week 9 & 10 notes

• Data input


  The operation of encoding the data and writing them to the database
• Aspects of data to be considered for GIS

  • Positional or geographical data necessary to define where the graphic or cartographic features occur
  • Associated attributes that record what the cartographic features represent
• Methods of entering spatial data into a GIS


  • Manual digitising and scanning of analogue maps
  • Image data input and conversion to a GIS
  • Direct data entry including Field Surveys and Global Positioning Systems (GPS)
  • Transfer of data from existing digital sources
  • Photogrammetric
• Manual input to a vector system

  1. Coordinates of the data are obtained from the reference grid already on the map or from the reference to a graticule or overlaid grid

  2. The values can be simply fed into a computer
• Manual input to a grid system

  1. Selection of grid cell (raster) of appropriate size

  2. Laying a transparent grid of that size over the map

  3. Writing down the values of a single map attribute for each cell and feeding into the computer

  4. Run-length coding (RLC) can be used to overcome the disadvantages
• 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 is placed at the beginning of the line, computer records coordinates at equal intervals as the cursor is moved

  3. Point digitizing: Operator tells the computer to record every coordinate by pressing a button
• Automated scanning


  1. Raster scanners: Work on the principle that a point or any part of the map may have one or two colours, black or white

  2. Vector scanners: Alternative to scanning lines using a raster device and then restoring the vector structure
• Non-spatial associated attributes


  Properties of a spatial entity that need to be handled in the GIS, but which are not of themselves spatial in kind
• Linking spatial and non-spatial data


  1. The digital representations of the points, lines and areas carry unique identifiers

  2. Both the identifier and the coordinate are stored in the database
• Data Verification


  1. Computer draws the spatial data again at the same scale as the original, then the two maps are visually compared

  2. Data editing and data updation are required when there are errors or data changes over time
• Data Storage


  • Digital databases have a useful life of 1-25 years, so timely backup is required
  • Data is stored in magnetic media like computer compatible tape (CCT), floppy disk, CD-ROM, or tape cassettes
• Data Output


  Presenting the results of data manipulation in a form understandable to a user or allowing data transfer to another computer system, e.g. maps, graphs, tables, CCTs
• Data input is the operation of encoding data for inclusion into a database
• Data collection and maintenance of databases remains the most expensive and time consuming aspect of setting up a major GIS facility, typically 60-80% of the overall costs
• Vector vs Raster format

  Considerations include the nature of the source data, predominant use, potential losses, storage space, and requirements for data sharing
• Data stored in a GIS does not have a scale, but may be equivalent to a certain map scale
• The ideal way is to fill the database with data with accuracies equivalent to very large scale maps, but compromises are often made due to data availability, cost, or lack of application need
• Methods of acquiring spatial data

  • Buy data
  • Steal data
  • Download data over the Internet
  • Collect data ourselves
• Methods of entering spatial data into a GIS


  • Manual digitising and scanning of analogue maps
  • Image data input and conversion to a GIS
  • Direct data entry including Field Surveys and Global Positioning Systems (GPS)
  • Transfer of data from existing digital sources
  • Photogrammetric
• Raster data set design considerations


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


  • Tracing map features in the form of points, lines or polygons with a mouse (puck) which relays the coordinate of each sample point to be stored in the computer
  • Point-mode: Operator selects and encodes "critical" points
  • Stream-mode: Device automatically selects points on a distance or time parameter
• On-screen digitizing


  Interactive process of creating a map using previously digitised or scanned information, by tracing features from a scanned map or image
• Digitizing errors will always occur (undershoots, overshoots, triangles)
• Editing of digitised features involves error correction, entering missing data, forming topology
• There are many issues to consider before digitising commences, including the purpose of the data, the coordinate system, and the accuracy of the layers
• On-screen digitising


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

  2. Use referenced information as a background
• On-screen digitising is similar to conventional digitising, but uses a mouse instead of a digitiser and cursor
• 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 to be used
  • Accuracy of the layers to be associated
  • Accuracy of the map being used
• Digitise as much as possible each time to make the technique more consistent
• Only one person should work on a given digitising project for more consistency
• If the source consists of multiple maps, select common reference points that coincide on all connecting sheets to avoid data not matching
• 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 map scale and geographic coordinates of control points

  3. Specify error limit
• RMS error


  Difference between the original control points and the new control point locations calculated by the 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)
• Common causes of high RMS error are - incorrectly digitised control points, careless placement of control points on the map sheet, and digitising from a wrinkled map