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

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.

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.

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

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

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

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

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.

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

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

Select overlapping features using the ‘Select Features’ button.

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

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

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

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

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 , therefore the feature should have ReachCode in Subbasin

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

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

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!

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 , therefore the feature should have ReachCode in Subbasin Use the NHDGeoEdit Tool to delete the one selected feature, ObjectID = 4472.

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

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.

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.

Overlap is between features ObjectID = 927 and ObjectID = 927 has ReachCode based on Subbasin , ObjectID = 3919 has ReachCode based on Subbasin

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

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

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

Overlap is between features ObjectID = 4493 and ObjectID = 4493 has ReachCode based on Subbasin , ObjectID = 5798 has ReachCode based on Subbasin

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

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

Overlap is between features ObjectID = 7 and ObjectID = 7 has ReachCode based on Subbasin , ObjectID = 5115 has ReachCode based on Subbasin

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

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

Overlap is between features ObjectID = 3030 and ObjectID = 3030 has ReachCode based on Subbasin , ObjectID = 2448 has ReachCode based on Subbasin

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

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

Overlap is between features ObjectID = 2448 and ObjectID = 6299 has ReachCode based on Subbasin , ObjectID = 2448 has ReachCode based on Subbasin

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

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

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

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

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.

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

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

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

Select overlapping features using the ‘Select Features’ button.

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

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 , while someone else compiled the feature ObjectID = 45 in subbasin

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

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.

I elected to delete ObjectID = 29 to eliminate the overlap. Use NHDGeoEdit Tool to delete ObjectID = 29. Remember to Validate Topology in Currrent Extent to verify you have resolved the issue.

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 = 966 and 36. Notice the error is an overlap issue between an NHDWaterbody (ObjectID = 966), and an NHDArea (ObjectID = 36).

Select the ‘Selection’ tab and set both NHDArea and NHDWaterbody as selectable layers by placing a checkmark in front of NHDArea and NHDWaterbody.

There is one selected NHDWaterbody and two selected NHDAreas. This overlap is between NHDWaterbody (966) and NHDArea (36).

ObjectID 966 is a Lake/pond, while ObjectID 36 is a Submerged Stream. This is a legal overlap! A submerged stream is allowed to overlap a Lake/Pond, in fact there must be a Lake/ Pond under the submerged stream.

You should mark this error as an ‘Exception’. In the ‘Error Inspector’, highlight the error you wish to mark as an exception with a left mouse click, then right mouse click and select ‘Mark as Exception’ from the drop down menu. This record will be moved to the Exceptions list and removed from the Error list.

To review the Exception list, uncheck ‘Errors’ in the Error Inspector, and place a checkmark in front of ‘Exceptions’, then select the ‘Search Now’ button. All exceptions will be displayed in the Error Inspector.

In the Error Inspector, uncheck ‘Exceptions’ and check ‘Errors’, then select the ‘Search Now’ button. This will display all remaining errors. Continue through all errors listed in the Error Inspector and attempt to resolve all errors, or mark legal errors as Exceptions.

The following will describe a process used to merge two overlapping features. In other words, you may find a small overlap between two separate polygons, and after review, you may decide to merge the overlapping error into one of the polygon, removing the overlap.

Notice the overlap between a Lake/Pond (shown in blue) and an Inundation Area (shown with hash marks). It is good practice to merge the overlap error into the predominant feature, in this case the Lake/Pond is more definable than an Inundation Area. The overlap is displayed in a salmon color.

NHD Geo Edit Tools: NHDAREA TOOLS, MODIFY NHDAREA GEOMETRY, Select NHDArea to Edit, Edit NHDArea Geometry

Open the Error Inspector: Right click over error, then select ‘Merge…’ from the drop down menu. Highlight the feature that will NOT be modified Select the ‘OK’ button.

NHD Geo Edit Tools: Apply NHDArea Rules, Save NHDArea Edits The overlap area is now removed from Inundation Area.

Revalidate Topology in Current Extent