Glossary of Defined Terms

Defilade and Defilading

Defilade was an arrangement of the trace and protective embankments of a field work to insure that an enemy could not see into and deliver an accurate fire into the interior of a field work. Defilading was the practical procedure for determining the proper height for parapets and traverses necessary to intercept enemy fire delivered from ground commanding the interior of a fortification.

There were two basic methods for defilading a field work: defilading by the trace and defilading by relief. A fortification defiladed by its trace was one in which the lines of the interior crest were directed toward ground that could not be occupied by an attacking force or could not be used by attacking force to gain a clear and unencumbered view of the interior of the work. A work defiladed by relief was one in which the parapets were raised to a height necessary to intercept enemy fire, traverses erected to shield interior spaces that could be seen from the exterior of the work, or the terre-parade lowered to hide it from the enemy's view.

Basic Elements of the Theory of Defilading by Relief

AB Plane of Site
C Point of Surface on hill commanding the fortified site (DE)
D Location of interior crest of parapet facing the hill
E Farthest point within field work to be defiladed by parapet at D
H Height of parapet necessary to defilade interior of the work from plane of site (7 feet)
G Altitude above plane of site enemy fire must pass for defilade of point E (7 feet)
F Altitude from which enemy fire could be delivered from top of hill (point of firing 3-5 feet above C)
FN Vertical height from plane of site to point of firing
FG Line of sight from firing point (F) to point of farthest defilade (G) or the Plane of Defilade
K Altitude to which parapet must be raised to defilade point G
GHM Horizontal plane of point of defilade and interior crest produced to FN used to calculate
actual height of K above H by similar triangles (FGM and KGH)

January, 2003