Data Models Vector and Raster Data

DATA MODELS VECTOR AND RASTER DATA

A vector based GIS is defined by the vectorial representation of its geographic data. The most common representation of map is using vector date that is consist of point, line and polygon

1.. VECTOR DATA

i.           Point Data -- layers described by points (or "event") described by x,y (lat,long; east, north)

ii.           Line/Polyline Data -- layers that are described by x,y points (nodes, events) and lines (arcs) between points (line segments and polylines)

iii.         Polygon Data -- layers of closed line segments enclosing areas that are described by attributes

Polygon  data  can  be  "multipart"  like  the  islands  of  the  state  of

Hawaii.

Vector Data Structures

• Fundamental unit is the x, y coordinate (vertex)

- points are explicitly defined

- lines, arcs and polygons are constructed

• Map area is presumed continuous

- position can be defined with great precision; limited by precision of software/hardware and source data

• Usually provides finer spatial resolution than raster model

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.

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 featurethey represent.

2.. RASTER DATA

Consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information, such as temperature. Rasters are digital aerial photographs, imagery from satellites, digital pictures, or even scanned maps.