When installing an above-ground pool in your backyard, you must ensure the ground beneath it is perfectly level. If a pool is installed on ground that is not level, the pressure exerted on the side of the pool that is at the bottom of the incline could cause the pool wall to give out. This could be extremely dangerous for anyone using the pool as the force of the water would push them out of it violently.
Check to see if the ground is out of level by more than 2 inches from the high to the low side. To do this, screw a couple of 2-by-4 boards together lengthwise so they are 1 to 2 feet longer than your pool's diameter. Lay the 2-by-4 boards on the ground where you plan on putting your pool and place a level on top. Lift one end of the 2-by-4 until it the level displays that the 2-by-4 is level. Measure the distance from the bottom of the 2-by-4 to the ground.
Drill a 3/8-inch hole through the center of the 2-by-4. Drive a metal rod through the hole to secure the 2-by-4 to the ground.
Pour sand over the area with the majority of the sand in the low area.
Placing the level on top of the 2-by-4, hold it level and spin it on its axis to distribute the sand to the lower areas. Add more sand as necessary and keep rotating the 2-by-4 until the level indicates the area is perfectly level.
Wet the area down once you have leveled it. Tamp it down with a tamper to compact the sand. Work your way over the entire circle until you barely leave impressions in the ground as you walk over it.