Chapter 4 topology Introduction Topology is a useful

Chapter 4 topology

Introduction Topology is a useful data structure concept in GIS, it allows great flexibility and powerful connections to be made among data. It is defined as an addition to a vector data structure in which each point, line , partial or whole polygon has the following characteristics: *”knows” where it is *”knows” what is round it * “understands” its environment * “ knows” how to get around

Topology • Topology is the mathematical procedure for defining spatial relationships • Topology is the mathematics of connectivity and adjacency for spatial features • Topology can be viewed as the programming that provides spatial relationships between node, chains , and geographic features. • When topology is applied to GIS , data structure tables are built for features, nodes and chains , the tables are used to determine what is connected , what is adjacent and the direction of chains. • By reading tables, GIS can track connections, make paths , measure distances & area, and determine a variety of spatial relationships.

APPLYING TOPOLOGY • Topology is applied by a simple operation usually after digitizing( digitizing is the digital tracing of maps). • Features are first digitized and the resulting lines are without connection or “intelligence”. This is the spaghetti model of data: lines that cross and form shapes without connection or relationship to other lines & shapes • Topology builds the spatial information by recognizing nodes at the ends of each digitized line and creating new nodes at intersections where lines cross, making true topological chains • When topology is applied , each node is numbered and a new node is established at the intersection

TOPOLOGY TABLES • Topology tables contains the data structure information used in feature definition and in determining spatial relationships. • The tables are created automatically when topology is applied to the data, nodes & chains define line and polygon features. • The chain topology table presents each chain and its start and end nodes, the node topology table lists the chains that connect to each node. • Coordinate tables for the chains and nodes present the real-world locations for each node and vertex. • A polygon topology table identifies the chains that make each polygon, The chain topology table identifies the polygon to the left and to the right.

MULTIPLE CONNECTIVITY • Topological connections can work on an integrated set of line features, such as a highway system, called a “network”. • Topology provides the connections and routing analysis. • When lengths & total distances are included , the shortest paths can be found among the complicated network. • Using topological links, the search follows the first chain until a node is reached , it then reads the node and chain to follow other connections. Eventually there may be numerous possible paths, but because distances have been calculated , it is simple to determine the shortest path.

Topology & Relational Query Combining the functions of a relational database and topology we can see a powerful GIS capability. Topology will provide the connections & the relational database will sort through the attributes.

TOPOLOGY’S CONTRIBUTIONS Topology offers a number of advantages and contributions to GIS: o. Spatial information: topology provides length, distance, perimeter, and area. o. Spatial relationships : topology creates connections which functionally link features that are adjacent. o. Multiple linkages : each feature is linked to other features, providing multiple connections (linkages). o. Network analysis : the functional connections , distance , and other spatial relationships , combined with the relational database, are ideal in interpreting network features, and performing specialized analysis ( ex, shortest route).

Topology’s Advantages 1. intelligent vectors 2. Automatic recognition of spatial relationships 3. multiple linkages 4. magic programming