signal numeric integration

[heang]

hello

i was having problem getting the area under the curve.to eliminate the noise, i set threshold level by using basic level trigger. then extract portion of the signal.
from the initial threshold level, i will get the t1 and t2 corresponding to the threshold value. but it does not represent exactly the area. i still left out the hatched area. so i need to define t3. (pls refer to diagram attached)
how to define t3?how to get the exact area curve. previously i used numeric integration(used simpson rule)

thanks

here attached with the diagram and offline data

[altenbach]

Integrating a noisy signal gets rid of a lot of noise automatically. Why don't you do a numeric integration of the signal, then pick the start and end point for the area? If the baseline is zero on the average, you can easily pick locations that are clearly outside and the area will still be correct.