1. NR 422: Topology Jim Graham Fall 2010 See: http://www.esri.com/library/whitepapers/pdfs/ge odatabase-topology.pdf

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

3. Topology Wyoming Colorado

4. Topological Wyoming Colorado

5. Topological Colorado Wyoming

6. 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

7. Arc Data Storage ShapefilesCoveragesGeoDatabases Collection of datasets Folder of Shapefiles ArcInfo Workspace GeoDatabase DatasetFolder of Shapefiles CoverageFeature dataset Collection of features ShapefileFeature class FeaturesPoint, Multipoint, Polygon, Polyline Point, Polygon, Polyline, etc. Point, Multipoint, Polygon, Polyline, Network Annotation

8. Data Storage Tradeoffs Coverages –Data is tightly defined GeoDatabases –Data is loosely defined –Special editing tools –Addition of tolerances and rules –Run validations and fix or except errors Shapefiles –Data is loosely defined –Can use topology tools!

9. 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!

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

13. 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

14. Introduction to Databases Tables like Excel Relationships between the tables XYDBHType -105.3421040.324501.2PP -105.3034539.93954.92LP -105.2321040.02132.82LP -105.3429240.128431.1LP AbrveNameBFCost PPPonderosa Pine.98 LPLodge Pole1.10

15. GeoDatabases Why? –Puts an entire collection into one place –Easy to distribute data –Make it relatively easy to edit and maintain topologies Why not? –Complex, buggy, slow Advice: –Use them when told to or need topology –Otherwise, use standard files

16. Types of GeoDatabases Personal GeoDatabase: –Microsoft Access file (*.mdb) File GeoDatabase: –Folder There are non-ESRI spatial databases: –SQL Server 2008 –Oracle –PostgreSQL w/PostGIS

17. GeoDatabase Feature dataset (folder) Feature class (Shapefile) Relationship class Table Raster class

18. 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

19. 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

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 –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. 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

24. Fixing lots of overlaps 1.Merge the shapes into a single Feature Class in a GeoDatabase using ArcToolbox -> Data Management Tools -> General -> Merge 2.Create a new Topology for the Feature Class with the rule “Must not overlap” 3.Validate the Topology 4.Load the Topology into ArcMap with it’s associated feature class 5.Start an edit session 6.Click on the Map Topology icon in the Topology task bar 7.Select the merged layer and click OK 8.Select the layer in the Topology tool bar (it will have been reset) 9.Select the Fix Topology Error Tool 10.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 features 13.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 session 15.Select Toolbox -> Generalization -> Dissolve and dissolve the topology features into one of the main features

25. 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

26. 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