Cartographic representation of the earth requires the knowledge about the shape, size and area of the earth. The early thought about the shape of the earth ranged from the flat disc to the sphere. As the spherical shape was proved theoretically by Clairaut in 1743, only minor changes have been made till recently. The recent satellite measurements brought two small modifications in the shape of the earth -the Southern Hemisphere having larger circumference than the Northern Hemisphere (the pear shape) and the equatorial cross-section is slightly elliptical and triaxial. The second fundamental measurement is gravity of the earth as the combination of mass gravitation and centrifugal force. Based on the gravitational force, the density of the earth appears to be 5.52 times greater than that of water. It is a problem in cartography to reduce the earth to a convenient size and to transfer the spherical shape of the earth - how to shrink the earth down to usable map size and the second problem is how to pin the spherical earth down on a flat surface.
Characteristics of maps
Every good map must have a scale; a suitable scale has to be selected to represent specific features on a desired degree of clarity. In the compilation process, maps are often reduced or enlarged. 'G' scale (a standard of geographical measurement) proposed by Haggett, Chorley and Stoddart is used to indicate the relative size of the area studied with the total area of the earth as the base. A map must have the base direction. All directions, are expressed as angles measured clockwise from the North. Mil and Grad are the other units of measurement. A Mil is the tangent subtended by one yard across a distance of 1,000 yards (approximately of 0.001) and is used for military purposes. A circle is divided into 400 Grad but this is not commonly used. The magnetic and grid declination are given in the maps by which the true North can be determined. A cartographic representation of the whole or a part of the earth needs the systematic reference network of lines known as geographic coordinates. There are two systems of representing geographic coordinates namely hexagesimal and centesimal. The hexagesimal system is commonly adopted where a degree is subdivided into 60 minutes and a minute into 60 seconds. This denotes 60 as an even multiple of 12 and divider of 360 number represents 12 30 days and 360 degrees for circle.
Statistical Data
The geodetic and plane surveying offer only first and second type of data. Socio-geographic survey provides reports, statistical handbooks, international and national, regional and local information publications. The accuracy of data depends on the methods adopted, namely census method and sampling method. The census method is more reliable than the sampling method. The sampling method is less suitable for areas marked by irregularities. Random arid stratified sampling are commonly adopted. The secondary sources are of two types -1) published and unpublished records. 2) published and unpublished maps. A cartographer should have the knowledge of the organizations, which can supply the needed information. Encyclopedia and professional journals are the other sources. Data collected are qualitative and quantitative. The quantitative data needs to be grouped and summarized. Analytical steps like deviation, correlation, and regression may be adopted. These analyses lead to statistical mapping. Computers now process the data according to given instruction and give the results in the form of numbers, words or graphic patterns.
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