GIS topology is a set of rules and behaviors that model how geographic features share coincident geometry. It ensures data quality by managing how points, lines, and polygons connect and intersect. Topology defines connectivity, area definition/containment, and contiguity. It establishes 25 rules for relating different data layers, such as requiring polygons to not overlap, lines to not self-intersect, and points to be properly contained within other features. Topology is useful for detecting and correcting errors in spatial data and enabling certain types of GIS analysis and queries.

1. TOPOLOGY
(geographical information system)
DR. VENKATESWARLU GOGANA

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. GIS Topology is a set of rules and behaviors that model how points, lines, and polygons share coincident
geometry.
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