Split Polygon Using GeoEdit Tool

1. Split Polygon Using GeoEdit Tool BJ Smith – 1 April 2009 – 4 August 2009

2. Split Polygon Using the NHDGeoEdit Tool This function may be used to split an existing polygon into two separate polygons. This may be required to correct a WaterArea Topology issue. Or this technique may be used to split an existing polygon at or near a subbasin boundary. In either case, you will be using the NHD GeoEdit Tool to complete the effort, which will edit the appropriate feature tables, and track all edits in the ‘Status’ table. All user’s MUST exercise care when splitting Polygons, simply because USGS has observed many user’s splitting polygons, then deleting the portion of the polygon outside of their particular area of interest (subbasin or subregion). This action has created missing polygon features in the National Geodatabase.

3. Open ArcMap. Load NHD data as required. Symbolize NHD data as required. Load NHD GeoEdit Tool, if required. Set up Metadata Session, this is required (if you intend to use the NHD GeoEdit Tool)! Remember: Your data must be ‘projected’ to use the GeoEdit Tool!

4. Use standard ArcMap tools to zoom to area of interest.

5. Select the ‘Selection’ Tab in the Table of Content. Set Selectable Layers to NHDAreas only (for this example). Use standard ArcMap tools to examine the issue. In this case, notice two overlapping polygons, both NHDAreas representing the same StreamRiver.

6. Notice how ObjectID = 39 is the polygon that only extends over the east portion, while ObjectID = 38 extends over the entire area. The goal will be to edit BOTH polygons, so they join together at the subbasin boundary, as opposed to overlapping. Further, we shall keep the east portion of ObjectID = 39, and the west portion of ObjectID = 38, which will later be edge matched at or near the subbasin boundary.

7. Hint: Prior to starting any edit using the USGS GeoEdit Tool, a user should use standard ArcMap tools to analyze the issue, decide exactly the approach to take, then use the USGS GeoEdit Tool to complete the process. User’s should avoid mixing the use of the standard ArcMap tools and the USGS GeoEdit Tool, as much as possible. In this case, we used standard ArcMap tools to zoom and pan. We determined exactly the process we wish to use to eliminate the overlapping polygons. Now, we shall use the GeoEdit Tool to complete the edit.

8. In this case, we have two (2) NHDArea’s that overlap. This error was probably created when the original data was created. One person collected one polygon from the subbasin on the right, while a second person collected the second polygon from the subbasin on the left. The result is overlapping polygons. This edit will split the overlapping polygons, then edge match at or near the subbasin boundary.

9. Open the ‘NHDArea Tool’ Select ‘Modify NHDArea Geometry’ Select ‘Select NHDArea to Edit’ Use the mouse to select the right (east) polygon. Notice you only have one NHDArea feature selected!

10. Select ‘Split NHDArea Geometry’ tool. Use mouse to draw a straight line across the selected polygon, using the subbasin boundary as a guide. Remember to select the ‘F2’ Key (on your keyboard) to ‘Finish Sketch’ to complete the edit. The selected polygon should split into two separate polygons.

11. You should see three (3) polygons, the original ObjectID = 38, which was not modified during this step. ObjectID = 38 maintained the same ComID = 142772924. Notice, ObjectID = 39 has been split into two (2) polygons, ObjectID = 49 and ObjectID = 39. Notice, ObjectID = 39 maintained the same ComID = 121531218. Notice, ObjectID = 49 has a new ComID = -201

12. Open the “NHDStatus’ table. Notice the ‘Add Metadata’ record, ComID = -200. This was added in Slide 3, ‘Setup Metadata Session’. Notice the ‘Add Feature’ record, ComID = -201. This is the new, split polygon. Notice the ‘Modify Feature Geometry’ record, ComID = 121531218. This record will modify the geometry for the original polygon, ObjectID = 39, indicating the split feature. The ‘Modify Feature Attribute’ record for ComID = 121531218 will modify the attribute fields for AreaSqKm, Shape_Length, and Shape_Area.

13. Unselect ObjectID = 38 by placing your cursor over the gray box to the left of the record ObjectID = 38, right mouse click, and select ‘Unselect Highlighted’ from the drop-down menu. The highlighted record should be removed from the ‘Selected’ table. Do the same for ObjectID = 49. You should only have one selected record ObjectID = 39 in the ‘Selected’ table.

14. Use the NHDGeoEdit Tool to delete the west portion of the split polygon ObjectID = 39. Open the NHDArea Tools. Select the ‘Delete NHDArea’ tool. Select ‘Select NHDArea to Delete’. Use mouse to select the desired feature. You will probably select more than the desired polygon. Use above technique to unselect other records. Use the ‘Selection’ tab to verify only one NHDArea record is selected. Verify no other feature is selected! Delete the selected NHDArea feature using The ‘Delete NHDArea feature(s)’ button. Save by selecting ‘Save NHDArea Delete’.

15. Open ‘NHDStatus’ Table Notice the new record, ‘Delete Feature’ for ComID = 121531218. This deleted the west portion of original polygon ObjectID = 39 You should still have the east portion Of the original polygon ComID = 39 (now ObjectID = 49).

16. Use the NHDGeoEdit tool to split Polygon ObjectID = 38. We shall split the polygon at the Subbasin boundary, then delete the east portion of ObjectID = 38. Select the ‘NHDArea Tool’ Select ‘Modify NHDArea Geometry’. Select ‘Select NHDArea to Edit’. Use mouse to select ObjectID = 38. This should be the only polygon selected.

17. Select the ‘Split NHDArea Geometry’ tool. Use mouse to draw a straight line across the selected polygon, using the subbasin boundary as a guide. Remember to select the ‘F2’ key to finish the edit. The selected polygon should split into two separate polygons.

18. Open the “NHDStatus’ table. Notice the ‘Add Feature’ record, ComID = -202. This is the new, split polygon. Notice the ‘Modify Feature Geometry’ record, ComID = 142772924. This record will modify the geometry for the original polygon, ObjectID = 38, indicating the split feature. The ‘Modify Feature Attribute’ record for ComID = 142772924 will modify the attribute fields for AreaSqKm, Shape_Length, and Shape_Area.

19. We should preserve the west portion of this polygon split. Unselect ObjectID = 38. You should only have one (1) NHDArea selected, ObjectID = 50. This would be the east portion of the split polygon. This is the portion that will be deleted.

20. Use the NHDGeoEdit Tool to delete the Unwanted, east portion of the split polygon. Select the ‘NHDArea Tools’ Select the ‘Delete NHDArea’ tool. Select the ‘Select NHDArea to Delete’ button. The polygon, ObjectID = 50, should be deleted! Save the edit by selecting ‘Save NHDArea Delete’ button.

21. Open ‘NHDStatus’ Table Notice the new record, ‘Delete Feature’ for ComID = -202. This deleted the east portion of original polygon ObjectID = 38 You should still have the west portion of the original polygon ComID = 38.

22. We are now down to two (2) polygons, ObjectID = 49 and ObjectID = 38. The final step in the polygon edit process will match the edges of the two split polygons.

23. Use the NHD GeoEdit tool to ‘Modify Geometry’ of ObjectID = 38, so ObjectID 38 Matches polygon with ObjectID = 49. Select the ‘NHDArea Tools’. Select ‘Modify NHDArea Geometry’ tool. Select ‘Select NHDArea to Edit’ button. Select ‘Edit NHDArea Geometry’ button. Notice all vertices are displayed.

24. Set your Snap Options: Select the ‘Editor’ button. Select ‘Snapping ….’ from the Drop-down menu. In the Snap window, set snapping to ‘NHDArea’ and ‘Vertex’.

25. Use the ‘Edit Tool’ to move two (2) vertices to make the two separate polygons edge match. You should notice the vertices ‘Snap’ to the adjoining NHDArea vertices. Select the ‘F2’ key to ‘Finish Sketch’. Save the edit by selecting the ‘Save NHDArea Edits’ button.

26. Open the ‘Status’ table. One new record is indicating the ‘Modify Feature Geometry’ for the last edit.

27. The final part of this edit would be to ‘Modify NHDFlowline Geometry’ to edit the artificial path to match the polygon edits. In this case, one NHDFlowline feature (ComID = 142771647) would be split into two separate artificial paths, one assigned to one polygon, the other assigned to the second polygon.

28. The purpose of this portion of the PowerPoint is to demonstrate a method (not the only method) of clipping two polygons so they join together without a gap or overlap, as opposed to an existing polygon overlap. The above techniques showed a user how to “split” two overlapping polygons, and delete the unwanted portion of the split polygon. The split and the delete were “tracked” in the ‘Status’ table. The following technique will describe how to use a Vertex Editing method to accomplish a similar goal.

29. Cut Polygon using Vertex Edit Method Bill J Smith - 2 July 2008 - 4 August 2009

30. The purpose of this PowerPoint is to demonstrate a method (not the only method) of clipping two polygons so they join together without a gap or overlap, as opposed to an existing polygon overlap. In this case, we have two overlapping NHDAreas (ComID = 113925023 and ComID = 142772901). Both are 2-dimensional Canal Ditch features representing the same Canal Ditch. This 2-dimensional topology error was found during the 2-d topology check completed on the NHDArea and NHDWaterbody feature classes based on the rule ‘No Overlapping NHDAreas’ allowed. Using the ‘Error Inspector’ we ‘zoomed to’ this error. At this point in the editing process, the NHD subregion has been reprojected from Geographic to Albers Equal Area Conic, the ‘Setup NHD Edit Database’ step has been completed, the ‘Task Assistant’ and ‘NHD GeoEdit Tool’ have been opened, a metadata session has been initialized, and the NHDProcessingParameters table has been modified.

31. A quick analysis of the issue indicates both polygons (NHDAreas) are the same feature collected at two separate times by two separate compilers, one for subbasin 18020109 (the northwest subbasin), the second for subbasin 18040005 (the southeast subbasin). Notice ObjectID = 2 (ComID = 113925023) traverses the entire length of the overlap.

32. While ObjectID = 32 (ComID = 142772901) only traverses the southern portion of the overlap.

33. Also notice, at the southern end of the overlap, both features share the exact same spatial area.

34. One method (not the only method) of resolving this issue might be to split both polygons at the subbasin boundary, marking one half of each split as a ‘Modify Feature Geometry’ and the other half of each split as a ‘Delete Feature’. In this example, we shall use the ‘NHDGeoEdit Tool’ along with the ‘Sketch Properties’ window to complete a vertex edit of the two polygons. We shall ‘Modify Feature Geometry’ for ComID = 113925023 to remove any vertice south of the subbasin boundary, and we shall ‘Modify Feature Geometry’ for ComID = 142772901 to remove any vertice north of the subbasin boundary. In either case, you must remember to go back and ‘Modify Feature Geometry’ for any affected artificial path associated with the edited NHDArea feature.

35. Remember, a Metadata Session has already been initialized. You have ‘Set Selectable Layers’ to NHDAreas only! All selected features have been cleared!

36. Open the ‘NHDArea Tools’, Open the ‘Modify NHDArea Geometry’ tool.

37. Select ‘Select NHDArea to Edit’ button, then select both NHDAreas, using the ArcMap ‘Select Feature’ button. Notice two NHDAreas are currently selected. You can tell by the ‘NHDAREA (2)’ seen in the Table of Contents, and by noticing two features are selected in the ‘Selected Attributes of NHDArea’ table.

38. Open and use the ‘Attributes’ window to unselect ObjectID = 2. Right mouse click on ObjectID = 2, then select ‘Unselect’ from dropdown menu.

39. Notice, feature ObjectID = 2 was removed from the ‘Attributes’ window, the number of selected features in the Table of Content changed to one (1), but the number of selected features in the ‘Selected Attributes of NHDArea’ table remained at two (2).

40. By selecting the ‘All’ button in the NHDArea table, then selecting the ‘Selected’ button in the NHDArea table, you will see the number of selected records change to one (1) selected record, should be ObjectID = 32.

41. You may accomplish the same goal by placing the cursor over the right gray box in the NHDArea Attribute table, right mouse click, and select “Unselected’ from the drop down menu.

42. Select the ‘Edit NHDArea Geometry’ button. Notice the selected feature (ObjectID=32) changed to show all vertices.

43. Open the ‘Sketch Properties’ window by selecting the ‘Sketch Properties’ button.

44. Zoom into the area north of the subbasin boundary. Notice the vertices displayed? Notice the one (1) vertice displayed in white, as opposed to green?

45. Randomly select vertices in the ‘Edit Sketch Properties window. Once selected, notice the selected vetices are displayed in white. I have selected vertices 0 – 11 by selecting vertice 1, holding shift key and selecting vertice 11. They are north of the subbasin boundary and will be deleted.

46. To delete selected records from ‘Edit Sketch Properties’ window, hold curser over Selected records, right mouse click and select ‘delete’ from drop down menu. Make sure to select the ‘Finish Sketch’ button after the delete!

47. Select other records (vertices 48 – 58) to delete all vertices north of subbasin boundary. Make sure to select the ‘Finish Sketch’ button after the delete!

48. Notice all vertices north of the subbasin boundary have been deleted.

49. I decided to add two vertices to better shape the NHDArea.

50. Moved two new vertices to better shape the final polygon.