Topographic maps provides the graphical portrayal of objects present on the surface of the earth. These maps provide the preliminary information about a terrain and thus very useful for engineering works. For most part of India, topographic maps are available which are prepared by the Survey of India. To identify a map of a particular area, a map numbering system has been adopted by Survey of India. The system of identification is as follows:

An International Series (within 4° N to 40° N Latitude and 44° E to 124° E Longitude) at the scale of 1: 1,000,000 is being considered as base map. The base map is divided into sections of 4° latitude x 4° longitude and designated from 1 (at the extreme north-west) to 136, covering only land areas and leaving any 4° square if it falls completely in the sea (Figure 3.8).

For Indian Topographic maps, each section is further divided into 16 sections (4 rows by 4 columns), each of 1° latitude x 1° longitude (1:250,000), staring from a letter A (North-West corner) and ending on P, column-wise. These degree sheets are designated by a number and an alphabet such as 53 C (Figure 3.9).

These degree sheets are further sub-divided in the following ways:

Each sheet is divided into four parts (2 rows by 2 columns),, each of 30' latitude x 30' longitude (1:100,000) designating them by cardinal directions NW, NE, SW, and SE. Such sheets are identified as 53 M/SE (Figure 3.10).

Degree sheets have also been divided into 16 sheets (4 rows by 4 columns), each 15' latitude x 15' longitude (1:50,000) and numbered from 1 (at the north-west corner of the particular degree sheet) to 16 columnwise and are identified as 53 B/3 (Figure 3.11).

Each 1:50,000 scale sheet contains four (2 rows by 2 columns) 1:25,000 sheet ( 7' 1/2 latitude x 7' 1/2 longitude ) which are numbered NW, NE, SW, and SE. Such sheets are identified as 53 O/14/NE (Figure 3.12).

In this way, the topographic map of most of the area of India may be acqiured at the scale available and subsequently can be updated and upgraded as required for a particular project. For large scale maps, further surveying needs to carried out.

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