Vector and Raster overview (writing)

Two coded elements of GI data: discrete object & continuous fields

  - discrete objects: objects with well-defined boundaries in otherwise empty space

  - continuous fields: when data do not have well-defined boundaries or no boundaries

                            representation of a finite number of variables, each one defines every possible position

                            it can be distinguished by what is being measured at each point

  - there are practically strong relationship between discrete objects and vector, and continuous fields and raster

 

Vector

  - All lines are captured as points connected by precisely straight lines (approximated from curved real line) -> specify the location of points  

  - Three components features: point, line, polygon 

    Point: 0-dimensional object = Node ,

            single coordinate pairs 

    Line: 1-dimensional object = Curve (OGC/ISO standards), Chain (early GIS standards), Edge  

                                                  (computer graphics), Polyline (common usage), or Arc (Esri)  

           A series of ordered coordinate pairs 

    Polygon: 2-dimensional object = Face

                one or more line segments that close to form a polygonal area / must fully close 

  - Each object has 2(x,y), 3(x,y,h<height>), or 4(x,y,h,m<time or other properties>) dimensions

 

Raster

  - It divides the world into arrays of the equally sized rectangular cells (usually square) and assigns attributes to the cells          *cell = pixel   

  - Common forms come from satellite (remote sensing) 

  - each pixel has a single value (an attribute value + location) 

  - raster representation: largest-share rule, central-point rule 

  - Point = single cell / line = connected sequence of cells / area = cluster of connected cells 

  - Many analysis tasks simpler with raster 

 

Issue	Raster	Vector
Volume of data	Depends on cell size  (require effective compression)	Depends on the density of vertices
Spatial analysis	Faster than vector	slower than raster
Sources of data	Remote sensing, imagery	Social and environmental data
Applications	Resources, environmental	Social, environmental, administrative
Software	Raster GI systems, image processing	Vector GI systems, automated cartography
Resolution	Fixed	Variable

Contrast between vector and raster

The apparent precision of vector is often not real because many geographic phenomena simply cannot be located with high accuracy -> increase uncertainty, so raster may be honest to the inherent quality of the data