Topology

• Relationships between features:– Polygons can share parts of boundaries– Polylines can share endpoints

• Supposed to prevent:– Gaps– Slivers– Overlaps

• http://www.esri.com/library/whitepapers/pdfs/geodatabase-topology.pdf

Topology

Wyoming

Colorado

Non-Topological

Wyoming

Colorado

Topological

Colorado

Wyoming

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

Integrating Datasets

Datasets Don’t Line Up!

Gaps and Slivers

• Gaps and slivers appear when operating on datasets that have topological errors

GapSliver

Levels of Topological Control

• No Control

• Snapping with manual editing

• Topological Rules– GeoDatabase

• Fixing large numbers of errors– GeoDatabase

• Topological Structure– Only available in coverage's which are

going away!

Snapping

• Turn on snapping!

• Task: Modify Edge

• Tool:

• Move first edge

• “Snap” second edge to first

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

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

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

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

Fixing lots of overlaps1. Merge the shapes into a single Feature Class in a GeoDatabase

using ArcToolbox -> Data Management Tools -> General -> Merge2. Create a new Topology for the Feature Class with the rule “Must not

overlap”3. Validate the Topology4. Load the Topology into ArcMap with it’s associated feature class5. Start an edit session6. Click on the Map Topology icon in the Topology task bar7. Select the merged layer and click OK8. Select the layer in the Topology tool bar (it will have been reset)9. Select the Fix Topology Error Tool10. Select the overlaps 11. Right click on one of the selected overlaps and select “Create

Feature…”12. Open the attribute table and select the new features13. 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)

14. Make sure you clear the selection AND stop the editing session15. Select Toolbox -> Generalization -> Dissolve and dissolve the

topology features into one of the main features

Fixing lots of gaps

1. Follow the instructions for overlaps except select the rule “Must not have gaps”

2. When selecting features to create only select the gaps that should be filled (not the outside of the boundaries or lakes between shapes)

3. You will need to close ArcMap to create the topology if ArcMap has had the Features Classes loaded into it

Additional Slides

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

Topology

• Cluster Tolerance– 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

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

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

2 Levels of Topology

• Within one layer– 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

Advantages

• Makes data consistent

• Editing can be done on shared geometries

• “Snapping” does the same thing but requires moving both points.