Topology basically refers the relationship
between things, and in the realm of GIS,
Topology refers to the relationship between
spatial features or objects.
Importance In GIS
In terms of functionality, topology is
important to GIS in (at least) three important
First, topology is necessary for certain spatial
functions such as network routing through linear
networks. Here the idea is that if line features do
not share common nodes, that routes cannot be
established through the network.
Second, topology can be used to create
datasets with better quality control and
greater data integrity. Topology rules can
be created so that edits made to a dataset
can be 'validated' and show errors in that
dataset. An example would be the
creation of a new manhole/sewer access
feature outside a polygon dataset of road
Third, by creating topological relationships between
feature classes, features can be shared across feature
classes. In other words, if you open one dataset and
edit/move a line feature that is shared between two
feature classes, then both feature classes will be
updated to reflect the edits. This is massively helpful
for keeping datasets synchronized. An example would
be a river feature that defines a administrative
boundary (where the river moves over time), or the
boundary of a municipal area and zoning polygons.
There are many topology rules that you can use in
creating spatial datasets. : Point in Feature Class X must
lie within polygons in Feature Class Y, Polygons in
Feature Class X must completely cover polygons in
Feature Class Y, Lines in Feature Class X must intersect
lines in Feature Class Y.
For a more complete list of rules, see the ArcGIS
Desktop Help (or Online Help), open the index and
navigate to Topology Rules : Topology Rules (Editing in
With "coverages", a data format we haven't used very
much, topology had to be "built" after each edit. But
once topology was built, the dataset stored natively the
node-id's shared by line features, and the line-ID's shared
by polygons. Coverages are rarer and rarer now that
most data providers use shapefiles and to a lesser extent
geodatabases, but you still see them occasionally.
Shapefiles have a relatively unsophisticated implementation
of topology. In fact, shapefiles do not store topological
relationships. Rather, you can do topology-like operations
(e.g. network routing) where the topological relationships are
generated on the fly (e.g. connectivity between line features).
Here is a nice document summarizing topology and
Geodatabases provide the greatest topological functionality
among the data formats supported by ArcMap.
Here is a summary of how topology is implemented:
1. Topology exists between Feature Classes in a Feature Dataset
2. A Feature Dataset can be considered an association of Feature Classes,
but a Feature Dataset itself has spatial properties such as a spatial reference
system and XY domain. Any Feature Class in a Feature Dataset must adhere
to the same spatial properties as defined in the Feature Dataset.
3. Topology rules reside within a Feature Dataset. These rules have two
characteristics: 1) the type of rule (within, intersecting, overlapping...) and
2) the feature classes that are members of that rule.
4. Most of the topological operations in ArcMap are available from two
a. ArcToolbox : Data Management : Topology
b. ArcMap : Editor toolbar : Advanced Editing : Topology
Topology rules help create datasets with greater integrity -
(i.e. no slivers between polygons, no unsnapped nodes
between lines that should be connected, no twisted line
Topology facilitates the editing of shared features between
different spatial layers.
Different data format have different implementations of
topology with varying degrees of functionality. With data
formats supported by ArcMap, Geodatabases have the
greatest topological functionality.
Not every GIS project really requires topology. If you're
just making a map of city locations and roads, then you
don't really need topology. If you want to find the
optimal path between five different cities, then topology
is useful, but there are plenty of GIS projects where you
don't really need topology (or at least topology built into
The most common topological data structure is the arc/node
data model. This model contains two basic entities, the arc
and the node. The arc is a series of points, joined by straight
line segments that start and end at a node. The node is an
intersection point where two or more arcs meet. Nodes also
occur at the end of a dangling arc, e.g. an arc that does not
connect to another arc such as a dead end street. Isolated
nodes, not connected to arcs represent point features. A
polygon feature is comprised of a closed chain of arcs.
Data Organization And Storage
In GIS software the topological definition is
commonly stored in a proprietary format. However,
most software offerings record the topological
definition in three tables. These tables are analogous
to relational tables. The three tables represent the
different types of features, e.g. point, line, area. A
fourth table containing the coordinates is also
The node table stores information about the node and the arcs
that are connected to it. The arc table contains topological
information about the arcs. This includes the start and end
node, and the polygon to the left and right that the arc is an
element of. The polygon table defines the arcs that make up
each polygon. While arc, node, and polygon terminology is
used by most GIS vendors, some also introduce terms such as
edges and faces to define arcs and polygons. This is merely the
use of different words to define topological definitions. Do not
be confused by this.
Since most input data does not exist in a topological data
structure, topology must be built with the GIS software.
Depending on the data set this can be a CPU intensive and
time consuming procedure. This building process involves the
creation of the topological tables and the definition of the arc,
node, and polygon entities. To properly define the topology
there are specific requirements with respect graphic elements,
e.g. no duplicate lines, no gaps in arcs that define polygon
The topological model is utilized because it effectively models the
relationship of spatial entities. Accordingly, it is well suited for
operations such as contiguity and connectivity analyses. Contiguity
involves the evaluation of feature adjacency, e.g. features that touch one
another, and proximity, e.g. features that are near one another. The
primary advantage of the topological model is that spatial analysis can be
done without using the coordinate data. Many operations can be done
largely, if not entirely, by using the topological definition alone. This is a
significant advantage over the CAD or spaghetti vector data structure
that requires the derivation of spatial relationships from the coordinate
data before analysis can be undertaken.
The major disadvantage of the topological data model is
its static nature. It can be a time consuming process to
properly define the topology depending on the size and
complexity of the data set. For example, 2,000 forest stand
polygons will require considerably longer to build the
topology that 2,000 municipal lot boundaries. This is due to
the inherent complexity of the features, e.g. lots tend to be
rectangular while forest stands are often long and sinuous.
This can be a consideration when evaluating the topological
building capabilities of GIS software.
Topology is very important in GIS because it effectively
models the relationship of spatial entities. Moreover, it is well
suited for operations such as contiguity and connectivity
analyses. Without a topologic data structure in a vector based
GIS, most data manipulation and analysis functions would not
be practical or feasible. Topology facilitates the editing of
shared features between different spatial layers and is a
mechanism to ensure integrity with spatial data.
However despite its importance in GIS, one major drawback
of the topological data model is its static nature.