1. A method of estimating the value of an attribute at a principal point, the method comprising: identifying one or more secondary points where sample attribute data was obtained that are within a predetermined distance to the principal point; sequentially selecting the one or more secondary points identified as being within the predetermined distance to the principal point; for each secondary point selected: allocating a masking zone to the secondary point; determining whether a more distant secondary point is within the area of the masking zone; and if the more distant secondary point is inside the masking zones assigning less significance to the data obtained at the more distant secondary point than other secondary points when estimating the value of the attribute at the principal point.