This document discusses vector GIS database structures. It explains that vector GIS represents the world using points, lines, and polygons. Vector models store discrete data like country borders and streets. Polygons are the basic unit and are created by connecting points with straight lines. Topology encodes spatial relationships between objects to accurately model real-world geometry. Topological rules govern connectivity and adjacency. Building topology involves calculating relationships between points, lines, and areas digitized in a GIS. Topological errors can occur if features do not perfectly connect. Vector databases are widely used in applications like transportation, utilities, and resource management.

1. Vector GIS database structure.
Prepared by;
DAVIS MUJUNGI
BRYSON ATUPELE
NAJA MWAKIBASI

2. INTRODUCTION;
 Vector GIS database refers to the representation of
the world using points (nodes), lines (chains or
arc), and areas (vector/polygons) and continuous
surfaces.
 Vector models are useful for storing data that has
discrete boundaries, such as country borders, land
parcels, and streets.
 Vector data models are based on vectors or polygons,
the polygon is the primitive or basic unit of the vector
data model.

3. Points as basis for objects;
Polygons are created by connecting points with
straight lines.
Some systems points to be connected using arcs of
circles.
Areas are defined by sets of lines.
The term polygon is synonymous with area in vector
database because of the use of straight-line
connections between points.

4. Point Feature
• A point layer: a collection of records with (x,y) coordinates
0 1 2 3 4 5 6
0
1
2
3
4
5
6
2,2
6,3
5,5
3,6
1
2
3
4
4,1
10

5. Line (Arc) Feature;
One or several segments are used to define an arc
(straight or curved)
2 points are connected to define a line segment
Line end points are nodes; angle points are vertices
(singular; vertex)
Arcs meet at nodes
Feature is the ARC, not the line segment
Each point has a unique location (Kumar, 2005).

6. Line (Arc) Cont….;
Line segment
Node
Vertices
Node

7. Polygon Feature;
Line segments bound the polygon
In a polygon layer, lines (arcs) define areas
A polygon therefore appears as the closed
region

8. Polygon Feature;
Lines (Arcs)
Points

9. Topology;
 Definition1; explicit encoding of spatial
relationships between objects.
 The spatial location of each point, line and
polygon is defined in relation to each other.
 Definition2; Topology is a collection of rules and
relationships that enables the geo-database to
more accurately model geometric relationships
found in the world (Kumar, 2005).

10. What is for ? & why topology in GIS?
Topology is basically information about
relationships, about connectivity between areas
on a map.
Allows for powerful analysis tools
Quality control mechanism

11. Types of vector topology;
Arc node-topology

12. Polygon-arc topology

13. Route topology

14. Region topology

15. Point events

16. Node topology

17. Connectivity and adjacency;
Topological characteristics are those which describe
connectivity.
When thinking about topology the qualitative
relationships of connectedness and contiguity are
more important that the quantitative attributes such
as length and area.

18. What makes a spatial database topological?
A spatial database is often called topological if one or more of
the following relationships have been computed and stored.
Connectedness of links at intersections for example; which roads
intersect?
Ordered set of lines (chains) forming each polygon boundary , for
example what lines or arcs are next to each other.
By contrast, a data base is called cartographic if the above
conditions are absent. Objects can be manipulated individually.
Relationships between the objects are unavailable or are
considered unimportant.

19. Limitations of cartographic database;
They are less useful for analysis of spatial data.
They are satisfactory for simple mapping of data.
Many packages designed for mapping only use
cartographic database models.
A cartographic database can usually be converted to a
topological database by computing relationships-the
process of “building topology”

20. Building topology;
Building topology is often done when digitizing.
Once points are entered and geometric lines are created,
topology must be built.
This involves calculating and encoding relationships
between the points, lines and areas.
This information may be automatically coded into
attribute tables.

21. Topology errors;
 There are different types of topological errors and they
can be grouped according to whether the vector feature
types are polygons or poly-lines.
 Topological errors with polygon features can include the
following (Gentemann, 2004).
-unclosed polygons
-Gaps between polygon borders or overlapping
polygon borders.

22. Topology errors cont….;
 A common topological error with poly-lines features
is that they do not meet perfectly at a
point(node).
 These types of error are:-
-Undershoot, if a gap exists between the lines.
-Overshoot, if a line ends beyond the line it
should connect to.

23. Illustration of topology errors;
 Undershoots (1) occur when digitized vector lines that should connect
to each other don't quite touch.
 Overshoots (2) happen if a line ends beyond the line it should connect
to.

24. Application of vector database;
Very large vector databases have been built for
different purposes.
Vector database tend to dominate in transportation,
utility and marketing applications.
Vector and raster are both used in resource
management applications.

25. “There is the GIS world that is largely managing
authoritative data sources, supporting geocentric
workflows like fixing roads, making cities more livable
through better planning, environmental management,
forest management, drilling in the right location for oil,
managing assets and utilities”
Jack Dangermond