Resolving Topology Errors

1. Resolving Topology Errors Created by: BJ Smith 19 February 2008 Revised: 15 May 2009

2. Resolving Topology Errors NHD was created by Subbasins, then later several Subbasins were joined to create a Subregion. In some cases this might lead to the same feature being collected in two Subbasins, which leads to ‘Topology Errors’. After creating the ‘Topology Rules’ in ArcCataog, then validating the ‘Topology Rules’ in ArcMap, these topology errors will be listed in the ‘Error Inspector’. The NHD user/editor should ‘zoom to’ each error listed in the ‘Error Inspector’, analyze the issue, determine the appropriate resolution, then fix each error.

3. Special Notes: Topology rules should be created in ArcCatalog, but validated in ArcMap. If for some reason your ‘Topology’ window is grayed out (not active), in ArcMap, you must use the ‘Add Data’ button to add your topology layers you created in ArcCatalog. These topology layers can be found in the Hydrography feature dataset in the geodatabase file you are editing.

4. Open ArcMap Load personal geodatabase (.mdb) Start Editing, by selecting the ‘Editor’ button, then selecting ‘Start Editing’ from the drop down menu.

5. Locate ‘Topology’ tool in ArcMap main menu area. If tool is not present in ArcMap main menu area, see ‘Add Tools’. Verify ‘FlowLine’ Topology is present in ‘Topology’ window. To validate Topology in ArcMap, select ‘Validate Entire Topology’ button. ‘Yes’, validate the full extent.

6. Open ‘Error Inspector’ by selecting the ‘Error Inspector’ button. The ‘Error Inspector’ window will open.

7. Uncheck ‘Visible Extent only’ Left Click on ‘Search Now’ ‘FlowLine’ Topology errors are displayed for entire dataset

8. Left click on first error in list. Error will be highlighted in blue. Right click on highlighted error, and select ‘Zoom To’ from drop down menu.

9. NHDFlowline Topology error (overlap) will be shown in black. In this case, an overlap of two vectors, ObjectID = 1806 and 4472.

10. Set ‘Selectable Layers’ to NHDFlowline only by selecting the ‘Selection’ tab, then verifying only NHDFlowline is checked.

11. Select overlapping features using the ‘Select Features’ button.

12. Zoom to Selected Features using ‘Zoom to Selected Feature’ button. ObjectID = 4472 ObjectID = 1806 Notice the two overlapping features (the small black dot at the t-intersection is the overlap).

13. Open the ‘NHDFlowline’ table and display the selected records. Notice, ObjectID = 1806 has ReachCode in Subbasin 18040012, while ObjectID = 4472 has ReachCode in Subbasin 18040005. ObjectID = 4472 Area of overlap ObjectID = 1806

14. Notice, ObjectID = 4472 has a ReachCode based on Subbasin 18040005 Notice, ObjectID = 4472 overlaps at the intersection.

15. Using the ‘Select Feature’ button, select two features to the north of the intersection, ObjectID = 4472 and 4650. Notice, ObjectID = 4472 has ReachCode in Subbasin 18040005, while ObjectID = 4650 has ReachCode in Subbasin 18040012. 18040012 18040005

16. Because Subbasin boundary (solid black line) is ‘Non-Certified’, a user could use either ReachCode. Notice, the majority of the selected feature is in Subbasin 18040012, therefore the feature should have ReachCode in Subbasin 18040012. 18040012 18040005

17. I am electing to delete ObjectID = 4472, ReachCode = 18040005. This decision resolves two issues: a) will eliminate topology error between ObjectIDs 4472 and 4650, placing feature in Subbasin 18040012, based on most of ObjectID = 4472 is in subbasin 18040012, and b) will eliminate initial topology error between ObjectIDs 1806 and 4472. 18040012 18040005 Use NHDGeoEdit Tools to delete ObjectID = 4472.

18. In many cases, an editor/user will have two features (in this case arcs) selected. In order to delete only one of the two selected features the editor/user must unselect one of the selected features. 18040012 18040005

19. 18040012 18040005 To unselect one of the two selected features, hold the cursor over the gray box to the left of the feature to be unselected, right mouse click, and select ‘Unselect’ from the drop down menu. Care should be exercised to unselect the correct feature!

20. This action will unselect the feature to remain, leaving only the feature to be deleted as selected. Notice, the majority of the selected feature is in Subbasin 18040012, therefore the feature should have ReachCode in Subbasin 18040012. 18040012 Use the NHDGeoEdit Tool to delete the one selected feature, ObjectID = 4472. 18040005

21. Validate ‘Topology in Current Extent’ by selecting ‘Validate Topology in Current Extent’ button in ‘Topology’ window.

22. Open the ‘Error Inspector’ Set ‘Show’ to ‘NHDFlowline-Must Not Overlap’ Place Checkmark in front of ‘Visible Extent Only’ Select the ‘Search Now’ button. The overlap error between ObjectID 1806 and ObjectID 4472 has been resolved! Go to the next ‘FlowLine’ topology error.

23. Open Error Inspector window, uncheck ‘Visible Extent Only’, Select ‘Search Now’ button. Error list should appear in Error Inspector’ window. Select and zoom to second error, ObjectID = 927 and 3919.

24. Overlap is between features ObjectID = 927 and 3919. ObjectID = 927 has ReachCode based on Subbasin 18040012, ObjectID = 3919 has ReachCode based on Subbasin 18040005. 18040005 18040012

25. Zooming out indicates the feature correctly belongs in Subbasin 18040005, and subbasin boundary will probably be modified when Certified to move north of headwater drains. 18040005 18040012 Use NHDGeoEdit Tools to delete ObjectID = 927.

26. Remember to ‘validate Topology in Current Extent’ to verify the topology error has been eliminated.

27. Open Error Inspector window, select and zoom to third error, ObjectID = 4493 and 5798.

28. Overlap is between features ObjectID = 4493 and 5798. ObjectID = 4493 has ReachCode based on Subbasin 18040012, ObjectID = 5798 has ReachCode based on Subbasin 18040005.

29. Zooming out indicates the feature correctly belongs in Subbasin 18040005, since entire feature resides in Subbasin 18040005. 18040012 18040005 Use NHDGeoEdit Tools to delete ObjectID = 4493.

30. Open Error Inspector window, select and zoom to fourth error, ObjectID = 7 and 5115.

31. Overlap is between features ObjectID = 7 and 5115. ObjectID = 7 has ReachCode based on Subbasin 18040012, ObjectID = 5115 has ReachCode based on Subbasin 18040005.

32. Zooming out indicates the feature correctly belongs in Subbasin 18040005, based on end vertice being in Subbasin 18040005. 18040012 18040005 Use NHDGeoEdit Tools to delete ObjectID = 7. Please note this decision will also remove the next topology error, between ObjectID 7 and 871.

33. Open Error Inspector window, select and zoom to next error, ObjectID = 2448 and 3030.

34. Overlap is between features ObjectID = 3030 and 2448. ObjectID = 3030 has ReachCode based on Subbasin 18040012, ObjectID = 2448 has ReachCode based on Subbasin 18040005.

35. Zooming out indicates the feature correctly belongs in Subbasin 18040005, and subbasin boundary will probably be modified when Certified. 18040013 18040012 18040005 Use NHDGeoEdit Tools to delete ObjectID = 3030.

36. Open Error Inspector window, select and zoom to next error, ObjectID = 2448 and 6299.

37. Overlap is between features ObjectID = 2448 and 6299. ObjectID = 6299 has ReachCode based on Subbasin 18040012, ObjectID = 2448 has ReachCode based on Subbasin 18040005.

38. Zooming out indicates the feature correctly belongs in Subbasin 18040005, and subbasin boundary will probably be modified when Certified. Use NHDGeoEdit Tools to delete ObjectID = 6299.

39. All NHDFlowline errors have been corrected. Change ‘FlowLine’ topology to ‘WaterArea’ topology, and complete topology checks for WaterArea. Change ‘Topology’ from ‘FlowLine’ to ‘WaterArea’ topology: To validate Topology in ArcMap, select ‘Validate Entire Topology’ button. ‘Yes’, validate the full extent.

40. Open ‘Error Inspector’ by selecting the ‘Error Inspector’ button. The ‘Error Inspector’ window will open.

41. Uncheck ‘Visible Extent only’ Left Click on ‘Search Now’ ‘WaterArea’ Topology errors are displayed for entire dataset

42. Left click on first error in list. Error will be highlighted in blue. Notice, the overlap is between two NHDArea features, ObjectID = 29 and 45.

43. Right click on highlighted error, and select ‘Zoom To’ from drop down menu.

44. ArcMap will zoom to the overlapping features, in this case ObjectID = 29 and 45, both NHDArea features.

45. Set ‘Selectable Layers’ to NHDArea only by selecting the ‘Selection’ tab, then verifying only NHDArea is checked.

46. Select overlapping features using the ‘Select Features’ button.

47. Zoom to Selected Features using ‘Zoom to Selected Feature’ button. Notice the two overlapping NHDArea features (shown in black).

48. Open the ‘NHDArea’ table and display the selected records. It appears this overlap is a result of subbasin collection of the data. In other words, someone compiled the feature ObjectID = 29 in subbasin 18040005, while someone else compiled the feature ObjectID = 45 in subbasin 18040003.

49. By highlighting ObjectID = 29 in the NHDArea table you can see the feature in yellow in the ArcMap display.

50. By highlighting ObjectID = 45 in the NHDArea table you can see the feature in yellow in the ArcMap display. Notice both features are an exact overlap, except ObjectID 45 has much additional data.