This document discusses GIS topology, which establishes rules for how geographic features share geometry and spatial relationships. Topology ensures data quality, enhances analysis, and manages coincident geometry. It has three components: connectivity between nodes and arcs, area definition using polygon boundaries, and contiguity to determine adjacent features. Topological rules prevent errors like overlaps, gaps, dangles and ensure proper containment of points and boundaries.

1. TOPOLOGY
(GEOGRAPHICAL INFORMATION
SYSTEM)
UNIT – III
Dr. Rambabu Palaka, Professor, BVRITOct. 2020

2. Topology
GIS Topology is a set of rules and behaviors that model how points, lines, and
polygons share coincident geometry.
For example:
 Adjacent features, such as two counties, will have a common boundary between
them. They share this edge.
 The set of county polygons within each state must completely cover the state
polygon and share edges with the state boundary.
Topology is often explained through graph theory. Topology has least two main
advantages.
 The assurance of data quality
 Enhance GIS analysis

3. Why Topology
Why Topology?
 Topological or topology based data are useful for detecting and correcting
digitizing error in geographic data set and are necessary for some GIS
analyses.
 Topologic data structures help insure that information is not unnecessarily
repeated. The database stores one line only in order to represent a boundary
(as opposed to two lines, one for each polygon). The database tells us that the
line is the “left side” of one polygon and the “right side” of the adjacent polygon.

9. Generally, topology is employed to do the following:
 Manage coincident geometry (constrain how features share geometry). For example,
adjacent polygons, such as parcels, have shared edges; street centerlines and the
boundaries of census blocks have coincident geometry; adjacent soil polygons share
edges; etc.
 Define and enforce data integrity rules (such as no gaps should exist between parcel
features, parcels should not overlap, road centerlines should connect at their endpoints).
 Support topological relationship queries and navigation (for example, to provide the
ability to identify adjacent and connected features, find the shared edges, and navigate
along a series of connected edges).
 Construct features from unstructured geometry (e.g., the ability to construct polygons
from lines sometimes referred to as "spaghetti").
Topology

10. Components of Topology
Topology has three basic components:
1. Connectivity (Arc – Node Topology):
o Points along an arc that define its shape are called Vertices.
o Endpoints of the arc are called Nodes.
o Arcs join only at the Nodes.
2. Area Definition / Containment (Polygon – Arc Topology):
o An enclosed polygon has a measurable area.
o Lists of arcs define boundaries and closed areas are maintained.
o Polygons are represented as a series of (x, y) coordinates that connect to define an area.
3. Contiguity:
o Every arc has a direction
o A GIS maintains a list of Polygons on the left and right side of each arc.
o The computer then uses this information to determine which features are next to one another.

11. Topology Rules
The existence of errors or inconsistency in the vector data could easily by setting topological
rules
There are 25 topology rules than can be used to relate different layers.
A. Polygon Rules
1. Must Not Overlap
2. Must Not Have Gaps
3. Must Not Overlap With
B. Line Rules
1. Must Not Overlap
2. Must Not Intersect
3. Must Not Intersect With
4. Must Not Have Dangles
5. Must Not Intersect Or Touch Interior
6. Must Not Overlap With

12. Topology Rules
C. Point Rules
1. Must Be Covered By Boundary Of
2. Must Be Properly Inside
3. Must Be Covered By Endpoint Of
4. Must Be Covered By Line