18 February 2011

Principal Functions of GIS

Data Capture: Data used in GIS often come from many types, and are stored in different ways. A GIS provides tools and a method for the integration of different data into a format to be compared and analysed. Data sources are mainly obtained from manual digitization and scanning of aerial photographs, paper maps, and existing digital data sets. Remote-sensing satellite imagery and GPS are promising data input sources for GIS.

Database Management and Update
: After data are collected and integrated, the GIS must provide facilities, which can store and maintain data. Effective data management has many definitions but should include all of the following aspects: data security, data integrity, data storage and retrieval, and data maintenance abilities.

Geographic Analysis:
Data integration and conversion are only a part of the input phase of GIS. What is required next is the ability to interpret and to analyze the collected information quantitatively and qualitatively. For example, satellite image can assist an agricultural scientist to project crop yield per hectare for a particular region. For the same region, the scientist also has the rainfall data for the past six months collected through weather station observations. The scientists also have a map of the soils for the region which shows fertility and suitability for agriculture. These point data can be interpolated and what you get is a thematic map showing isohyets or contour lines of rainfall.

Presenting Results:
One of the most exciting aspects of GIS technology is the variety of different ways in which the information can be presented once it has been processed by GIS. Traditional methods of tabulating and graphing data can be supplemented by maps and three dimensional images. Visual communication is one of the most fascinating aspects of GIS technology and is available in a diverse range of output options.

Data Capture an Introduction:
The functionality of GIS relies on the quality of data available, which, in most developing countries, is either redundant or inaccurate. Although GIS are being used widely, effective and efficient means of data collection have yet to be systematically established. The true value of GIS can only be realized if the proper tools to collect spatial data and integrate them with attribute data are available.

Manual Digitization:
Manual Digitizing still is the most common method for entering maps into GIS. The map to be digitized is affixed to a digitizing table, and a pointing device (called the digitizing cursor or mouse) is used to trace the features of the map. These features can be boundary lines between mapping units, other linear features (rivers, roads, etc.) or point features (sampling points, rainfall stations, etc.) The digitizing table electronically encodes the position of the cursor with the precision of a fraction of a millimeter. The most common digitizing table uses a fine grid of wires, embedded in the table. The vertical wires will record the Y-coordinates, and the horizontal ones, the X-coordinates.

The range of digitized coordinates depends upon the density of the wires (called digitizing resolution) and the settings of the digitizing software. A digitizing table is normally a rectangular area in the middle, separated from the outer boundary of the table by a small rim. Outside of this so-called active area of the digitizing table, no coordinates are recorded. The lower left corner of the active area will have the coordinates x = 0 and y = 0. Therefore, make sure that the (part of the) map that you want to digitize is always fixed within the active area.

Scanning System:
The second method of obtaining vector data is with the use of scanners. Scanning (or scan digitizing) provides a quicker means of data entry than manual digitizing. In scanning, a digital image of the map is produced by moving an electronic detector across the map surface. The output of a scanner is a digital raster image, consisting of a large number of individual cells ordered in rows and columns. For the Conversion to vector format, two types of raster image can be used.
  • In the case of Chloropleth maps or thematic maps, such as geological maps, the individual mapping units can be separated by the scanner according to their different colours or grey tones. The resulting images will be in colours or grey tone images.

  • In the case of scanned line maps, such as topographic maps, the result is a black-and-white image. Black lines are converted to a value of 1, and the white areas in between lines will obtain a value of 0 in the scanned image. These images, with only two possibilities (1 or 0) are also called binary images.
The raster image is processed by a computer to improve the image quality and is then edited and checked by an operator. It is then converted into vector format by special computer programmes, which are different for colour/grey tone images and binary images.

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