vantage points for triangulation lines to distant points. U.S. elevation benchmarks were often placed along rail lines or roads that provided good sight lines for leveling. All active and some abandoned railroad rights of way are private property, actively patrolled by railroad police. A common location is the top level of the abutment of a highway bridge or its wing walls. As part of triangulation networks, intersection stations were sighted. They are typically tall, prominent, and well-defined points like a smokestack, the peak of a water tower, or a church spire. However, many of these objects have been altered or replaced by similar nearby structures and no longer mark the original location, so careful identification is required. U.S. benchmarks were commonly placed on public buildings, such as courthouses, post offices, city halls, and older schools. Benchmarks are often located on private property, or access to them can only be achieved by crossing privately owned land, so permission is needed for access. In the UK UK benchmarks tend to be on the corners of pubs, churches, and other public buildings, as well as farm buildings, railway bridges, and private houses especially those near a road junction. However, any building may be used, as well as natural features such as a rock outcrop. In the United Kingdom, trig pointing is a recreational activity similar to benchmarking. Searching for trig points is more popular than hunting benchmarks in the UK, but there is considerable overlap in participation. "Trig points" is the common name for "triangulation pillars". These are concrete pillars, about 4 feet tall, which were used by the Ordnance Survey in order to determine the exact shape of the country in a project known as the retriangulation of Great Britain, which was carried out from 1936 to 1962. They are generally located on the highest bit of ground in the area so that there is a direct line of sight from one to the next. By setting a theodolite on the top of the pillar, accurate bearings to nearby trig points could be taken. This process is called triangulation. There are 6550 such pillars listed in the T:UK database. While most of them have fallen into disuse, about 184 of them are currently used in the Passive Station network. These are maintained so they can be used as accurately located anchor points for differential GPS comparisons, and are re-surveyed every five years