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Spatial Data
The absolute and relative location of geographic features.
There are two types of spatial (or GIS) data:
Vector Data Model
The vector data model represents geographic space as 2-d Cartesian (x,y) coordinates.
- 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
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Raster Data Model
The raster data model represents geographic space as a matrix of cells or pixels
- 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
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-- AprilWebster - 05 Mar 2007 |