Rounded coping (semi circled) is a style that has been around for approximately 10 years. However, it is a terrible design and creates many obvious problems for pool liners.

Here is the reasoning: The pool liner overlaps the pool wall. A metal piece sits directly on top of the Overlap pool liner and the rounded coping sits on top of the metal piece. Simply put, the rounded coping is not protecting the liner from the metal and the metal piece causes many rips in the pool liner. This is an awful design and creates many problems.

The best solution is to go with the flat coping and reverse the order of how it is applied. In this case, the pool liner will overlap the pool wall, then the coping will sit on the liner, lastly the metal piece will lie directly on top of the coping. This design will let the coping protect the pool liner from the metal and will create much less wear and tear on the liner.