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# Lecture+12+topology+2013 (3)

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### Lecture+12+topology+2013 (3)

1. 1. Lec #12 Fall 2013 LDA 150/ABT 150 4-Units Introduction to Geographic Information Systems (GIS) Instructor: Stavros G. Vougioukas, Ph.D. Lecture: Tuesday and Thursday, 3 Kleiber, 3:10-4:30 PM Lab Sections: 1137 PLESC Lab--Friday (01 & 04), Monday (02), Wednesday (03 & 06), Thursday (05) 11/6/2013 6:05 PM 1
2. 2. Lecture contents • Topology in GIS. • Topology errors. • Fixing topology. 11/6/2013 6:05 PM 2
3. 3. Limitations of geometry • Consider the geometric description of three polylines. – Both descriptions are geometrically equivalent. (6.2,3.3) (5.1,2.3) 1 Polyline 1 2 3 Coordinates (6.6, 1.4), (6.2, 3.3), (5.1, 2.3), (4.8, 1.3) (4.8, 1.3), (6.6, 1.4) (4.8, 1.3), (5.7, 0.2), (6.6, 1.4) (6.6,1.4) 2 (4.8,1.3) 3 (5.7,0.2) 11/6/2013 8:52 PM Polyline Coordinates 1 (4.8, 1.3), (5.1, 2.3), 6.2, 3.3), (6.6, 1.4) 2 (4.8, 1.3), (6.6, 1.4) 3 (4.8, 1.3), (5.7, 0.2), (6.6, 1.4) 3
4. 4. Limitations of geometry • If lines are a road network and polygons are land parcels: – What about one way streets? – Which road is between parcels? – Which parcel is on the ‘left’? (5.1,2.3) (6.2,3.3) 1 • Something more than geometry is needed… (6.6,1.4) b a 2 (4.8,1.3) 11/6/2013 8:57 PM 3 (5.7,0.2) 4
5. 5. Topology • The representation of the arrangements of geometric objects and the relationships among them. • Some fundamental relationships – Connectivity: what is connected to what. – Area definition: containment. – Contiguity: borders and left/right adjacent areas. 11/6/2013 9:24 PM 5
6. 6. Topology • Topological relationships are invariant under: – Affine transformations – Bending – Stretching. 11/7/2013 11:37 AM 6 http://www.indiana.edu/~gisci/courses/g338/lectures/introduction_vector.html
7. 7. Directed graph (digraph) • In GIS vector data topology is implemented using graph theory. • Graph: A set of – nodes, and – ordered pairs of nodes, called arcs • Each arc connects two nodes and has direction from the 1st node to the 2nd node. 11/7/2013 8:54 AM 7
8. 8. Connectivity: Adjacency • If an arc joins 2 nodes, the nodes are called adjacent. • Represented by the adjacency matrix. – Encodes direction. – No arc identity. 11/6/2013 10:20 PM 8
9. 9. Connectivity: Incidence • If an arc joins 2 nodes, the nodes are called incident to the arc. • Incidence matrix – Encodes direction, and – Arc identity. 11/6/2013 10:35 PM 9
10. 10. ESRI Coverage: Topological properties • Connectivity. – Arcs connect to each other at nodes. • Area definition. – Connected arcs that surround an area define a polygon. • Contiguity. – Arcs have direction and left and right sides). 11/7/2013 9:18 AM Lo and Yeung p. 86 10
11. 11. Coverage Connectivity: Arc-node topology • Beginning and ending node of each arc. • Remember: A node is either the end of a line or the intersection of lines. 11/7/2013 9:33 AM 11
12. 12. Coverage Area definition: Polygon-arc topology • Area: series of connected arcs; polygon-arc list. • Containment: – 0 precedes internal polygon. 11/7/2013 8:57 AM 12
13. 13. Coverage Contiguity: Left-right topology • Relationships between arcs and their left and right polygons. • A background polygon (100) must be defined. 11/6/2013 10:59 PM 13
14. 14. Topology creation • Can we build topology from geometry? a b 1. 2. Arc Coordinate List Arc # Coordinates 1 (4.8,1.3), (5.1,2.3), (6.2,3.3) (6.6, 1.4) (6.6, 1.4) 2 (4.8,1.3), (6.2,3.3) 3 (6.2,3.3), (5.7,0.2), (4.8,1.3) (6.6, 1.4) (6.2,3.3) 1 (5.1,2.3) (6.6,1.4) b a 2 (4.8,1.3) 3 (5.7,0.2) 11/7/2013 10:07 AM 2. Arc-Node List 1. Arc # From-node To- node 1a b 2a b 3b a 14
15. 15. Topology creation 2. Arc-Node List 1. Arc # 1 A From-node To- node 1a b 2a b 3b a b a B 3 11/7/2013 9:59 AM 2 3. Polygon-Arc List Poly # Arcs A 1, 2 B 2, 3 15
16. 16. Topology & errors • Building topology from geometry involves identifying all nodes, arcs, and polygons. • What if there are errors in the geometry? (6.19,3.3) (6.2,3.3) 1 (5.1,2.3) – No problem in topology… –Only in geometry. (6.6,1.4) b a 2 (4.8,1.3) Polyline Coordinates 1 3 (5.7,0.2) 11/7/2013 10:41 AM (4.8, 1.3), (5.1, 2.3), (6.19, 3.3), (6.6, 1.4) 2 3 (4.8, 1.3), (6.6, 1.4) (4.8, 1.3), (5.7, 0.2), (6.6, 1.4) 16
17. 17. Topology & errors • What about this error? Polyline Coordinates (6.2,3.3) (5.1,2.3) 1 1 2 3 (4.8, 1.3), (5.1, 2.3), 6.2, 3.3), (6.599, 1.4) (4.8, 1.3), (6.6, 1.4) (4.8, 1.3), (5.7, 0.2), (6.6, 1.4) (6.599,1.4) – What happens to node b? 2 (6.6,1.4) –What happens to polygons? (4.8,1.3) 3 (5.7,0.2) 11/7/2013 11:44 AM 17
18. 18. Topology & errors • Nonsensical vertex order (ambiguity). Polyline Coordinates (6.2,3.3) (5.1,2.3) 1 1 2 3 (4.8, 1.3), (6.2, 3.3), (5.1, 2.3), (6.6, 1.4) (4.8, 1.3), (6.6, 1.4) (4.8, 1.3), (5.7, 0.2), (6.6, 1.4) (6.6,1.4) 2 (4.8,1.3) 3 (5.7,0.2) 11/7/2013 11:46 AM 18
19. 19. Geometry errors • Geometrical errors – May cause topological errors, i.e., violations of topological relathionships – May only affect spatial data accuracy. • Accuracy standards govern ‘acceptable’ geometrical errors (see Book, section 7.2). • Topological errors caused by very small geometry errors can cause severe effects! 11/7/2013 10:31 AM 19
20. 20. ‘Small’ error example • An ambulance carrying a stroke victim needs to take the fastest route from A to B. • Two arcs in a road network do not ‘exactly connect’. • The car navigator takes a long route… 20 miles B A 11/7/2013 11:08 AM 3 miles 14 miles 20
21. 21. Line feature topological errors • Undershoot – Gap between lines. • Overshoot – Overextended lines. • Both errors cause dangling nodes. – Such nodes could be OK though (when?) 11/7/2013 11:16 AM 21
22. 22. Line feature topological errors • Pseudonodes – Nodes not located at end of arc or at line intersection. • Direction of line. – Important in the case of one way streets or rivers/streams. 11/7/2013 12:32 PM 22
23. 23. Polygon feature topological errors • Unclosed polygons. • Gap between two polygons. 11/7/2013 12:07 PM 23
24. 24. Polygon feature topological errors http://scapetoad.choros.ch/help/v11/a-cartogram-creation.php • Overlap: Sliver polygons • Gaps 11/7/2013 12:07 PM 24
25. 25. Point feature topological errors • Each polygon requires a label point to link it to its attribute data. • If a polygon has zero or several label points, an error occurs. × 11/7/2013 12:34 PM × 25
26. 26. Error sources • Due to digitization errors within a data layer – E.g., ‘Heads-down’ manual map digitization using a digitizing table. – ‘Heads-up’ manual digitization of CAD or scanned rasters on the screen. 11/7/2013 12:17 PM 26
27. 27. Error sources • Due to digitization or alignment errors between layers. • Don’t need to be of the same feature type. E.g.: – Adjoining layer edges don’t match. 11/7/2013 12:15 PM – Distortion between overlapping layers. – Overlap, undershoot, 27 overshoot.
28. 28. Building topology in ArcGIS • Create topology 1. Define participating feature classes 2. Specify the topology rules between feature classes 3. Set cluster tolerance 4. Identify the accuracy ranks of the coordinates in each feature class. 5. Validate topology 6. Find and fix errors or mark exceptions 7. Iterate 11/7/2013 1:06 PM 28
29. 29. ArcGIS topology rules • Topology rules allow you to define spatial relationships between features in a single feature class or subtype or between two feature classes or subtypes. • Topology rules allow you to define the spatial relationships that meet the needs of your data model. • A pdf poster with all rules: http://help.arcgis.com/en/arcgisdesktop/10.0 /help/001t/pdf/topology_rules_poster.pdf 11/7/2013 1:18 PM 29
30. 30. Examples: ArcGIS topology rules • Data theme: Parcels • Feature classes: – Parcel polygons, – Parcel boundaries (lines) – Parcel corners (points) • Some topology rules – Parcel polygons must not overlap. – Parcel polygon boundaries must be covered by Parcel boundary lines. – Parcel boundary endpoints must be covered by Parcel corner points. 11/7/2013 1:15 PM 30
31. 31. Identifies errors in Arc-Node topology 11/6/2013 11:21 PM 31
32. 32. Identifies polygon containment errors Single 11/6/2013 11:23 PM 32
33. 33. Identifies adjacency errors Single 11/6/2013 11:25 PM 33
34. 34. Identifies point in polygon containment errors Multiple 11/6/2013 11:25 PM 34
35. 35. Cluster tolerance • ArcGIS analyzes the coordinate locations of feature vertices among features in the same feature class as well as between the feature classes that participate in the topology. • Those that fall within a specified distance of one another are assumed to represent the same location and are assigned a common coordinate value (in other words, they are collocated). 11/7/2013 1:22 PM 35
36. 36. Cluster tolerance • A cluster tolerance is used to integrate vertices. – All vertices that are within the cluster tolerance may move slightly in the validation process. • The default cluster tolerance is based on the precision defined for the dataset. • The default cluster tolerance is 0.001 meters in real-world units. 11/7/2013 1:22 PM 36
37. 37. XY tolerance 11/7/2013 1:28 PM http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//006200000003000000.htm 37
38. 38. Z-tolerance • If necessary, a z-tolerance is also defined to distinguish whether or not the z-heights or elevations of vertices are within the tolerance of one another and should be clustered. 11/7/2013 1:28 PM 38
39. 39. Coordinate accuracy ranks • Which vertices should move? • Coordinate ranks ensure that reliably placed vertices are the anchor locations toward which less reliable vertices are moved. E.g., – RTK-GPS vertices would have high rank (1) – Standard GPS vertices would have lower rank (2, 3)… • Equally ranked vertices are geometrically averaged. 11/7/2013 1:32 PM 39
40. 40. Validate topology: algorithmic • Algorithmic clustering and cracking. • Insertion of common coordinate vertices into coincident features that share geometry. 11/7/2013 1:59 PM 40
41. 41. Validate topology: manual • Topology errors are violations of the rules that are initially stored as errors in an error log. • Error features record where topological errors were discovered during validation. – Example: Error features for the “must not have dangles” rule. 11/7/2013 1:43 PM 41
42. 42. Fixing errors • User intervention – Change vertex locations, trace new lines and replace old ones, etc. Fix! 11/7/2013 2:02 PM 42
43. 43. Mark exceptions • Certain errors may be acceptable, in which case the error features can be marked as exceptions. E.g.: 11/7/2013 1:45 PM 43
44. 44. Iterate • Topology validation is user driven. • Dirty areas are areas that have been edited, updated, or affected by the addition or deletion of features; they are maintained automatically by ArcGIS. • This allows selected parts, rather than the whole extent of the topology, to be validated. 11/7/2013 1:50 PM 44