how to detect rough boundary of a object in a image

I have a BW(black and white) image like this, and I am trying to detect the boundary in the center (as I have drawn in the second image). How should I program to get it?

[Frank Jiang]

The original image can be found as attached.

Answer 1

[Arun Mathamkode]

I think the morphological operations like erode and dilate will be useful in separating the central bell-shaped region from the rest white region. First, you may need to use a combination of imerode and imdilate for this. Once you have separated the central region you use the bwboundaries function to find the boundary.