1. A condition monitoring method comprising: storing information representing a plurality of boundary points of a boundary that circumscribes an area of interest in which the boundary points are defined by at least two prescribed parameters; obtaining via an on-board controller at least one condition point defined by current values of the prescribed parameters; determining via the on-board controller a first boundary point from the plurality of boundary points that is closest to the condition point; generating via the on-board controller geometric data representing a geometric relationship between the first boundary point, the condition point and a second boundary point of the boundary points, the geometric relationship including a first straight line connecting the first boundary point and the condition point, a second straight line connecting the second boundary point and the condition point, and a third straight line connecting the first boundary point and the second boundary point; calculating via the on-board controller reference point data representing a reference point based on the geometric data; determining via the on-board controller coordinate condition data based on an angle between a predetermined direction and a reference line connecting the first boundary point and the reference point; and determining via the on-board controller whether the condition point lies within the area of interest based on a comparison between coordinates of the condition point and the coordinate condition data.