Spatial Data

There are two types of spatial (or GIS) data:

• Vector Data
• Raster Data

Vector Data Model

• Type of data represented: DISCRETE data
  • Data consisting of clearly defined/delineated entities
  • Examples: road networks, houses, population counts, etc
• Basic data units:
  • Points - identify locations (e.g., hospitals, retail stores, etc)
  • Lines (or Arcs) - connect points (e.g., roads)
  • Polygons (or Areas) - consist of connect line segments
• File types:
• Issues:
  • Vector data MUST have a projection
    • A projection is a mathematical model that translates the locations of features on the Earth's surface (3-dimensional) to locations on a 2-dimensional surface; in essence, a projection converts the location of a spatial entity from spherical coordinates (i.e., latitude, longitude) to Cartesian coordinates.
    • It is imporant for spatial data to have a projection as to be able to use the basic trigonometric functions

Raster Data Model

• Type of data represented: continuous data (data that exists everywhere in space and varies continuously)
  • Examples: satellite images, aerial photographs, digital elevation models (DEMs), etc
• Basic data unit: Cell
  • Note: lines and areas in the raster data model are represented by a collection of cells
• Each cell contains its location (co-ordinate) and an attribute value
• Unlike the vector data model, topology is stored implicitly in a raster file
  • The spatial location of each cell is embedded in the ordering of cells in the matrix
• The cell is the MINIMUM MAPPABLE UNIT in raster data

-- AprilWebster - 05 Mar 2007