We have a natural form pool (above concrete floor) so we could cut the tarp perfectly to fit the unique contours of our basin. FYI good technique is to spread the tarp over the basin, position it well to make sure it covers everything and cut it with a good quality cutter without seeking to cut close to the water but well stay high. It's simple, fast and perfect. Regarding the heating capacity we are delighted with the results. The pool Ganges within 4-5 degrees. I said that we are in Nîmes in the south and the sun beats down in the day. One last point, despite not possible wind the cover is not loose, the suction effect is powerful.