Barometric pressure affects the atmosphere's density, and thus, the drag on the bullet. The program uses pressure values "reported" by radio and T.V. stations, which are always corrected to sea level. Thus, reported values only measure the effect weather systems have on the atmosphere's pressure. The program assumes the value you enter is the reported barometric pressure. Reported pressure is independent of altitude, so you can change the Altitude parameter's value without having to change pressure. Some users have access to absolute pressure readings from accurate on-site instruments. These values can be entered by clicking on the Options button and then clicking on the Calc Reported Press button.