# Finding the full width half maximum (FWHM) of a rounded part of peak instead of sharp peaks

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Manny Kins on 18 Nov 2021
Commented: Adam Danz on 18 Nov 2021 Hi, I have this plot where I want to find the FWHM around the main "bulge" of the plot (around the red line I have crudely drawn), ignoring the other sharper peaks. I have attached a file of the XY coordinates and would appreciate any help! Many thanks

Adam Danz on 18 Nov 2021
Edited: Adam Danz on 18 Nov 2021
This uses rmoutliers to remove the spike and then computes the FWHM of the resultant curve. You can play around with rmoutliers to get the expected curve or use a different method if you find something more suitable.
Other approaces:
• fit the curve to a gaussian and use the fit parameters to compute FWHM.
• Use findpeaks along with the half-height width reference. See demo.
% Plot raw data
h(1) = plot(data.X, data.Z, 'DisplayName', 'RawData');
hold on
% Remove outliers
[Zm,idx] = rmoutliers(data.Z, 'movmedian', 300);
h(2) = plot(data.X(~idx), Zm, 'DisplayName', 'rmOutlier');
% Compute FWHM
halfMax = max(Zm)/2; % simple since the baseline is near 0
lrIdx = [find(Zm >= halfMax, 1, 'first'), find(Zm >= halfMax, 1, 'last')]; % L/R index of width
fwhm = diff(data.X(lrIdx));
% Show bounds
h(3) = yline(halfMax, 'k--', 'DisplayName', 'HalfHeight');
h(4) = xline(data.X(lrIdx(1)), 'DisplayName', 'WidthBounds');
xline(data.X(lrIdx(2)));
legend(h)
title(sprintf('FWHM = %g', fwhm)) ##### 2 CommentsShowHide 1 older comment
Adam Danz on 18 Nov 2021
It was equally fun for me 😎

R2021a

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