Why does cumtrapz return negative values?

Hi!

I am trying to calculate the area underneath a small piece of line using cumtrapz. The line has a positive slope and has positive x and y value. However, cumtrapz returns negative values for the area:

 area = 0   -0.0145   -0.0290   -0.0435   -0.0580

I think it has something to do with the stepsize of the X-vector. How can I accurately calculate the area underneath the line?

Thanks!

Bas

% Example of data
X=[0.5056    0.5048    0.5040    0.5032    0.5024];
Y=[18.1277   18.1241   18.1205   18.1170   18.1134];

% Calculating the area under the data
area=cumtrapz(X,Y);

% Plotting data
figure
plot(X,Y)
grid on

Answer 1

[Stijn Haenen]

Your X and Y are from high value to low values, resulting in negative areas.

use fliplr() to mirror arrays X and Y.

X=[0.5056    0.5048    0.5040    0.5032    0.5024];
Y=[18.1277   18.1241   18.1205   18.1170   18.1134];
% Calculating the area under the data
X=fliplr(X);
Y=fliplr(Y);
area=cumtrapz(X,Y);
% Plotting data
figure
plot(X,Y)
grid on