Introduction to Geographic Information Systems
Reading: Understanding GIS: the Arc/Info Method, Chapters 1 and 2
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.