by David R. Maidment

This lecture introduces the subject of geographic information systems.

**Definition of Geographic Information Systems**

A geographic information system is a computer program that stores and operates on spatial data.**Coverage of the Landscape**A GIS depicts a region of the landscape by using a number of*themes*or*data layers*, in which each theme is comprised a a set of*geographic features*of a particular kind, for example, streams, roads or soils.**Geographic Features**Geographic features are defined by*location data*(where they are), and by*attribute data*(what they are). Each geographic feature with a particular theme has a unique*identity number*that is attached to both its location data and its attribute data and serves to connect them.**Vector and Raster Themes**Geographic features can be represented in*vector*data layers or in*raster*data layers. Vector data appear geographically as discrete points, lines or areas (polygons), each having a particular location in the landscape. Raster data defined on a smooth surface of fine mesh grid cells laid over the landscape.

**Vector Data Structures**

**Definition of a Vector**A vector is a quantity defined by both magnitude and direction. It can be represented by an arrow drawn from a point of origin whose length is proportional to the magnitude of the vector.**Point as a Vector**If we have a point at location (x,y), this can be thought of as a vector quantity since a vector can be drawn to this point from the origin of the coordinate system, (0,0).**Easting and Northing**In GIS, the x-coordinate of a planar or Cartesian or projected coordinate system is called the*Easting*, which the y-coordinate is called the*Northing*. It is important to note that (x,y) are defined in the horizontal plane. The vertical or z-dimension of a point is defined separately from its (x,y) coordinates, usually as a descriptive attribute. In this sense, GIS is a 2-D rather than 3-D data representation system.**Definition of a Line**A line or*Arc*is defined locationally by a sequence of points. In the Arc/Info GIS there is no such thing as a curved line -- curves are represented by a large number of short, straight segments joining closely spaced points. The beginning point is called the*from-node*and the ending point the*to-node*. Intermediate points are called*vertices*. The concept of a from-node and a to-node is very important in GIS and makes the line so defined a*directed line*. In Arc/Info, the set of vertices making up a line is called its Arc-Coordinate list. Each line has a unique identity number.**Definition of an Area**An area or*polygon*is defined as a closed set of directed lines. Each area has a unique identity number.**Topology**Topology is the term describing the set of rules that define geographic features locationally. In Arc/Info, there are three components of topology:*Connectivity*-- how points are sequenced to form lines,*Contiguity*-- which area is on each side of a line, and*Area Definition*-- how lines are sequenced to form polygons.

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