GEOGRAPHIC 

INFORMATION SYSTEM (GIS)          

Maps

Maps are representation of features on the surface of earth drawn to scale. Map scale is the ratio of the distance between any two points on a map to the actual distance between the points on the ground.

Map scale may be expressed in

·         by a statement.

·         by a numerical statement.

·         by a graphical section.

           Classification of Maps

Maps are mainly classified based on

·         Scale

·         Contents and purpose

Based on the scale, they are classified as

·         Cadastral Maps or Revenue maps

·         Topographical maps: Maps larger than 1:25000 scale.

·         Geographical maps: Maps on scale smaller than 1:250000. They are further divided into

·         International map of the world (IMW).

·         World aeronautical charts (ICAO).

·         India and adjacent countries, etc.

·         Atlas maps (They are of very small scale).

·         Wall maps.

Based on contents and purpose, they are classified into physical and cultural maps.

Map Numbering

The map numbering systems used in India are:

·         The International system (CIM)

·         India and adjacent Countries (IAC) System

CIM system

This system is used for international map on 1:1 million scale. Each sheet covers an area of 4⁰ latitude x 6⁰ longitude. The geographical position of the sheet is defined by two letters and a number. The first letter is N or S depending on whether the sheet is north or south of the equator. Next letter after the N or S indicates latitude of sheet alphabetically with the capital letters in succession of each 4⁰ band. Numbering starts from 180⁰ longitude and goes from west to east, the number changing after 6⁰ longitude, i.e. a map showing 0⁰ to 4⁰N and 0⁰ to 6⁰E will have a numbering of NA1.

IAC system

This is the system followed by the Survey of India maps. Each sheet is bound by 4⁰ latitude and 4⁰ longitude. The sheet numbering starts from NW corner 40⁰ latitude N and 44⁰ E as sheet no.1 and increases from North to South. Sheets falling in the sea are not numbered. Sheets covering India are numbered from 39 to 58. The sheets are called 1/M (or 1 Million) sheets.

The 1/M sheets are further sub-divided into 16 equal parts of 1⁰ x1⁰. The sheets are in 1:25000 scale. These sheets are numbered from A to P and each grid is called by the sheet number followed by the alphabet i.e. for the 1/M sheet 57, sixteen components are 57A to 57P.

                                57- 4⁰ x 4⁰ on 1:1M scale

Shaded cell shows 57G of scale 1: 250000.

The 1⁰ x 1⁰ map (degree sheet) is again subdivided into sixteen equal parts - each of fifteen minutes of latitude and longitude in extent. These sheets are numbered from 1 to 16 (e.g.: 57G/1, 57G/2 to 57 G/16) and on the scale 1: 50000 scale maps.

                57 G - 1⁰ x 1⁰ on 1: 250 000 scale

Shaded cell shows 57 G/12 of scale 1: 50000.

The 1:50000 sheet is further divided into four equal parts of 7.5' latitude x 7.5' longitude. The sheets are numbered A/1/NW, A/1/NE, A/1/SW, A/2/SE and scale is 1:25000.