week 5 notes

• Geographic Information System (GIS)


  A system designed to capture, store, manipulate, analyze, manage, and present all types of geographical data
• Components of GIS


  • Data
  • Structures
  • Representation
• Geographical data are complicated by the fact that they must include information about position, possible topological connections, and attributes of the objects recorded
• Geographical data


  Data referenced to locations on the earth's surface by using a standard system of coordinates

  
• Fundamental building blocks for analyzing and synthesizing complex geographical information

  • Points
  • Lines
  • Areas
• Map


  A set of points, lines, and areas that are defined both by their location in space with reference to a coordinate system and by their non-spatial attributes

  
• Region


  A set of pixels, areas or polygons that are described by a single legend unit
• Stages of representing geographical data in a computer

  1. User's perceived phenomenon structure

  2. GIS representation of phenomenon structure

  3. Database structure

  4. Hardware structure
• Database structure


  The organization of data to allow quick access and cross-referencing
• Database structures

  • Simple lists
  • Ordered sequential files
  • Indexed files
• Hierarchical data structure


  Data have a parent/child or one-to-many relation, allowing quick and convenient data access
• Relational database structure


  Data stored in simple records (tuples) in two-dimensional tables (relations), allowing flexible querying
• Explicit representation


  Spatial data represented by a set of points on a grid or raster
• Implicit representation


  Spatial data represented by a set of lines defined by starting and end points (vectors)
• Raster representation


  Spatial data represented by an array of grid cells, each with a value
• Vector representation


  Spatial data represented by points, lines, and areas defined by interconnected coordinates
• Methods of storing raster data

  1. Chain codes

  2. Run-length codes

  3. Quadtrees
• Raster GIS


  Spatial data stored and processed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels
• Pixel/cell


  The smallest unit of information in a raster dataset, with a discrete value
• Data capture methods for raster datasets

  • Remote sensing
  • Manual digitisation
  • Automatic digitisation
  • Scanning
• Vector to raster conversion


  Converting vector data (points, lines, polygons) into raster data (cells with discrete values)
• Raster to vector conversion


  Converting raster data (cells) into vector data (points, lines, polygons)
• Raster GIS is a cell-based data model, as opposed to a vector data model
• Geographical variation in the real world is infinitely complex, so data must be generalized or abstracted into discrete elements or objects
• Data model


  The rules used to convert real geographical variation into discrete objects
• Major choices of data model

  • Raster
  • Vector
• Data model


  A set of guidelines for the representation of the logical organization of the data in a database, consisting of named logical units of data and the relationships between them
• Current GIS differ according the way in which they organize reality through the data model
• Each model tends to fit certain types of data and applications better than others
• Factors influencing the data model chosen for a particular project or application


  • The software available
  • The training of the key individuals
  • Historical precedent
• Major choices of data model

  • Raster
  • Vector
• Raster model


  Divides the entire study area into a regular grid of cells in specific sequence, usually row by row from the top left corner. Each cell contains a single value, assumed to be distributed evenly throughout the cell
• Raster GIS


  • Space-filling, as every location in the study area corresponds to a cell in the raster
  • Layers - one set of cells and associated values is a layer, there may be many layers in a database (e.g. soil type, elevation, land use, land cover)
• Vector model


  Uses discrete line segments or points to identify locations, so discrete objects (boundaries, streams, cities) are formed by connecting line segments. Vector objects do not necessarily fill space, as not all locations in space need to be referenced in the model
• A raster model tells what occurs everywhere - at each place in the region covered by the GIS
• A vector model tells where everything occurs - it gives a location to every object
• Conceptually, the raster models are the simplest of the available data models
• Creating a Raster


  1. Lay a grid over a topological map

  2. Code each cell with a value that represents the elevation which best represents the elevation in that cell's area

  3. Assign values for other visible features on other layers

  4. Values are written into a file, often coded in ASCII, which can be created manually or semi-automatically
• Methods for creating raster databases


  • Cell by cell entry
  • Raster digitizing - selecting contours of a specific value and tracing them with the mouse pointer
  • Vector digitizing - storing the information as points, lines and polygons which can later be converted to a raster coverage
• Just because we have a raster GIS doesn't mean that we cannot use vector data for some tasks