Another way to say it:
Go out to the highest point of your airport and dig a hole. Dig that hole all the way down to sea level. Lower your barometer (one that's calibrated to read correctly at sea level, of course!) on a string down that hole. Read what the barometer says when it reaches the bottom. When you set your altimeter to that barometric pressure, it should read your altitude above the bottom of that hole - field elevation...or somewhat close.
Simply stated, your altimeter reads your approximate elevation above the pressure plane your have set in the Colesman window, so the Cloesman window setting should equal about the atmospheric pressure at sea level.
Example:
Your airport has a field elevation of 3,000 feet. The actual (true) barometric pressure at that airport is approximately 3"Hg lower than that at sea level. On a day when the actual (true) barometric pressure at the airport is 27.20"Hg, the atmospheric pressure at the bottom of that hole you dug is about 30.20"Hg. After all, there's an additional colum of air 3,000 feet high pressing down there! If you set 27.20"Hg in the Colesman window, you would read an altitude above *that* pressure plane in the atmosphere...or -0- feet. On the other hand, when you set the sea level pressure (30.20"Hg in this example) in the Colesman window, it will read your approximate elevation above *that* pressure plane...the one at sea level, or 3,000 feet.
