Level values represent logical levels above or below ground level and are not intended to correspond to any numbering scheme in use by the building itself. The ground floor of a building is always represented by the value 0. Floors above the ground floor are represented by positive integers, so a value of 1 represents the floor above ground level, a value of 2 represents two floors above ground level, and so on. Floors below the ground floor are represented by corresponding negative integers, with a value of -1 representing the floor immediately below ground level and so on. It is erroneous to use the user’s level in a building as an estimate of altitude.