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Splitting and Merging Polygons in ArcGIS Pro

Step-by-Step Guide to Splitting and Merging Polygons in ArcGIS Pro

ArcGIS Pro is a powerful tool for geographic information system (GIS) professionals, offering a wide range of capabilities. One of the essential tasks you might find yourself doing is splitting and merging polygons. This guide will walk you through the process step by step.

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1. Preparing Your Workspace:

  • Load up the polygon feature you wish to work on. For this guide, we’ll refer to it as the “land puzzle polygon”.
  • Once loaded, right-click on the polygon and head over to the attributes table. This will show you the attributes associated with your polygon. For instance, it might display the name of the parcel (e.g., “parcel A”), a unique parcel ID, and the area calculated in hectares.

2. Splitting the Polygon:

a. Ensure the layer containing your polygon is selected.
b. Navigate to the “edit ribbon” and click on the “select” button.
c. Use the select tool to choose the polygon you wish to split. Once selected, the polygon should be highlighted.
d. Expand the tools box. Here, you’ll find the “split” tool. There are multiple ways to use this tool:

  • Line: For straight splits.
  • Right Angle: If your split includes a right angle.
  • Arc: For splits that involve a curve. For this guide, we’ll use the “line” option.
    e. Begin your split by drawing a line across the polygon where you want the split to occur. If you want precision, enable the “snapping” option to snap directly onto the vertex of the existing line feature.
    f. Once you’ve drawn your split line, the polygon will divide into separate polygons.

3. Adjusting Attributes After Splitting:

After splitting, you might notice that the attributes (like area or parcel ID) of the newly created polygons might not update automatically. Here’s how to adjust them:
a. Right-click and go to the attributes table.
b. You’ll now see multiple records for each split polygon. Update the attributes as needed. For instance, you can rename the polygons (e.g., “parcel A”, “parcel B”), adjust the parcel IDs, and recalculate the area.
c. To recalculate the area, deselect all polygons, right-click, and choose “calculate geometry”. From here, you can perform an area calculation, select the unit (e.g., hectares), and choose the coordinate system.

4. Merging Polygons:

If you wish to combine two or more polygons into one, follow these steps:
a. Using the select tool, choose the polygons you want to merge.
b. Head over to the tools box and select the “merge” tool.
c. A prompt will ask which polygon’s attributes you wish to preserve since the merged polygon can only have one set of attributes. Choose the appropriate one.
d. Click on “merge”, and the selected polygons will combine into one.

5. Adjusting Attributes After Merging:

Similar to splitting, you might need to update the attributes of the merged polygon:
a. Head to the attributes table and update the necessary fields.
b. Recalculate the area if needed by right-clicking and selecting “calculate geometry”.

And there you have it! With these steps, you can efficiently split and merge polygons in ArcGIS Pro. Whether you’re working on land parcels, geographical regions, or any other polygon-based data, these tools will help you manipulate and manage your data with ease. Happy mapping!

Frequently asked questions about splitting and merging polygons in ArcGIS Pro:

Why would I need to split or merge polygons?

Answer: Splitting and merging polygons are essential GIS tasks used for various purposes. You might need to split a larger region into smaller administrative units or merge smaller units into a larger one for analysis. For instance, if you have a large forested area and want to divide it based on different species dominance or merge agricultural plots owned by the same entity.

How do I ensure that the attributes of my polygons are correctly updated after splitting or merging?

Answer: After splitting or merging, always check the attributes table. Ensure that the new polygons have the correct attributes, and manually update if necessary. Using the “Calculate Geometry” tool can help update area or length values. Regularly saving your work and keeping backups can also prevent data loss.

Is there a way to automate the splitting or merging process for multiple polygons at once?

Answer: Yes, ArcGIS Pro offers geoprocessing tools like “Split” and “Merge” that can be used in ModelBuilder or Python scripts to automate these processes for multiple polygons.

What happens to the original polygon data after splitting or merging? Is it retained or overwritten?

Answer: When you split or merge polygons, the original polygon data is altered. However, the original attribute data can be retained in the new polygons, but you might need to manually adjust some attributes, especially if they are area-dependent.

Can I undo a split or merge if I make a mistake?

Answer: Yes, ArcGIS Pro has an “Undo” feature (typically Ctrl + Z) that allows you to revert actions. It’s also wise to save your work regularly and create backups before performing significant operations.

How does the “snapping” feature work, and why would I use it during the splitting process?

Answer: The snapping feature ensures precision by allowing your cursor to “snap” to a specific point, edge, or vertex while drawing. This is useful to ensure that splits align perfectly with existing boundaries or points.

What’s the difference between the “Line”, “Right Angle”, and “Arc” options in the split tool?

Answer: These options dictate the shape of the split:

  • Line: Creates a straight line split.
  • Right Angle: Allows for splits that include right angles.
  • Arc: Enables curved splits.

How do I handle polygons with shared boundaries when merging?

Answer: When merging polygons with shared boundaries, ArcGIS Pro will automatically dissolve the shared boundary to create a single, unified polygon.

Is there a way to split or merge polygons based on specific attribute values automatically?

Answer: Yes, using the “Select by Attributes” tool, you can select polygons based on specific criteria. Once selected, you can then split or merge them as needed.

How does the software calculate the new area values after splitting or merging? Is it always accurate?

Answer: ArcGIS Pro uses geometric algorithms to calculate area values. While the calculations are generally accurate, discrepancies can arise due to projection issues or very complex polygon shapes. Always ensure you’re using an appropriate projection for area calculations.

Can I split or merge polygons using external data, like a CSV file or another layer?

Answer: Yes, you can use external data to guide your splits or merges. For instance, you can use a line layer to split polygons or use attributes from a CSV file to determine which polygons to merge.

What happens if I merge polygons with different attribute values? How does ArcGIS Pro decide which attributes to retain?

Answer: When merging, ArcGIS Pro will prompt you to choose which polygon’s attributes to retain. The software cannot automatically decide for you, so you’ll need to make a manual selection.

Are there any limitations to the number of polygons I can split or merge at once?

Answer: While there’s no strict limit, performance can become an issue with very large datasets. It’s recommended to work in smaller batches if you experience slowdowns or crashes.

Can I split or merge polygons in 3D views or only in 2D?

Answer: While most splitting and merging tasks are done in 2D views, ArcGIS Pro does support 3D editing. However, working in 3D can be more complex, and not all 2D tools and features are available in 3D.