Finding index (x value) of noisy data crossing a threshold value

Question

Jason 2019-12-4

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https://ww2.mathworks.cn/matlabcentral/answers/494776-finding-index-x-value-of-noisy-data-crossing-a-threshold-value

评论： ME 2019-12-4

采纳的回答： Jakob B. Nielsen

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Hello, I have some "noisy" data (xth_col) as shown by the red curve. I smooth this (black curve) first using:

win=200
d1=smooth(double(xth_col(:)),win);

I want to find out the width of this smoothed data by finding the x value where it crosses the horizontal red dotted line (=d80)

I have used:

xData=d1;
Value=d80
diff=abs(xData(:))-Value
[xshift, ind] = min(abs(diff(:)));

But this doesn't always work (as shown by the green data) . im going to need the first crossing coming from either end of the data. so I thought If I just list the diff, I can then find out where this swaps sign (i.e. crosses the horizontal line).

Not sure how to do this or if this is the best approach.

Thanks for any help

Jason

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Answer 1

Jakob B. Nielsen 2019-12-4

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https://ww2.mathworks.cn/matlabcentral/answers/494776-finding-index-x-value-of-noisy-data-crossing-a-threshold-value#answer_404679

编辑：Jakob B. Nielsen 2019-12-4

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Have you considered the find function? It will give you every index of the input array that meets a certain condition.

cross=find(d1 > yourthreshhold);
cross(1) %yields the first index of xth_col which is above your threshhold
cross(end) %yields the last index of xth_col which is above your threshhold

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Jason 2019-12-4

Thanks to both of you. I've accepted this answer only because it was first

ME 2019-12-4

No Worries! I'm glad you got the answer you wanted.

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Answer 2

ME 2019-12-4

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https://ww2.mathworks.cn/matlabcentral/answers/494776-finding-index-x-value-of-noisy-data-crossing-a-threshold-value#answer_404678

编辑：ME 2019-12-4

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idx = find(SmoothedData > Threshold);
Width = Xdata(max(idx)) - Xdata(min(idx));

Here the first line finds all indices of points above some threshold value. Then the second line calculates the time spent above this threshold by finding the difference between the X data points associated with the maximum and minimum indices.

Obviously, you'll need to adjust this to match your particular variable names, etc.

Is that what you wanted?