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Benchmark a survey mark inscribed on a permanent monument to provide a fixed reference of elevation above the datum plane. Benchmarks are basically planimetric control points on which the topographic survey of the country is based. A considerable number of high precision levelling lines have been observed with the construction of benchmarks. The survey of bangladesh (SOB) undertakes the responsibility to carry out the basic and fundamental survey of the country through construction of monuments on survey stations and preservation of records so that the survey marks can be used for many years.

There are four different types of benchmarks used in Bangladesh: (a) Great Trigonometric Survey (GTS), (b) permanent, (c) temporary, and (d) arbitrary. SOB establishes GTS benchmarks with high degree of precision at regular intervals throughout the country. Permanent benchmarks are the fixed points of reference of known elevations between the GTS benchmarks given by the Public Works Department of the Government of Bangladesh. Temporary benchmarks are generally established at the break point of any levelling work, and are usually marked on some permanent objects like bridges, and apparently long-lasting monuments. Arbitrary benchmarks are reference points whose elevations are assumed arbitrarily for small levelling work. Surveyors use benchmarks in project planning, for instance in water resources, measuring comparative levels eg, flood height, land elevation, as standard point of reference for making comparisons. Most large-scale maps of Bangladesh like topographical maps reveal locations of spot heights and benchmarks. [Mohd Shamsul Alam]