MATLAB: Element wise mean produces an incorrect result

MATLAB and Simulink Student Suitemean

Hi,
I have a x*n*t array. The array represent some atmospheric measure at location (x,y) and time t. 'field_example' (attached) is an example of field at t=1; I want to compute the time averaged field. So easy, just do
if true
  mean_array = mean(array,3)
end
My problem is that doing so produces the 'problem_field' plot. As you can see this can't be the mean of an atmospheric field as there's no spatial correlation between values. I found out that what was giving that problem was the last time field, that is when t=T. But my field at time T is actually pretty normal. Proof of it, and the strangest thing, is that if do
if true
  tmp = mean(array(:,:,1:end-1),3);
  mean_array = (tmp+array(:,:,end))./2;
end
It produces the 'correct_field'!
Any idea of why it is doing that?

Best Answer

hello manuel, What's going on is that for whatever reason, the sum in the third dimension of all your slices equals zero within numerical error. The 71x46 different sums are all down around 1e-15 and the elements themselves are of both signs, as if someone has subtracted off the mean already. If you sum all except for one of these you get a reasonable result, but if you sum them all you get a plot of autoscaled numerical confetti.
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