Topology

Question 1

what are the 3 basic components of vector data storage?

Answer 1

unique object ID - it is unique to the object.
n-term - states how many pairs of co-ordinates (set of them) there are. (1 = point, 2 = lines, 3= polygon)
co-ordinates

Question 2

What are the 2 types of structural models of storing vector data?

Answer 2

Topology and spagetti models.
difference is the level of structure and organisation.

Question 3

Topology model?

Answer 3

vector data an or cannot be assocaited with topology.
Much more complex.
relies on each line to have a certain information like start and end points. - info direction to how the points

Question 4

Define topology?

Answer 4

refers to the spatial relationships that exist between features in a feature layer.
geometric relationship of point/node, lines/edges and polygons/faces.

Question 5

What are the rules for topology?

Answer 5

Not enforced by default
VECTOR LAYERS

1. Connectivity
2. Adjacency
3. Containment
4. Continguity

Question 6

What is connectivity?

Answer 6

describes linking of points or polyogns to each other

Question 7

What is adjacency?

Answer 7

Summarises the sharing of a common boundary of two regions or polygons.

Question 8

What is containment?

Answer 8

features within another specified feature.

Question 9

What is the contiguity?

Answer 9

arc (lines) have direction.

only refers to the topology of lines

Question 10

What are the advantages of topology?

Answer 10

1. Ensures data quality
Prevents overlapping features, data error detection:
Undershoot, overshoot, polygons that don’t close, polygons that overlap.

2. Enhancs spatial analysis
Including topological data in addition to the spatial and attribute data
the advantage of topological data that allows spatial analysis
and queries expected from GIS is significant - compared to data
storage disadvantages.

3. Represent the spatial information as well as spatial relations.

4. Shows how objects correspond to one another.
❖ The node elements corresponding to point,
❖ Edge (arc) elements corresponding to the lines, and
❖ The face elements corresponding to the polygons.

Question 11

What are the disadvantages of topology?

Answer 11

1. Complex
2. Time consuming to build depending on the number of features and relationships to specify
3. Data integrity concerns.

Question 12

What are topological errors?

Answer 12

Breaks relationships between spatial features.
Errors need to be fixed to be able to analyse vector data like:
- network analysis
- meausuremnet

The applicability of a project directly depends on accuracy and
topological consistency of vector data.
Errors or inconsistency in the vector data = unreliable GIS analysis.

• Point features may not be at the intersection point of line features
  (bridge at the intersection of river and road).
• Polygon features are not closed properly
• Contact points do not coincide (e.g. river does not coincide on the edge
  of the lake)
• 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 (shared boundaries).
• 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.

Question 13

When do you need to use topology?

Answer 13

• Roads as a background layer in your GIS - NO
• Roads in an analysis of a transportation system – YES
• Used to check error - YES
  ❑ IN ARCGIS SOFTWARE:
• Shape files - No
• Coverages (Arc/Info) - Yes, if you specify
• Geodatabases
  ◼ Shapefile still has - Point (x;y); Line ; Poly (BUT NO TOPOLOGY)
• Two adjacent poly’s have separate boundaries
• Poly’s can overlap (what is the disadvantage of this?)
• Through complicated scripts one can ‘imitate’ topology