Reduced level in

surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...

refers to equating elevations of survey points with reference to a common assumed

datum In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...

. It is a vertical distance between survey point and adopted datum plane. Thus, it is considered as the base elevation which is used as reference to reckon heights or depths of other important places. Reduced here means equating and level means elevations. Datum may be a real or imaginary location with a nominated elevation of straight

zero 0 (zero) is a number representing an empty quantity. In place-value notation Positional notation (or place-value notation, or positional numeral system) usually denotes the extension to any base of the Hindu–Arabic numeral system (or ...

Datum used

The most common and convenient datum which is internationally accepted is

mean sea level There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...

. Countries take their nearby sea levels as datum planes for calculations of reduced levels. For example,

Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...

takes sea near

Karachi Karachi (; ur, ; ; ) is the most populous city in Pakistan and 12th most populous city in the world, with a population of over 20 million. It is situated at the southern tip of the country along the Arabian Sea coast. It is the former cap ...

as its datum while

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

takes sea near

Mumbai Mumbai (, ; also known as Bombay — the official name until 1995) is the capital city of the Indian state of Maharashtra and the ''de facto'' financial centre of India. According to the United Nations, as of 2018, Mumbai is the second- ...

as its datum for calculation of reduced levels. The term reduced level is denoted shortly by ‘RL’.

National survey A national mapping agency is an organisation, usually publicly owned, that produces topographic maps and geographic data, geographic information of a country. Some national mapping agencies also deal with cadastre, cadastral matters. According to ...

departments of each country determine RL’s of significantly important locations or points. These points are called ''permanent benchmarks'' and this survey process is known as Great Trigonometrical Surveying (GTS). The permanent benchmarks act as reference points for determining RL’s of other locations in a particular country.

Instruments

The instruments used to determine reduced level include: * Optical levelling instruments like automatic level, Y level, dumpy level, or Coke’s reversible level * Levelling staff *

Tripod A tripod is a portable three-legged frame or stand, used as a platform for supporting the weight and maintaining the stability of some other object. The three-legged (triangular stance) design provides good stability against gravitational loads ...

stand

RL calculation

RL of a survey point can be determined by two methods: # Height of instrument method # Rise and fall method

Significance

# For drainage of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

under

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

a suitable slope is required. Thus, roads are built in the fashion that their RL’s on sides are comparatively smaller than the RL at the mid-span of the road. This ensures proper drainage of water from roads. # For construction of buildings, roads, and

dams A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, ...

, a horizontal levelled surface is required. So, at construction sites, RLs of different points are obtained. The ground surface is then being levelled to the RL, which is obtained by taking the arithmetic mean of RLs of different points.{{cite book , title=A text book of Surveying Vol. 1 , author=Dr. B.C Punmia and Ashok Kumar Jain. , chapter=Levelling , year=2014

References

Surveying Civil engineering