Geographic Information System/Vector Model
A vector data model defines discrete objects. Examples of discrete objects are fire hydrants, roads, ponds, or a cadastral. A vector data model is broken down into three basic types:
- Points
- Lines
- Polygons
All three types of vector data are composed of coordinates and attributes.
Points
[edit | edit source]A point uses a single coordinate pair to define its location. Points are considered to have no dimension even though they may have a real world dimension. For the purposes of a GIS , no dimension is assumed. Each point has associated attribute information, and the information is attached to the center of the point. Examples of spatial phenomena that would be modeled well as points are light poles, manhole covers, and crime locations.
(picture of a point enlarged)
Lines
[edit | edit source]A line vector type is defined by an ordered set of coordinates. Each line, and curve is made up of multiple line segments, however, on occasion, curved lines are represented mathematically. There are two words that we need to define when discussing lines:
- node
- vertex
A node is where a line begins or ends. A vertex is where a line changes direction. The smallest possible line will have two nodes, a start node, and an end node. Longer lines will have at least two nodes, and many verticies in between where the line changes direction. Attributes may be attached to the entire line, individual node, or individual verticies, therefore, each line may have multiple rows of attributes in the attributes table.
For example, if a line represents a road, each road segment between two intersections may have its own address information, such as the start address and the end address for that block. An intersection may have an attribute that describes where the intersection has a stop sign or stoplight. The other option is for the entire line to have one row of attributes no matter how complex the line. Examples of spatial phenomena that are modeled well by lines are:
- roads
- pipelines
- outlines of objects
- power lines
Polygons
[edit | edit source]A polygon is formed by a set of connected lines where the start and end point have the same coordinate. Because the start and end p point have the same coordinate, the polygon will close and have an interior region. Attribute information is attached to the center of the polygon no matter how complex the polygon. Examples of spatial phenomena modeled well by polygons are:
- lakes
- cities
- tree stands
- political boundaries