Every safety covers must be measured to exact specifications in order to ensure proper fit. Rectangular pools are usually measured by overall width and length, while freeform pools and pools with raised wall areas, water fall features and special treatments need to be AB measured, which is a form of triangulation.
Professional Swimming pool technicians are trained to take exact measurements of your swimming pool to ensure that the cover you will have installed meets all of the ASTM safety standards. Never put a rectangle safety cover over a formfit type swimming pool. This scenario would create too much material lying directly on the deck around the swimming pool which can cause excess and extreme wear over a matter of months. A rectangle cover over a freeform pool will also not get the proper tensioning required to maintain safety standards. All warranties are void if you use a rectangle cover over a freeform pool.
Once the pool is measured properly, the second most important requirement for safety covers is the actual installation of the cover. Professional Swimming pool technicians are trained as to how far back and between the anchors must be placed in order to ensure proper tensioning.