I'm trying to purchase a new beaded liner for my Esther William's above ground pool. The pool measures 48" high and 19'7" across. I have measured the distance at the top, the bottom, the middle, and several distances across. Every distance measures 19'7". I cannot find anywhere to buy this size round liner. Am I measuring something wrong? Is this a common error? Do I order a custom liner - where from? Thanks in advance!