# WHAT IS THE NATIONAL GRID REFERENCE IN THE MAPPING

WHAT IS THE NATIONAL GRID

The national grid is a common reference point throughout the country with reference to the single prime meridian coincident with the point of origin at Greenwich.

The grid reference system of the map is the defined location in the map in the geographical coordinate system. The surface of the earth has nature in curved it is impossible to show on flat paper with the parallel meridian. It is shown on the map to a plane grid system coordinate system is referred to as a map projection.

The national grid reference system is different from the geographical coordinate system. It is referred to in the cartesian coordinate system. The grid reference system has required a datum for measuring the distance in a coordinate system.
The datum is defined as the reference datum is used to describe the location of unknown points on the earth.

The no of various datum is used for measurement. In India there are generally two datum is used for mapping WGS1984 and Everest 1956 or India Nepal datum. Now a day survey of India used Datum WGS 1984 and projection UTM.

`Projection is used to defined as the curved surface of the earth or part to represent a flat surface

There are three types of projection used in the mapping.

• Conic projection
• Cylindrical projection
• Planar projection
India used generally two types of projection Lambert conformal conic projection and Universal Transverse Mercator(UTM).

1. WGS1984 DATUM

The National Grid is started 500 km by 500 km squares and has 25 squares. All squares are denoted by an alphabet from A-Z.
 500 km grid reference

Each of these 500 km squares is then subdivided into a no. of squares 25, 100 km by 100 km squares, again each with a letter of the alphabet A-Z.  It has a scale of 1:250000
 100 km grid

It is represented by two alphabetical letters
The 100 km square is now divided by 100 in 10km by 10 km squares. These squares is represented by 0 to 99 numbers.
 10 km squares grid

Now 10 square is divided into 1km by 1km squares & it has a scale 1:50000.

 1 km by 1 km square grid map

How To Read National Grid reference numbers

In the National Grid Reference System, there are two types of numbers in use easting and northing. The number going toward the east is called the eastward number and the number going toward the north is called the northward number.

The digits used in locating a point as national grid reference numbers.
• Ten digits..........23371 06519.....Locating a point within a 1m square
• Eight digits.......2337 0651..........Locating a point within a10 m square
• Six digits...........233  065........... Locating a point within a 100m square
• Four digits.........23 06................Locating a point within a 1 km square
• Two digits.........2 0 ....................Locating a point within a 1 km square
Generally used four figure and six-figure grid reference.
 Four figure reference grid
In the above picture grid reference of

1=18 45
2=19 45
3=18 44
4=19 45

 Six-figure grid reference
Grey box grid reference=187 448
Red dot grid reference=185  443

 WHAT- IS- THE- NATIONAL- GRID -REFERENCE -IN- THE -MAPPING

In the above picture, one location is marked Dhandhpur. Dhandhpur is located between 258 km and 259 km in the eastward direction and is located between 2726 km and 2727.
Then the grid reference on UTM will be 2584 27264.

WGS 1984 coordinate of Dhandhpur = Langitude=72°36'48.207"
Latitude=24°38'1.318"

UTM Coordinate=Easting=258412.962m
Northing=2726486.8481m
2.Everest 1956 or India Nepal datum.

The Indian National Grid system uses the Everest Spheroid as a reference surface. It was originally defined in 1830 by Colonel George Everest (Surveyor-General Of India from 1830 to 1843) and updated in 1956. Kalianpur (Madhya Pradesh ) was chosen as the initial point of origin.
It is a local datum. The Centre of this reference surface is estimated to about 1 kilometer off from the center of mass of the Earth.

India Nepal datum the area of covered in nine zones.

 Grid reference zone

1. Zone0:-India and Pakistan North of 35°35' North
2. Zone IA:- India and Pakistan 28°N-35°N
3.zone IB:-Unknown parts of china.
4. ZoneIIA:-India21°-28°N & west of 82° and Pakistan south of 28°N
5.Zone IIIB:-Complete Bangladesh, India North 21°N & East of 82° and Myanmar North of 21°N.
6. Zone IIIA:- India 15°N-21°N.
7.ZoneIIIB:-Myanmar 15°N-21°N
8.Zone IVA:-India south of 15°N
9.Zone IVB:-Myanmar south of 15°
Note: The Zones area for the Zones is from freely available sources and not the official specifications.

In the above picture

WGS 1984 coordinate of Dhandhpur = Langitude=72°36'48.207"
= Latitude=24°38'1.318"
Indian Everest sphorid coordinate of
of Dhandhpur                                      =Langitude=72°36'50.4"
= Latitude=24°37'59.9"

Dhandhpur falls in ZONE IIA,
Indian Grid co-ordinate                      = Easting=2603019 m
= Northing=0763926 m