3.overview of surveying and indian topographic map

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Lecture-3

Overview of Surveying and Indian Topographic Map

Overview of Land Surveying

The fundamental objective of land surveying is to prepare aplanormapof an area. The mapthus prepared serves as the primary source of information about the surface of the earth for

further engineering works.The data required for making of a map gets collected through fieldsurveying. To start field surveying, it is required to know very accurately, the geographical

coordinates (latitude, longitude)of at least one point, known ascontrol pointand the length aswell as azimuth of a line, known asbaseline(Figure 3.1).
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The Latitude of the point and the azimuth of the line are determined through astronomical surveyand longitude from time measurement. The length of the line is measured with a distance

measuring instrument.

From thecontrol pointin association with the base line, a number of intervisible points are

selected such that on joining these points well shaped triangles are required to be formed. Thesetriangles carry forward points whose geographical positions are calculated from the

measurements (horizontal distance, horizontal angles)taken from the network of triangles. Thecalculated parameters undergo further adjustment by satifying the geometrical conditions

associated with the parameters as well as with figures. This helps in minimisation of errors whichmay creep in further surveying operation. Inside the big triangles formed by widely spaced

control points, network of smaller triangles get established (Figure 3.2).
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This process gets repeated materialising the basic priciple of surveying "to work from whole to

part". Thus the entire area to be surveyed gets covered with network of triangles.

The detail surveying is then carried out within the smallest triangle. During surveying,measurements (distance, direction, angles, height etc.) for important objects/ points are taken.

The measurements are then used for necessary calculations and adjustments resulting inidentification and finding positions (with respect to standard reference) of salient objects.

To prepare map, first a grid of medians and parallels of latitudes is being prepared to provide aframe work of map. Control points are then plotted by their spherical coordinates.

Then objects are graphically represented in a rectangular coordinate system for depicting


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planimetric position. Terrain height is depicted by usingcontoursat regular interval. The

identification of objects are depicted using conventional symbols and coloursFigure 2.1.

To prepare map of the area, inside the marked area inFigure 3.3,

first the surveying measurements and plotting are being carried out for plot number 1(Figure3.4).
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Next, detail surveying (Figure3.5)
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and plotting are being carried out for plot number 2 (Figure 3.6)


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and subsequently detail mapping is carried out for plot number 3 and plot number 4. Then, afinal map of the whole area is being prepared by mosaicing the component maps (Figure 3.7)


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Indian Topographic Maps

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 bythe 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 thescale 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, coveringonly land areas and leaving any 4 square if it falls completely in the sea (Figure 3.8).


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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 (Fig 3.9).


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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).


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Fig 3.10 Map Numbering System for India topographic map

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 column wise and are identified as 53 B/3 (Figure 3.11)
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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).


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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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3.overview of surveying and indian topographic map

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