In vector systems spatial data are represented by either points, lines, or polygons. A point is represented using a single coordinate pair. A line (or arc or segment) is represented by a string of coordinate pairs giving the start and end point of the line and the coordinates of all points where the line changes direction. A line's start and end points are often referred to as nodes. Polygons are created by completely enclosing an area by one or more lines. A basic polygon data model is shown in Figure 2.2. How lines are connected to create this model is known as topology. Both polygons and lines need topological information. A polygon needs to know the ID numbers of all the line segments that make up its boundaries, while a line segment usually knows which polygons are to its left and its right. For example in Figure 2.2, polygon p3 is bounded by line segments s4, s10, s11, s12, and s2. Line segment s10 has polygon p3 to its left and polygon p4 to its right. Source.