week 6-8

• Vector based GIS


  The vector representation of an object is an attempt to represent the object as exactly as possible. The co-ordinate space is assumed to be continuous, not quantized as with the raster space, allowing all positions, lengths and dimensions to be defined precisely.
• Point entities

  • Embrace all geographical and graphical entities that are positioned by a single XY co-ordinate pair
• Line entities


  • All linear features built up of straight line segments made up of two or more co-ordinates
• Area entities (polygons/regions)


  • Represented in various ways in a vector database to describe the topological properties of areas (their shapes, neighbours and hierarchy)
• Vector data


  Consist of lines or arcs, defined by beginning and end points, which meet at nodes. The locations of these nodes and the topological structure are usually stored explicitly.
• Vector storage


  Involves the storage of explicit topology, which raises overheads, however it only stores those points which define a feature and unlike raster structure all space outside these features is 'non-existent'.
• Vector based GIS


  Geographic objects are explicitly represented and, within the spatial characteristics, the thematic aspects are associated.
• Vector systems


  Composed of two components: one that manages spatial data and one that manages thematic data. This is a sort of hybrid organisation system, as it links a relational data base for the attributes with a topological one for the spatial data.
• Identifier


  Unique and different for each object, allows the system to connect both spatial and thematic data bases.
• Coordinates


  Pairs of numbers expressing horizontal distances along orthogonal axes, or triplets of numbers measuring horizontal and vertical distances, or n-numbers along n-axes expressing a precise location in n-dimensional space. Co-ordinates generally represent locations on the earth's surface relative to other locations.
• Point entities


  • A zero-dimensional abstraction of an object represented by a single X,Y co-ordinate. Besides the XY coordinates, other data must be stored to indicate what kind of 'point' it is and other information associated with it.
• Line entities


  • A set of ordered co-ordinates that represent the shape of geographic features too narrow to be displayed as an area at the given scale, or linear features with no area.
• Arc


  An ARC/INFO term that is used synonymously with line.
• Area entities (polygons)


  • Defined by the lines that make up its boundary and a point inside its boundary for identification. Polygons have attributes that describe the geographic feature they represent.
• Vector data models


  • List of coordinates "spaghetti"
  • Vertex dictionary
  • Dual Independent Map Encoding (DIME)
  • Arc / node
• List of coordinates "spaghetti" model is simple, easy to manage but has no topology and lots of duplication, hence need for large storage space. Very often used in CAC (computer assisted cartography).
• Vertex dictionary model has no duplication, but still does not use topology.
• Dual Independent Map Encoding (DIME) format


  Developed by US Bureau of the Census. Nodes (intersections of lines) are identified with codes. Assigns a directional code in the form of a "from node" and a "to node". Both street addresses and UTM coordinates are explicitly defined for each link.
• Arc / node structure


  Stores coordinates of nodes and vertex for all the arcs, as well as the topology information for arcs, polygons and nodes.
• Raster GIS may not be suitable for representing fine details like property boundaries, utilities and infrastructure, which require high resolution. Vector GIS is more suitable for these applications.
• Raster GIS is better for modelling and examining movement and flows over a surface, while vector GIS is more efficient for modelling flows along lines like roads, rails and pipelines.
• Vector representation


  Represents objects in the real world by points, lines and polygons in the GIS, using a 2-D coordinate system with continuous number system for the coordinates. This allows very high resolution storage very efficiently.
• Vector GIS comes largely from the digital cartographic side, with emphasis on boundaries and representing objects rather than phenomena and processes.
• Spaghetti model


  Stores a line in a vector GIS as a stream of coordinates, often with some additional information like how the line should be displayed.
• Spaghetti data model


  A way of representing data in a GIS where there is no underlying structure, just a collection of lines and points that look like a map or drawing when displayed
• In a spaghetti data model, to find out what other roads join a given road, we need to look at every single line and coordinate pair in the database and compare it to every line segment in the line we are starting from</b>
• Topology


  A way of structuring GIS data to make it easier for computers to analyse, by explicitly representing the relationships between the different components (points, lines, polygons)
• Topology


  • It deals with very basic geometric concepts that remain constant (invariant) under a range of transformations
  • It allows us to build a definite structure into all the objects in the vector GIS database
• Building topology in a GIS

  1. Give line strings a unique identifier

  2. Keep special note of nodes in a separate area, linked back to the original data

  3. Link line strings into chains, and keep note of these

  4. Take collections of chains that form a closed loop and reference these as the boundary of a polygon
• Ownership and component-ness


  The fundamental topological relationship, where objects 'own' or are 'a component of' other objects (e.g. lines own nodes, polygons own chains)
• Direction


  The direction of a line, from its 'from-node' to its 'to-node', which can be used to determine which polygons are to the left and right of the line
• Connectivity


  • Topological relationships allow quickly determining what lines meet at which nodes, and which polygons share boundaries
• Adjacency


  • Topological relationships allow quickly determining which polygons are adjacent, and the degree of adjacency
• Nestedness

  • Topological relationships allow easily identifying which polygons are nested inside others
• In a topologically sound GIS database, there are only two fundamental relationships between objects: they either touch along a common boundary, or they do not touch at all
• Improper topological relationships


  Situations where there are overlapping polygons or ambiguous spatial relationships, which need to be eliminated through proper topological structuring
• Even with GPS, it is difficult to achieve data precision better than 0.1 meters, yet GIS can display coordinates to fractions of a millimeter, which is misleading about the true accuracy of the data
• GIS users need to be aware of the difference between scale, accuracy, precision and resolution, and not be misled by the apparent precision of GIS data
• There are no standards for the quality of the rest of the map
• We can measure objects on the Earth with GPS and get precision to 0·1 meter. With good surveying gear, you may even be able to get to 0·01 meter. We are still a long way from a millimeter, let alone a fraction of a millimeter.