6 - Topology Flashcards
Spatial data model
Simplified representation of spatial
features or spatial phenomena on the Earth’s surface
How is vector data prepared??
- Geometrical data are stored as sets of xy coordinates.
- The data & their spatial relationships are organised into digital data files that the computer can access, interpret and process.
Components of general format for storage of vector data
- The object Id-number, which has to be unique
- The n-term
- The co-ordinate pairs as defined by n-term
The object Id-number use
- Has to be unique
- Used to identify the object and to link attributes to it
The n-term use
States how many pairs of co-ordinates are used to build the object
Difference bwtn spaghetti and topology models
The level of structure and organisation of the data
Topology def
- The spatial relationships that exists between features in a feature layer in the vector model.
- Set of rules of how points, lines and polygons share their geometry.
- The geometric relationship between edges (line), nodes (point) and the faces (polygon), they created.
What does topology do?
Checks and validates the spatial relationship of neighbouring and overlapping features.
Nodes def
Connections at the intersection points and start at end points of edges
Edge def
Set of coordinate pairs starting with a node and ending with a node making up the boundaries of
polygons.
Face def
The largest two-dimensional space restricted by the edges
Some topology rules
- Municipal boundaries must not overlap.
- Municipal boundaries must not have gaps (slivers).
- Polygons showing property boundaries must be closed.
- Undershoots (lower than final value) or overshoots (higher than the final value) of the border lines are not allowed.
- Contour lines in a vector line layer must not intersect.
Main topology rules
- Connectivity
- Adjacency
- Containment
- Contiguity
Connectivity
Describes the linking of points or polygons to each other.
- Adjacency
Summarizes the sharing of a common boundary of two
regions or polygons.
- Containment
Features within a another specified feature
Contiguity
Arcs (lines) have direction
Arc-node topology
- a from-node (starting node) indicating where the arc begins
- a to-node ( ending node) indicating where the arc ends.
- Between each node pair is a line/arc segment = link
Polygon-arc (area)
- Arc that connects to surround an area defines a polygon.
- arcs are used to construct polygons, and each arc is stored only once.
Polygons that share an arc are deemed…?
Adjacent or continuous
Advantages of topology
- Ensures data quality/ Enables error detection
- Enhance spatial analysis
- Represent the spatial information as well as spatial relations
- Shows how objects correspond to one another
Disadvantages of topology
- Complex
- Time consuming to build depending on the number of features
and relationships to specify - Data integrity concerns
Ensures data quality - Enables error of detection of:
nsures data quality - Enables error detection of:
❖ Undershoot - lines that don’t meet
❖ Overshoot
❖ Polygons that don’t close
❖ Polygons that overlap
Enhances spatial anaylyse
Include spatial and attribute data (increase in volume/storage)
Examples of analyzing vector data
❖ Network analysis (e.g. finding the best route across a road network) or
❖ Measurement (e.g. finding out the length of a river)
Topological error examples
- Point features may not be at the intersection point of line features
- Polygon features are not closed properly
- Contact points do not coincide
- Overlaps or gaps in polygon features . Since neighbouring or same location features are represented twice, there is no full coincidence at the point and line features.
- Neighbourhood relations are unclear
- Due to exclusion of information, polygon, point or line features included in polygons are unclear.
- Navigation is not possible since there is no direction concept in the line features.
Why use shapefiles if it does not include topology?
- Displays more rapidly
- Format is non-proprietary and interoperable - can be used
across different software packages.
Industry standard for vector data files
SHAPEFILES
Non-proprietary def
freedom from exclusive ownership / not registered or protected as a trademark or brand name; generic.
Interoperable
able to exchange and make use of information
