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GIS

What is geographic data?

Geographic data is concerned with a representation of the world, in spatial terms that is independent of any particular visualization of that data. When we talk about geographic data we are trying to capture information about the properties and geometry of the objects which populate the world about us.

Maps for Representation

The main method of identifying and representing the location of geographic features on the landscape is a map. A map is a graphic representation of where features are, explicitly and relative to one another. A map is composed of different geographic features represented as either points, lines, and/or areas. Each feature is defined both by its location in space (with reference to a coordinate system), and by its characteristics (typically referred to as attributes). Quite simply, a map is a model of the real world.

Maps are the traditional method of storing and displaying geographic information. A map portrays 3 kinds of information about geographic features:

  • Location and extent of the feature
  • Attributes (characteristics) of the feature
  • Relationship of the feature to other features

Geography has often been described as the study of why what is where? This description is quite appropriate when considering the three kinds of information that are portrayed by the traditional map;

  • the location and extent of a feature is identified explicitly by reference to a coordinate system representing the earth's surface. This is where a feature is.
  • the attributes of a feature describe or characterize the feature. This is what the feature is.
  • The relationship of a feature to other features is implied from the location and attributes of all features. Relationships can be defined explicitly, e.g. roads connecting towns, regions adjacent to one another, or implicitly, e.g. close to, far from, similar to, etc. Implicit relationships are interpreted according to the knowledge that we have about the natural world. Relationships are described as how or why a feature is.

GIS

There are many definitions of GIS and so finding one which summarises a GIS is problematic:

  • What is a system? A system is a group of connected entities and activities which interact for a common purpose. e.g. A car is a system in which all the components operate together to provide transportation.
  • What is an information system? An information system is a set of processes, executed on raw data, to produce information which will be useful in decision-making. An information system must have a full range of functions to achieve its purpose, including observation, measurement, description, explanation, forecasting and decision-making.
  • What are geographical data? Geographical data include those which are spatially referenced. Geographical data contains four integrated components, namely, location, attribute, spatial relationship and time.

A GIS uses geographically referenced data as well as non-spatial data and includes operations which support spatial analysis.

Some standard GIS capabilities include:

  • integrating maps based on different scales, map projections, or legends
  • changing of scale, projections, legend, annotations, etc.
  • overlaying different types of maps of a particular area to make a new map that combines the attributes of the individual maps. For example, a vegetation map could be overlaid on a soil map. This in turn could be overlaid on a map showing length of growing season, thereby producing a land suitability map for a given crop
  • generating buffer or proximity zones around lines or polygons on a map. This technique is used to find areas within a given distance from roads, rivers, etc., or from certain thematic conditions. These buffer zones can in turn be used as another layer in overlay operations
  • querying spatial and attibute databases.