week 9 & 10

• Data input


  The operation of encoding the data and writing them to the database
• Spatial data and associated attributes

  • Spatial data necessary to define where the graphic or cartographic features occur
  • Associated attributes that record what the cartographic features represent
• Data input to a GIS

  1. Entering the spatial data (digitizing)

  2. Entering non-spatial associated attributes
• Manual input to a vector system

  Source data are envisaged as points, lines or areas, coordinates obtained from reference grid or graticule, values fed into computer
• Manual input to a grid system

  All points, lines and areas envisaged as sets of cells, values of a single map attribute for each cell written down and fed into computer, can be overcome by run-length coding
• Digitizing


  1. Using a digitizer to encode the X and Y co-ordinates of the desired points, lines, areas or grid cells

  2. Stream digitizing - cursor placed at beginning, computer records coordinates at intervals

  3. Point digitizing - operator tells computer to record each coordinate by pressing a button
• Automated scanning


  • Raster scanners - work on principle that a point or part of map may have one or two colours, black or white
  • Vector scanners - alternative to raster scanning to restore vector structure
• Entering non-spatial associated attributes

  Non-spatial properties of a spatial entity that need to be handled in the GIS, but are not spatial in kind, can be efficiently linked to spatial data by giving each a common identifier
• Linking spatial and non-spatial data


  Digital representations of points, lines and areas carry unique identifiers, both identifier and coordinate stored in database
• Data verification


  Computer draws spatial data again at same scale, compared visually to original, data editing and updation required for errors or changes
• Data storage


  Digital databases stored on magnetic media like computer compatible tape or floppy disk, CDROM or tape cassettes now used for better storage capacity
• Data output


  Presenting results of data manipulation in understandable form for user or data transfer, includes displays and permanent images
• Data input is the operation of encoding data for inclusion into a database, creation of accurate databases is very important but most expensive and time consuming part of GIS
• Considerations in developing a GIS database

  • Whether to store data in vector or raster format
  • Nature of source data
  • Predominant use
  • Potential losses in transition
  • Storage space
  • Requirements for data sharing
• Scale in GIS

  Data stored in GIS does not have a scale, refers to accuracy equivalent to a map scale
• Ideal to fill database with data of very large scale accuracy, but may not be practical due to availability, cost, or lack of application requiring that accuracy</b>
• Methods of acquiring spatial data


  • Manual digitising and scanning of analogue maps
  • Image data input and conversion
  • Direct data entry including Field Surveys and GPS
  • Transfer of data from existing digital sources
  • Photogrammetric methods
• Considerations in designing a raster database

  • Physical extent of database
  • Resolution (grid size)
  • Themes to be included
  • Classifications to be used within themes
  • Appropriateness of input data scale to preferred grid size
• Manual digitizing


  Tracing map features with a cursor on a digitizing table (heads-down) or computer screen (heads-up), in point-mode or stream-mode, coordinates transformed to real world system
• Digitizing errors will always occur, editing of digitized features involves error correction, entering missing data, forming topology
• Many issues to consider before digitizing, including purpose of data use, coordinate system, accuracy of layers
• On-screen digitising


  1. Create map layer up on the screen with the mouse

  2. Use referenced information as a background
• There is always a requirement to transform coordinates from the digitiser system to the real world system (e.g. national map grid)
• Digitising errors will always occur (undershoots, overshoots, triangles)
• Editing of digitised features


  1. Error correction

  2. Entering missing data

  3. Forming topology
• Issues to consider before digitising

  • Purpose of the data
  • Coordinate system
  • Accuracy of layers
  • Accuracy of map
• Digitise as much as possible each time, only one person should work on a given digitising project
• If the source consists of multiple maps, select common reference points that coincide on all connecting sheets
• Include attributes while digitising to save time later
• Appropriate map registration or georeferencing is essential if the data is to be merged with a larger database
• Map registration or Georeferencing

  1. Digitise control points

  2. Specify error limit

  3. Calculate RMS error
• RMS error


  Difference between original control points and new control point locations calculated by transformation process
• To maintain highly accurate geographic data, the RMS error should be kept under 0.004 inches (or its equivalent measurement in the coordinate system being used)
• Scanning


  1. Convert data to raster representation

  2. Raster to vector conversion
• Scanning requires that the map scanned is of high cartographic quality, with clearly defined lines, text and symbols; be clean and have lines of 0.1mm width or wider
• Scanning


  1. Scanning to produce regular grid of pixels

  2. Binary encoding to separate lines from background
• Editing of scanned data

  1. Pattern recognition

  2. Line thinning and vectorisation

  3. Error correction

  4. Supplementing missing data

  5. Forming topology
• Surveying and manual coordinate entry

  1. Field surveying

  2. GPS data collection

  3. Total station data collection
• Manual data entry is slow, tedious and expensive
• GPS-based and total station data collection are making the data entry process faster and better