Detecting plateau in a data.

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Abhi Ove
Abhi Ove 2017-2-12
编辑： Abhi Ove 2017-2-12
Hi All, more info and better clarification in reply.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Hi All,
I have this profile presented in image.
I need to detect the point where the value falls to zero. Forms a plateau. This plateau changes for various experiments so the moving average technique is not efficient.
Currently I can carry out a linear fitting whether I threshold a gradient. However the small number of data points in this set makes it difficult to determine the true value. I was wondering if anyone can guide towards the right direction.
Many thanks

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Image Analyst
Image Analyst 2017-2-12
Why is 81-85 not also a plateau?
You can use diff() or stdfilt() to determine where the variability of your data is low. This will find a plateau at any level. Combine this with ANDing of your data to get plateaus that are low, like less than 0.5 or whatever.
Attach your data for more help.
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Image Analyst
Image Analyst 2017-2-12
How about this:
clc; % Clear the command window.
% close all; % Close all figures (except those of imtool.)
% clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;
y = [9.1382875;9.6605644;9.5544062;9.1403189;8.3296022;7.1736870;5.9494658;4.8966575;4.0415068;3.1646166;2.5283923;1.9758664;1.5139828;0.98420805;0.83403659;0.52328163;0.30553690;0.14569098;0.17771824;0.065542839;0.053332146;0.041690052;-0.057317857;0.044289846;-0.0055398578;-0.066662073;-0.032653969;-0.072456174;0.077120677;0.064909987;0.042586125;0.057097465;0.13626869;0.055818811;-0.10009535;0;0;0;0;0;0;0];
subplot(2, 1, 1);
plot(y, 'b.-', 'MarkerSize', 11);
grid on;
xlabel('Index', 'FontSize', fontSize);
ylabel('Y Signal', 'FontSize', fontSize);
title('Standard Deviation', 'FontSize', fontSize);
filteredSignal = stdfilt(y, ones(5, 1));
subplot(2, 1, 2);
plot(filteredSignal, 'b.-', 'MarkerSize', 11);
grid on;
xMax = length(filteredSignal);
xlim([1, xMax]);
xlabel('Index', 'FontSize', fontSize);
ylabel('Standard Deviation of Y', 'FontSize', fontSize);
title('Standard Deviation', 'FontSize', fontSize);
% Find out where the standard deviation is less than 0.2
index = find(filteredSignal < 0.2, 1, 'first');
% Draw a green patch over it
yl = ylim
hold on;
patch([index, index, xMax, xMax, index], ...
[yl(1), yl(2), yl(2), yl(1), yl(1)], ...
'g', 'FaceAlpha', 0.3);
message = sprintf('The plateau indexes start at %d', index)
uiwait(helpdlg(message));
Abhi Ove
Abhi Ove 2017-2-12
编辑：Abhi Ove 2017-2-12
That's amazing. Thanks. I will try it out with the rest of my data sets. Many thanks. I have spent a week trying different fittings/codes and this is just so simple. Many thanks.

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