(isnan(AmplMatr)) = 0 ; %Because NaN values cause issues
contourf(xq,yq,AmplMatr);