1 : 31680 A triangulation diagram appears on the reverse of this diagram, enabling the draughtsman to plot features of the landscape with precision. An area calculation is also written on the back of the sheet. At Northop, pencil lines radiate from a dot within a red triangle. This is a trigonometrical station from which angular measurements were taken. The ancient Watts Dike is named in gothic script; the standard presentation after 1816, when the inclusion of archaeological features became obligatory. A note in the bottom left of the drawing gives the date on which the drawing was received in the Tower - one of the few plans to do so. Giles, John C.