GIS toolglossary

 

 
vector based GIS
  general definitions
  vector representations of data
  vector models
  advantages/disadvantages of raster and vector data models
  data bases
  data entry

general definitions

Vector is a data structure, used to store spatial data. Vector data is comprised 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. Features are defined by their boundaries only and curved lines are represented as a series of connecting arcs. Vector storage involves the storage of explicit topology, which raises overheads, however it only stores those points which define a feature and all space outside these features is 'non-existent'.

A vector based GIS is defined by the vectorial representation of its geographic data. According with the characteristics of this data model, geographic objects are explicitly represented and, within the spatial characteristics, the thematic aspects are associated.

There are different ways of organising this double data base (spatial and thematic). Usually, vectorial systems are composed of two components: the one that manages spatial data and the one that manages thematic data. This is the named hybrid organisation system, as it links a relational data base for the attributes with a topological one for the spatial data. A key element in these kind of systems is the identifier of every object. This identifier is unique and different for each object and allows the system to connect both data bases.


Figure 4.Vector representation
Vector representation of data
In the vector based model (figure 4), geospatial data is represented in the form of co-ordinates. In vector data, the basic units of spatial information are points, lines (arcs) and polygons. Each of these units is composed simply as a series of one or more co-ordinate points, for example, a line is a collection of related points, and a polygon is a collection of related lines.
 
  co-ordinate
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
A zero-dimensional abstraction of an object represented by a single X,Y co-ordinate. A point normally represents a geographic feature too small to be displayed as a line or area; for example, the location of a building location on a small-scale map, or the location of a service cover on a medium scale map.
  line
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 (contours, street centrelines, or streams), or linear features with no area (county boundary lines). A lines is synonymous with an arc.
  arc
An ARC/INFO term that is used synonymously with line.
  polygon
A feature used to represent areas. A polygon is 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 models
There are different models to store and manage vector information. Each of them has different advantages and disadvantages.
 list of coordinates "spaghetti" (figure 5)
vertex dictionary (figure 6)
Dual Independent Map Encoding (DIME) (figure 7)
arc / node (figure 8)


Figure 5. List of coordinates "spaghetti"

  • simple
  • easy to manage
  • no topology
  • lots of duplication, hence need for large storage space
  • very often used in CAC (computer assisted cartography)
 

Figure 6. Vertex dictionary

  • no duplication, but still this model does not use topology


Figure 7. 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
 
 
File 1. Coordinates of nodes and vertex for all the arcs
ARC F_node Vertex T_node
1
 3.2, 5.2 1, 5.2 1,3
2
 1,3 1.8,2.6 2.8,3 3.3,4 3.2, 5.2
3
 1,2 3.5,2 4.2,2.7 5.2,2.7
 
File 2. Arcs topology
ARC F_node T_node R_poly L_poly
1
1
2
 External A
2
2
1
 A External
3
3
4
 External External
 
File 3. Polygons topology  
Polygon Arcs
A 1, 2
 
File 4. Nodes topology  
Node Arcs
1
1,2
2
1,2
   
3
3
4
4
5
5
Figure 8. ARC / NODE structure or POLYVRT
see the Vector Overlay module for information on vector based GIS overlay operations
advantages/disadvantages of raster and vector data models
 
raster
vector
precision in graphics
traditional cartography
data volume
topology
computation
update
continuous space
integration
discontinuous

data bases

The elements in a vector based GIS are then the DBMS (Data Base Management System) for the attributes and the system that manages the topological data. In some GIS packages, the DBMS is based in an existing software, i.e. dBASE.


entity-relation model
Three elements are considered in this approach: (a) Entities as the relevant objects for the data base. In a GIS, an entity is any fact that can be localised spatially. (b) Attributes or characteristics attached to the entities. Each attribute has a limited domain of possible values, i.e. the quality of a road can be bad, average, good, very good. (c) Relations or mechanisms that allow to relate entities. Some examples are: ‘located in’, ‘contained in’, ‘crossed with’, etc.

DBMS
The data bases used in GIS are most commonly relational. Nevertheless, Object Oriented data bases are progressively incorporated.

relational data bases
 In a relational data base, data is stored in tables where rows represent the objects or entities and columns the attributes or variables. A data base is usually composed of several tables and the relations between them is possible through a common identifier that is unique for each entity. Most of the relational data bases in GIS present two variables with identifiers; one of them is unique and correlative, it could be numeric or alphabetic, and the second one might be repeated and helps to organise the attribute table.
The advantages of using this kind of data base are:
  The design is based in a methodology with heavy theoretical basis, which offers confidence in its capacity to evolve.
It is very easy to implement it, specially in comparison with other models such as hierarchical, network, and object oriented.
It is very flexible. New tables can be appended easily.
Finally, many powerful DBMS using this approach contains query languages (like SQL) which makes easy to include this tool in a GIS. Thus, some commercialised GIS packages include a DBMS pre- existent.

object oriented data bases
 Based on objects, it can be defined as an entity with a localisation represented by values and by a group of operations. Thus, the advantage in comparison with relational data bases is based on the inclusion, in the definition of an objet, not only its attributes but also the methods or operations that act on this object. In addition, the objects belong to classes that can have their own variables and these classes can belong to super-classes.
 
data entry

see the Spatial Data Entry module for information on this

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