I build a potato cannon with my friend and we were having fun shooting it safely in a clearing. I decided to shoot is straight up and measure the time it is in air. Out of three measurements I got 9, 9, and 7 seconds which is a long time. Taking and average and Using 2 dim eqs I got 2v=at, v=40.875 for initial and final velocity, Then using vf^2=Vo^2 +2ax I got a height of 170m or about 558 feet. How much of a error could I expect to receive. It was at about 90 degrees and I figure if the wind is blowing horizontal then there would be no error considering that it wouldn't affect gravity. Thanks.