How to unmerge polygons in ArcMap?

In ArcMap, "unmerging" polygons typically refers to two main actions: separating a single multipart feature into individual features, or splitting a single, continuous polygon into two or more distinct polygons. The most direct method for unmerging multipart features is using the Explode tool.

Separating Multipart Polygons (Unmerging with Explode Tool)

If you have a single feature that visually appears as multiple separate polygons (e.g., islands representing one country, or two adjacent polygons that were previously merged into a single record), this is known as a multipart feature. The Explode tool allows you to convert each part into an independent feature with its own record in the attribute table.

What is a Multipart Feature?

A multipart feature is a single entry in your attribute table that corresponds to multiple geometric shapes on the map. For example, if you merge two adjacent polygons, they might become one multipart feature, or if you have a country composed of many islands, all islands could be represented as a single multipart polygon feature. Unmerging, in this context, means breaking these parts back into individual, distinct polygon features.

Step-by-Step Guide to Explode Polygons

Follow these instructions to separate multipart polygons into individual features:

Start an Editing Session:
- Navigate to the Editor Toolbar. If it's not visible, go to Customize > Toolbars and check Editor.
- Click Editor > Start Editing. Select the data source (folder or geodatabase) containing the polygon layer you wish to edit.
Select the Multipart Feature:
- On the Editor toolbar, locate and click the Edit Tool (often represented by a black arrow).
- Click directly on the multipart polygon feature you want to separate. It will highlight in cyan.
Access the Advanced Editing Toolbar:
- If the Advanced Editing toolbar is not visible, go to Customize > Toolbars and check Advanced Editing.
Execute the Explode Command:
- On the Advanced Editing toolbar, click the Explode tool. This tool typically looks like a polygon breaking into smaller parts.
- ArcMap will immediately convert each part of the selected multipart feature into a new, individual feature. Each new feature will inherit the attributes of the original multipart feature.
Save and Stop Editing:
- After exploding the features, click Editor > Save Edits to apply the changes permanently.
- Click Editor > Stop Editing to exit the editing session.

Each formerly connected or grouped part will now be an independent polygon feature, selectable and editable on its own, with its own unique record in the attribute table.

Splitting a Single Polygon into Multiple Parts

Sometimes, "unmerge" might refer to dividing a single, continuous polygon (not a multipart feature) into two or more separate polygons. For this, you would use the Cut Polygons Tool.

Using the Cut Polygons Tool

The Cut Polygons tool allows you to draw a line across an existing polygon, effectively splitting it into two or more new polygons.

Start an Editing Session: Just like with the Explode tool, start an editing session for your polygon layer.
Select the Polygon: Use the Edit Tool to select the single polygon you wish to split.
Activate the Cut Polygons Tool: On the Editor toolbar, click the Cut Polygons Tool (often depicted as scissors cutting a polygon).
Draw the Cut Line:
- Click inside the selected polygon to start drawing your cut line.
- Draw the line across the polygon to define where you want to split it.
- Double-click to finish the line.
Outcome: The original polygon will be divided into two or more new polygons along the line you drew. Each new polygon will inherit the attributes of the original feature.
Save and Stop Editing: Remember to save your edits and stop the editing session.

Understanding Key Concepts: Merge vs. Explode vs. Dissolve

To clarify the "unmerging" process, it's helpful to understand related editing operations in ArcMap:

Action	Description	Effect on Polygons	Use Case
Merge	Combines two or more selected features of the same layer into one feature.	Can create a multipart feature or combine geometries into a single, continuous polygon.	Combining adjacent land parcels into a single property.
Explode	Separates a single multipart feature into individual features.	Each part of the multipart feature becomes a new, independent polygon.	Breaking an island archipelago (one feature) into individual island features.
Dissolve	Combines features based on common attribute values or spatial proximity.	Simplifies geometry by removing internal boundaries between features with shared attributes.	Merging all counties with the same administrative code into a single region.

For more information on these tools, you can refer to ESRI's official documentation on editing features.

Best Practices for Polygon Editing

Backup Your Data: Always create a backup of your geodatabase or shapefiles before undertaking significant editing tasks. This safeguards against accidental data loss or irreversible mistakes.
Work in an Edit Session: All geometric modifications in ArcMap must be performed within an active editing session.
Understand Your Data: Be clear about whether you are dealing with a multipart feature or a single, continuous polygon before choosing your editing tool.
Attribute Management: When features are exploded or cut, the new features inherit attributes from the original. You may need to manually update or adjust these attributes for the new, individual polygons.

Troubleshooting Common Issues

Tool Not Visible/Grayed Out: Ensure you have started an editing session and the correct toolbar (Editor or Advanced Editing) is active and visible. Some tools are only active when a feature is selected.
Unexpected Results: If the Explode tool doesn't yield the expected results, it might be that your feature is not a multipart feature but rather a single, continuous polygon. In such cases, the Cut Polygons Tool would be more appropriate.

By following these steps and understanding the distinct functions of these editing tools, you can effectively "unmerge" or split polygons in ArcMap to manage your geospatial data with precision.