Contour detection from area geometry coordinates txt

[sincho]

I need to solve a contour detection problem. I define it as follows.

A contour detection is to recognize the outer rim of the given neighbor areas.

OK. So I have a txt file that defines the area geometry with their corner (boundary) points. Points have coordinates. With this, the need is to find the contour.

Please check the image in attachment to understand better.

I would like to use an existing algorithm if there is one already. If there is no algorithm that can solve this then I would like to hear algorithmic ideas about the steps that will lead to the solution.

I appreciate any help here..

Thanks in advance. Kind Regards.