Numbering System & Concept of Grid Sheets

Chapter II
NUMBERING SYSTEM & CONCEPTS OF GRID SHEET
2.1.
Global Positioning Systems has provided us with new measurement
techniques to define a point on the surface of the earth. Earlier, each
country has adopted its own local best fitting spheroid, which relates to
their part of globe. But now world is switching over to Geocentric Coordinate system in which the earth's center is taken as the origin. The
most recently developed and widely used datum is WGS-84. It serves as
the framework for locational measurement worldwide. In 2005, as per
Mapping Policy of Government of India, the Survey of India--the
National Mapping Agency -- has switched over from Everest to WGS84 datum.
2.2.
Due to the fast advancements and versatile application of the GPS/GIS
technologies as a governance tool, most of the countries of the world
have switched over to WGS-84 datum from their local datum. Kerala,
therefore, has consciously decided to follow the WGS-84 datum and will
have a direct linkage with the National Geodetic Framework being
established Survey of India.
2.3.
The total Stations are incapable of accepting spherical coordinates.
Hence it is necessary to convert the spherical coordinates i.e. the
latitudes and longitudes, into rectangular coordinates. Kerala has
decided to use cylindrical projection called Transverse Mercator, which
is best suited for areas having north-south extent like Kerala. (Figure 1)
2.4.
Kerala lies between 74°E and 78° E longitude and 8° N and 13° N
latitude. Hence it falls in zone 43 of UTM (Universal Transverse
Mercator) projection Refer Figure 2. The centre of the above extent is
76° E and 10° 30’ N. This centre is presumed to be a false (center)
origin of Kerala Grid system and has been assigned the value 70,00,000
Easting and 40,00,000 Northing.
2.5.
The values assigned to the centre are arbitrary in nature and the value of
70 lakhs as Easting will differentiate from the Northing value which is
40 lakhs . The scale factor has been selected as 0.999600. See figure 3.
All the control points, vectorised FMBs are brought to this rectangular
coordinate system. The coordinates of the origin point are fixed, and all
other points are calculated in relation to it.
Table 3. Kerala – Geographical parameters
Minimum latitude
:
8°
Maximum Latitude
:
13°
Minimum Longitude
:
74°
8
Maximum Longitude
:
78°
Figure 1: Mercator projection system
2.6.
The world has been divided into various UTM zones. A zone is
basically an area encompassed by 6° longitude and 8° latitude. Refer
Figure 2. Kerala falls into UTM 43P zone. But in our country, Survey
of India has retained the same longitude zone but subdivided the 8°
latitude zone into 2 equal division of 4°. As per the Survey of India
nomenclature, Kerala falls mostly into C- 43 zone mostly. Thus zone
C-43 has an area of 6° longitude and 4° latitude. Probably some portion
of Kannur & Kasargode district may lie in D-43
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Figure 2: UTM Zones of the World
2.7.
The Kerala Land Information Mission has selected an arbitrary origin
for the purpose of conversion into rectangular coordinates. The latitude
of origin is 10.5° and Longitude of Origin is 76°. Further an arbitrary
value of 70 lakhs as Easting and 40 lakhs as Northing
has been
assigned to the point (10.5 N, 74 E).Refer Figure 3
Figure 3: Kerala’s arbitrary origin and coordinates
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2.8.
Kerala has adopted the same concept of numbering systems as adopted
by Survey of India (SOI), but only up to 1:2000 scale. The numbering
system is unique in its concepts where each part of Kerala can be
uniquely defined. This 1:2000 scale grids are further subdivided as
shown in figure to arrive at the primary grid having an approximate area
of 180 meter X 180 meter (approximately 3.24 Ha area). Refer figure (4)
below:
12°
8°
12°
11°
11
12°
11°45’
11°54
11°51’
12
11°53’24”
11°52’48
Figure 4- Disintegartion of SOI Grids to Field Drawing Sheets)
2.9.
It can be seen that with a disintegration of the C-43 zone into different
smaller grids, the number changes from C-43 to a 11 digit figure which
is again unique. (refer figure 4). The SOI grid with 4° X 6° changes into
a Kerala Grid of 6” x 6”. If one second is taken as approximately, 30
metres, this will encompass an area of 180 m x 180 m or approximately
3.24 Hectares, an area to be surveyed and records prepared by a survey
team.
2.10. The primary grid sheet with the unique grid sheet number with the
following details is printed in thicker drawing sheet of good quality duly
laminated on the backside is issued to a survey team for the field work.
This is called the Field Drawing Sheet-Toposheet 1.(Refer Figure 5).
The sheet will contain the following
2.10.1.
2.10.2.
2.10.3.
2.10.4.
2.10.5.
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All the available control point and their values
Details of adjoining grid sheets.
Old FMB7 approximately geo-referenced and placed.
Grid values (Easting and Northing) at the periphery.
And the other details like village, taluk, district and symbology etc
If old FMBs are available in proper form and without any errors.
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Figure 5 : Model of Field drawing sheet of 6” X 6” (Topo sheet-1)
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