GIS10

Vector GIS Basics
Vector Data map elements:  
Point features : Single X-Y coordinate pair
Line features  : String of X-Y coordinate pairs
Polygon features : Closed loop of X-Y coordinate pairs, last pair are the same.
Features are usually named uniquely in the geographic database, termed the “primary key” in database, Data Attributes are linked to features by a common field,
Vector GIS and Relational Data Management
 Each line feature has a string of X-Y coordinate pair locations, it would be inefficient to store all coordinates for all lines, because several lines have “common nodes” where the lines “intersect”, the line, feature table is therefore “related” to the node feature table through the “common Node TAG”.
Unique IDs or TAGs are assigned to every feature element.
The list of coordinates for each feature are associated by the GIS with each TAG and separate tables are created for points [ nodes ],  lines [ arcs ] and polygons
 Similarly, every polygon consists of a closed set of line features, it  would be inefficient to store all this information, because  “adjacent” polygons would “share line segments” ,the polygon  feature table is therefore “related” to the line feature table through  the “common Line TAG”  , note that the vertices for the polygon can be derived by the previous node-line table association.
Consider the following example, where we have an area represented by two streets,. Several parcels and two blocks  also have to be represented in the vector data structure.
All point [ node ] features would be given unique identifiers or TAGs, which are then associated with the X-Y coordinate location
All line [ arc ] features would be given unique identifiers or TAGs, and the line table would be related to the node table, using the common Node TAG
All polygon features would be similarly identified by TAGs, and the polygon table would be related to the line table, using the common Line TAG, Thus nodes “build” arcs, and arcs “build” polygons