TOPOLOGY & LAYERS (THEMES)
Topology refers to the spatial relationships between geographic features. It describes the relationships between connecting or
adjacent coverage features. Topological relationships are built from simple elements into complex elements: points
(simplest elements), arcs (sets of connected points), areas (sets of connected arcs), and routes (sets of sections, which are
arcs or portions of arcs).
Topology is useful in GIS because many spatial modeling operations don’t require coordinates, only topological information.
For example, to find an optimal path between two points requires a list of the arcs that connect to each other and the cost to
traverse each arc in each direction. Coordinates are only needed for drawing the path after it is calculated.
Components of Topology:
Topology has three basic components:
I. 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.
II. 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.
III. 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.
Explanation of Topology:
TOPOLOGY & LAYERS (THEMES)
Topology refers to the spatial relationships between geographic features. It describes the relationships between connecting or
adjacent coverage features. Topological relationships are built from simple elements into complex elements: points
(simplest elements), arcs (sets of connected points), areas (sets of connected arcs), and routes (sets of sections, which are
arcs or portions of arcs).
Topology is useful in GIS because many spatial modeling operations don’t require coordinates, only topological information.
For example, to find an optimal path between two points requires a list of the arcs that connect to each other and the cost to
traverse each arc in each direction. Coordinates are only needed for drawing the path after it is calculated.
Components of Topology:
Topology has three basic components:
I. 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.
II. 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.
III. 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.
Explanation of Topology:
yers (Themes):
Maps produced traditionally or by automation involved the separation of data into layers, that each contained different types of
features: rivers, roads, etc. These are then combined to form a map where layers have usually been printed in different colors.
To present all the data collected for a given area might require the production of several maps, as they could not all be printed together. Equally in GIS, data for an area are divided into layers or themes, divided by type but here for the dual purposes of display and analysis.
Themes
The database can be divided into as many layers as is necessary, where each layer contains one characteristic such as soils, land use, drainage, etc. The layers ‘overlay’ each other perfectly as a result of Georeferencing and enabling analysis between layers.
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