Introduction to Indian Grid System
General
The Indian Grid System is a system of geographic grid references different from the Geographic Coordinate System (Longitude/Latitude). This system is used specifically by Survey of India Maps and consequently by a large number of Indian Government organisations.
The Indian Grid System was designed during British days (pre-Independent India) and is consequently similar to the British National Grid System. It covers the Indian subcontinent: India, Afghanistan, Bangladesh, Bhutan, Cambodia, Laos, Myanmar (Burma), Nepal, Pakistan and Vietnam. It also covers certain parts of China (Tibet), Iran and Thailand.
Datum
Datum is very important to correctly denote a location, as the Earth is not a perfect sphere. The radius of the Earth is about 6,378 Kilometres from the centre to a point on equator vs about 6,357 Kilometres from the centre to the poles. Thus it is flattened from the poles i.e. ellipsoid/spheroid in shape. Mapping the Earth becomes a little more complicated as the ellipsoid/spheroid Earth does not fit completely to a grid system, as well as a perfect spherical shape would. Datums are known geographic shapes of the Earth, which we can apply to maps so that coordinate systems work perfectly. Datums are broadly of two types: Local Referencing Datums and Global Referencing Datums. Local Referencing Datums are ones developed for a set local area (generally at national/regional level). A Global Referencing Datum best approximates the size and shape of the Earth as a whole (at an international level) but is not good at predicting accurate coordinates on a national/regional level.
The Indian Grid System uses the Indian Geodetic Datum (sometime written as Indian Datum), which uses the Everest Spheroid as reference surface (i.e. it defines the shape of the Earth). It was originally defined in 1830 by Colonel George Everest (Surveyor General of India from 1830 to 1843), and subsequently updated in 1956. Kalianpur (Madhya Pradesh) in central India was chosen as initial point of origin. Due to the Indian Datum's Geodetic nature and its antiquity, it is not very accurate. It is a local datum. Centre of this reference surface is estimated to be about 1 Kilometre off from the centre of mass of the Earth. Compare this to just ±02 centimetres for the Geocentric WGS84 Datum (which is a globally adopted Datum).
Projection System
The Earth's spherical (or rather ellipsoid/spheroid!) surface needs to be depicted as a flat surface for maps. There are a number of ways to do this, known as map projection systems. All of them have their own advantages and disadvantages. The Indian Grid System uses the Lambert Conformal Conic Projection system (also known as Conic Orthomorphic Projection) with 2 Standard Parallels. This was developed by Johann Heinrich Lambert around 1772. In this an imaginary conical surface is placed on the Earth, in such a way that it intersects the Earth's surface at two Standard Parallels (a pair of latitudes), i.e. the slopes of the cone form secants to the Earth's surface. The spherical surface of the Earth is then projected conformally onto the conical surface i.e. the angles are preserved so longitudes are at right angle to the latitudes. The conical surface thus formed is then unrolled to give a triangular sheet which is then flattened out (stretching the area towards the apex), in the form of a flat rectangular sheet. With the Lambert Conformal Conic Projection, distortions east and west are minimized but increase with distance north and south of the Standard Parallels. The Standard Parallels are given a set scale and other parallels are scaled up or down accordingly. Shapes are portrayed more accurately than area in this manner. The major advantage is that angles between locations on the surface of the Earth are correctly shown. A graphical representation of the same is given below :-
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| Graphical depiction of Lambert Conformal Conic Projection with 2 Standard Parallels being applied to create a map of Indian Sub Continent |
Zones
The complete Indian Grid consists of a total of nine zones. Each of the zones has its own unique set of parameters. The zones are :
Zone 0 - India and Pakistan North of 35°35'N.
Zone IA - India and Pakistan 28°N-35°35'N.
Zone IB - Unknown parts of China (Tibet).
Zone IIA - India 21°N-28°N & West of 82°E and Pakistan South of 28°N.
Zone IIB - Complete Bangladesh, India North of 21°N & East of 82°E and Myanmar (Burma) North of 21°N.
Zone IIIA - India 15°N-21°N.
Zone IIIB - Myanmar (Burma) 15°N-21°N.
Zone IVA - India South of 15°N.
Zone IVB - Myanmar (Burma) South of 15°N.
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| Approximate layout of Zones in Indian Grid System Dashed lines are unknown bounds |
Note: The area of use given for the Zones are from freely available sources and not the official specifications.
Expressing a Location
Any location in the Indian Grid System is donated numerically by a Grid Reference (written in short as GR), given as a pair of Eastings and Northings (x, y; with Eastings increasing from West to East and similarly Northings increasing from South to North). Originally these were in yards, but was changed to meters subsequently. Eastings and Northings each is a seven digit number giving a accuracy of 01 meter. Thus the complete GR with the Eastings first and the Northings next. For e.g. 12123456767890 i.e. Eastings: 1212345; Northings: 6767890. Also, do keep in mind, only knowing the Eastings and Northings is not adequate. One also needs to know the Zone (there are a total of nine of them as brought out earlier) to which the GR belongs, as without knowing the correct Zone, there are total of nine totally different possible locations! So it is equally important to be aware of the Zone.
At times, the GR may be expressed with lesser than the full seven digits of the Eastings and Northings for brevity. This can be done in two ways...
The first common practise is to do away with the first two digits and use the last five digits each. This is as the first two digits correspond to 105 meters i.e. 100,000 meters = 100 Kilometres. So this can be resorted to if we are concerned with a specific area smaller than 100 Kilometres. But for large distances this will lead to confusion. The same is explained in the figure below. All the four marked positions (13400006820000, 13400006720000, 12400006720000 and 12400006820000) have the same GR (4000020000) with the first two digits omitted!
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| Confusion while omitting first two digits of Eastings and Northings |
So, it is preferable to use the complete GR. Omitting the first two digits (or even a single for that matter) of the Eastings and Northings will not pinpoint the exact location, as there will be a number of possible combinations within the Zone.
The second common practise is to do away with equal number of last digits of the Eastings and Northings. This leads to lesser accuracy in the location. For e.g. if the GR is 67123451299987. It can be expressed as 6712312999 (67123451299987) i.e. with a accuracy of only 100 meters. So the GR 6712312999 would be taken as 67123001299900. Note this is off the actual GR by almost 98 meters. Thus, the accuracy will decrease in case this practise is adopted. However, this is less of concern as the resolution of maps and methods of determining position have limited accuracy. Even with standard commercial GPS receivers errors of up to 10m are common.
These omissions can be made in combination. For e.g. the GR 67123451299987 can be expressed as 123999 (67123451299987). These again suffer from the limitation as discussed for omission of the first two digits of Eastings and Northings and also reduced accuracy.
Parameters
The exact parameters of the Indian Grid System Zones are not freely available due to obvious reasons of national security. There are third party parameters which are freely available online/in various softwares. However, these values seem to have been approximated and using them for conversion will yield error(s) :-
Note: Only values for Lambert Conformal Conic Projection with 1 Standard Parallel are available and not for the actual Lambert Conformal Conic Projection with 2 Standard Parallels
1. For Indian Grid System, 'Indian (India, Nepal)' Datum is used with following parameters :-
Everest 1956 Ellipsoid :-
Semi-Major axis - 6377301.243m.
Semi-Minor axis - 6356100.228m.
Parameters for transformation to WGS84 :-
Delta X - 295m.
Delta Y - 736m.
Delta Z - 257m.
2. For Geographic Coordinate System, 'WGS84' Datum is used with the following parameters :-
Semi-Major axis - 6378137m.
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