How to crop a matrix based on its image?

I have a large matrix(hundreds x hundreds) that can be shown in the image below. What I want is to cut the matrix by the line in the second picture. I don't care if I have to do it by drawing or by some algorithm(I don't think there will be an algorithm that can cut exactly as desired, especially when the map is very uneven). I want to get the two matrix that corresponds to the two parts in the picture. The matrix can be of the same size, with the remaining part padded with zeros. Can anyone help me with this?

Answer 1

[Image Analyst]

See my freehand drawing demos, attached.