Anyone attempting to cross the control strips would leave footprints which were quickly detected by patrols. This enabled the guards to identify otherwise undetected escape attempts, recording how many individuals had crossed, where escape attempts were being made and at which times of day escapees were active. From this information, the guards were able to determine where and when patrols needed to be increased, where improved surveillance from watchtowers and bunkers was required, and which areas needed additional fortifications. Anti-vehicle barriers were installed on the other side of the primary control strip. In some locations, chevaux-de-frise barricades, known in German as Panzersperre or Stahligel ("steel hedgehogs"), were used to prevent vehicles being used to cross the border. Elsewhere, V-shaped anti-vehicle ditches known as Kraftfahrzeug-Sperrgraben (KFZ-Sperrgraben) were installed along of the border and were absent only where natural obstacles such as streams, rivers, gullies or thick forests made such barriers unnecessary. Outer fence, walls and minefields The outer fences were constructed in a number of phases, starting with the initial fortification of the border from May 1952. The first-generation fence was a crudely constructed single barbed-wire fence (Stacheldrahtzaun) which stood between high and was built very close to the actual border line. This was replaced in the late 1950s with parallel rows of more strongly constructed barbed-wire fences, sometimes with concertina wire placed between the fences as an additional obstacle. A "third-generation" fence, much more solidly constructed, was installed in an ongoing programme of improvements from the late 1960s to the 1980s. The fence line was moved back to create an outer strip between the fence and the actual border. The barbed-wire fences were replaced with a barrier that was usually 3.2–4.0 metres (10–13 ft) high. It was constructed with expanded metal mesh (Metallgitterzaun) panels. The openings in the mesh were generally too small to provide finger-holds and were very sharp. The panels could not easily be pulled down, as they overlapped, and they could not be cut through with a bolt- or wire-cutter. Nor could they be tunnelled under easily, as the bottom segment of the fences was partially buried in the ground. In a number of places, more lightly constructed fences (Lichtsperren) consisting of mesh and barbed wire lined the border. The fences were not continuous but could be crossed at a number of places. Gates were installed to enable guards to patrol up to the line and to