Chapter 3 - Vector Data Model

The vector model is a map of x-y coordinates depicting discrete data which is data that can be displayed as points, lines, and polygons with empty space around them. This type of data is different than continuous data which is not object defined but instead is ever flowing and changing. Elevation is mostly depicted as continuous data.

The U.S Geological survey (USGS) has established the use of a vector line to represent a river 40 feet wide or smaller. Lines are called arcs and points where arcs meet or intersect are called nodes.

Topology is a feature held by some vector models which ensures data quality and integrity by assigning an exact location to each point, line, and polygon. By doing this you can detect lines that do not meet and decide rather to make them intersect. In this way the topology feature is used for error checking and for ensuring that lines meet correctly and polygons are closed properly in vector models. For example two polygons representing two countries  would have to border each other if using topology.

TIGER or the Topologically Integrated Geographic Encoding and Referencing database is a product of the U.S. Census Bureau. The TIGER database has assigned the number 0 for points, 1 for lines, and 2 for polygons.

Not all vector models have topology though. There are three types of vector models. Coverage, Shapefile, and Geodatabase. Coverage has topology, Shapefile does not, and in Geodatabase files users can choose to have topology or not. Coverage displays topology by giving established polygons their own outlined location, defining polygon areas as distinguished lines (arcs) connecting with distinguished points (nodes), and establishing a rule that every arc has a direction and a polygon to the left and right of it. Shapefiles do not have topology, but by so doing they can be opened and used with multiple GIS software platforms. Still there are lines, points, and polygons represented in x-y coordinates but polygons sharing boarders each have their own arcs. By doing this empty space is allowed to exist. This makes files smaller as they do not hold topology data. The third vector model is called geodatabase which can turn topology on or off. In this model data is stored in a hierarchical structure which groups data in categories and subcategories. This is helpful if you want to represent an object twice with different attributes each time. Also this model allows for custom rules to be written for the model so the user can better create the graph they envision.

Regions and Subclasses of data create overlapping areas as well as joint and disjoint areas. This can not be done in shapefile or geodatabase but what can be done is creating multipart polygons which allow for polygons to have parts that join other polygons and parts that dis-join from the original, as well as polygons that overlap.

A route can be put on top of road polylines in geodatabase. This seems to break the rules but in fact is possible.

TINs are triangulated irregular networks that use vector data combined with elevation to show a 3-d map of an area. In this sense a mountain range is constructed with non-overlapping triangles. Esri has a tool that can automatically (on the fly) create a TIN when nodes,x, y, and z coordinates are known.