1. Topology Relationships between features: Supposed to prevent:
Polygons can share parts of boundaries
Polylines can share endpoints
Supposed to prevent:
Gaps
Slivers
Overlaps

2. Topology
Wyoming
Colorado

3. Non-Topological
Wyoming
Colorado

4. Topological
Wyoming
Colorado

5. Problems without Topology
Common problems:
Slivers
Gaps
Caused by:
Reprojecting
Different sources
Editing or digitizing without snapping
Any tool that changes the values of coordinates in vector data

6. Integrating Datasets

7. Datasets Don’t Line Up!

8. Gaps and Slivers
Gaps and slivers appear when operating on datasets that have topological errors
Gap
Sliver

9. Levels of Topological Control
No Control
Snapping with manual editing
Topological Rules
GeoDatabase
Fixing large numbers of errors
Topological Structure
Only available in coverage's which are going away!

10. Snapping Turn on snapping! Task: Modify Edge Tool: Move first edge
“Snap” second edge to first

11. Editing Topological Structures
Open the topology tool bar
Task: Modify edge
Tool: Topology Edit Tool
Click on the vertex to edit
Drag to new location

12. New Polygons Turn on Snapping! Task: Auto Complete Polygon
Tool: Sketch Tool
Click to start polygon on existing edge
Click for each new point
Click back on old boundary to complete
Note: Remember to check task, target, and folder we are editing in

13. Topologies in GeoDatabases
Live within Feature Dataset
Can have multiple per dataset
Can affect multiple feature classes
Each feature class can be in only one topology
Note: All feature classes in a feature dataset must have the same spatial reference

14. Fixing Many Topological Errors
The following procedures will allow you to remove large numbers of overlaps and gaps when merging datasets
The instruction must be followed closely
Notes:
You cannot create or edit topologies when ArcMap is open if it has contained the layers
You cannot use ArcToolbox functions during an edit session (bad things happen)
You cannot delete a Feature Class that is participating in a Topological Class
Only one topology problem can be removed at a time

15. Fixing lots of overlaps
Merge the shapes into a single Feature Class in a GeoDatabase using ArcToolbox -> Data Management Tools -> General -> Merge
Create a new Topology for the Feature Class with the rule “Must not overlap”
Validate the Topology
Load the Topology into ArcMap with it’s associated feature class
Start an edit session
Click on the Map Topology icon in the Topology task bar
Select the merged layer and click OK
Select the layer in the Topology tool bar (it will have been reset)
Select the Fix Topology Error Tool
Select the overlaps
Right click on one of the selected overlaps and select “Create Feature…”
Open the attribute table and select the new features
Set an attribute in the new features to match the same value in the feature they should be merged into (i.e. set the state name to Minnesota)
Make sure you clear the selection AND stop the editing session
Select Toolbox -> Generalization -> Dissolve and dissolve the topology features into one of the main features

16. Fixing lots of gaps
Follow the instructions for overlaps except select the rule “Must not have gaps”
When selecting features to create only select the gaps that should be filled (not the outside of the boundaries or lakes between shapes)
You will need to close ArcMap to create the topology if ArcMap has had the Features Classes loaded into it

17. Additional Slides

18. GIS Lessons Don’t give up! Keep trying different approaches
Write down what works and what to avoid
Break processes into simple steps
Look for folks who can help
Arc is just a tool – it’s up to you to figure out how to use it to get the task at hand completed
PS: Arc is not the only tool out there

19. Topology Cluster Tolerance Relative ranks for each feature class Rules
Distance that vertices and line segments will be snapped together
Make it about 1/10 the accuracy of the data
Relative ranks for each feature class
Features in classes with larger ranks will be snapped to features in classes with lower ranks
Rules
Must not overlap
Must be contained within

20. Validating Topology Dirty areas – have not been checked
Validate Topology
Vertices and boundaries are snapped
Checks data against rules
Marks errors
User can:
Fix errors
Mark as exceptions

21. Fixing Topological Errors
Gap:
Create Feature
Merge features
Overlap:
Merge...
For lots:
Open "Error Inspector“
Search for errors
Set a length/area ratio
Anything over 0.2 is probably a sliver (and have higher IDs)
Create features for all
Data Management Tools -> Generalization -> Eliminate
Based on largest shared border

22. 2 Levels of Topology Within one layer Between multiple layers
States share boundary
Streams and roads should meet at ends
Between multiple layers
National Park and Forest share part of a boundary
Interstates join with highway
Park boundary follows the center of a river

23. Advantages Makes data consistent
Editing can be done on shared geometries
“Snapping” does the same thing but requires moving both points.