18 February 2011

Maps in GIS

The early concept of map handling had a serious difficulty, as they could not handle the tabular data or attribute data in conjunction with the spatial information. This let the development of additional methods and techniques wherein both the spatial and attribute data could be handled and integrated so that the outputs were more meaningful and dynamic for planning and decision-making.

GIS technology is relatively recent and over the past 30 years, there has been a rapid increase in theoretical, technological and applications level. Today the advancement of mapping, modeling, spatial analysis, and time series analysis has overcome the limitations of the traditional manual process buy powerful GIS toolsi.

Recent advancement in processing technology provides an unprecedented opportunity for computer based mapping and spatial information management. Present mapping and analytical technology, known as Geographic Information System (GIS), is a very advance tool that facilitates the large scale data analysis, manipulation and visualization of natural, social, physical, economic and cultural landscape.

In many developing countries in Asia, GIS has been used as an integral part of the decision making process for last 10 years. In the field of planning and monitoring of the natural resources it is now clear that the spatial information technology provide a powerful organization and analytical tool for decision-making. Moreover the ever-increasing volume of Earth observation data intensifies the usefulness of GIS as a tool for integration different data types and enhancing our capacity to manipulate and analyze this information for planning and monitoring.

What is a Map?
According to the International Cartographic Association, a map is: a representation, normally to scale and on a flat medium, a selection of material or abstract features on, or in relation to, the surface of Earth. Map is the most powerful visualization tool for any implementation and monitoring processii. Briefly we can say: A graphic representation of features on the earth's surface or other geographically distributed phenomena is called a map.

Types of Maps
Maps are usually classified in two types:

Topographic map is a reference tool, showing the outlines of selected natural and represented by contours and/or shading, but topographic maps also shows roads, rivers, contours and other prominent features (Figure 1.1).

Thematic map is a tool to communicate geographical concepts such as the distribution of population densities, climate, movement of goods, land use etc. (Figure 1.1)

Coordinate system
A coordinate system specifies the units used to locate features in two-dimensional space and the origin point of those units. Latitude and longitude is a coordinate system (often called the "geographic" coordinate system).

A reference system used to measure horizontal and vertical distances on a plan metric map. A co-ordinate system is usually defined by a map projection, a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions to locate x,y positions of point, line, and area features. In ARC/INFO, a system with units and characteristics defined by a map projection. A common co-ordinate system is used to spatially register geographic data for the same area.

Geographic Coordinates
A measurement of a location on the earth's surface expressed in degrees of latitude and longitude.

Types of Coordinate Systems uses in Afghanistan are:
1. DD (Decimal Degree)
2. DMS (Degree Minute Seconds)

DD Coordinate System
Values of latitude and longitude expressed in decimal format rather than in degrees, minutes, and seconds.

DMS Coordinate System
Degree, Minute and Seconds (DMS): Values of latitude and longitude expressed in degrees, minutes, and seconds.

In the most general sense, any set of numeric or geometric constants from which other quantities, such as coordinate systems, can be defined. A datum defines a reference surface. There are many types of datums, but most fall into two categories: horizontal and vertical.

In surveying, a reference system for computing or correlating the results of surveys. There are two principal types of datums:

Vertical datum: is a level surface to which heights are referred. In the United States, the generally adopted vertical datum for leveling operations is the national geodetic vertical datums of 1929 (differing slightly from mean sea level).

Horizontal datum: The horizontal datum, used as a reference for position, is defined by: the latitude and longitude of an initial point, the direction of a line between this point and a specified second point, and two dimensions which define the spheroid.

Map Projection
A map projection translates the locations on the globe onto the flat surface of your map. Projection is a fundamental component of mapmaking. A projection is a mathematical means of transferring information from the Earth's three-dimensional, curved surface to a two-dimensional medium—paper or a computer screen. Different projections are used for different types of maps because each projection is particularly appropriate for certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.

A projection is a method by which the curved surface of the earth is represented on a flat surface. The Earth’s surface is curved but it must be shown on a flat sheet, some distortion is inevitable. Distortion is least for when the map only shows small areas, and greatest when a map attempts to show the entire surface of the Earth. A complex mathematical transformation is involved in representing three dimensional earth surfaces to a two-dimension paper map. Considering the size and shape of Afghanistan, projection would be critical for measuring the surface of the land, because map projection influences the underlying database of an area.

There are several types of projections. Below two example types:
1. Universal Transverse Mercator (UTM)
2. Geographic

Map Scale
The scale of the map is the ratio between distances on the map and corresponding actual distances on the earth. If a map has a scale of 1:50,000, then 2cm on the map equals 1km on the Earth’s surface.

The Most Popular way to define map scales are: “Small scale” and “Large scale” maps. The easiest ways to remember a large scale map that shows great detail, small features representative fraction is large, e.g. 1:10,000 and a small scale map shows only large features representative fraction is small, e.g. 1:250,000.

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