Cut Polygon by Line

What Is Cut Polygon by Line?

Cut Polygon by Line is a vector editing operation that splits a single polygon into two or more pieces along a user-drawn cutting line. It is one of the most common digitizing operations in GIS: subdividing a parcel, splitting a zoning district, dividing an agricultural field, carving a right-of-way out of a larger area, or creating sub-compartments in a forest block. Every major GIS has a version of this — QGIS calls it "Split Features," ArcGIS Pro uses the Cut tool, PostGIS does it with ST_Split, and Turf.js has turf.polygonSlice.

The idea is simple: draw a line that crosses the polygon boundary at least twice, and the polygon is divided along that line into two parts. In practice it is tricky because the cut line must fully cross the polygon (entry point on the boundary to exit point on the boundary), the resulting pieces must remain valid, and attributes from the parent polygon need to be carried to the children. Our in-browser Cut Polygon tool handles all of these details.

This tool is particularly valuable for planners and cadastral analysts who regularly subdivide parcels, for wildfire response teams carving evacuation zones, and for environmental consultants separating study areas. Because it runs entirely in your browser, confidential property boundaries and response-critical geometries never leave your machine.

How Polygon Cutting Works

Line-Polygon Intersection

The first step is computing where the cut line crosses the polygon boundary. Each crossing is an intersection point where a new vertex is introduced into both the line and the polygon. For a simple cut (one entry, one exit) there are exactly two crossings.

Ring Splitting

Once the crossings are identified, the polygon's outer ring is split at those points into two arcs. The cut line (restricted to the interior) is added to each arc to form two new closed rings. Each new ring becomes a new polygon.

Hole Handling

If the polygon has holes, the tool determines which holes fall entirely in the left piece, entirely in the right piece, or are themselves cut. Cut holes become two new holes in their respective parent polygons.

Attribute Inheritance

All child polygons inherit the parent's attributes by default. Fields that represent aggregate quantities (population, area, count) may need manual recalculation — the tool flags these for review.

Key Parameters and Options

Cut Line Input

Draw the cut line interactively on the map, or import a LineString from a GeoJSON file.

Multi-Cut Mode

A single cut line can split a polygon into two pieces. For multiple cuts, chain operations or use a MultiLineString cutter.

Attribute Handling

Choose whether to inherit all attributes, split numeric attributes proportionally by area, or leave fields blank for manual editing.

Output Mode

Keep original, or replace the original polygon with the cut pieces. The tool supports both.

Practical Applications

Parcel Subdivision

Land developers subdividing a large parcel into smaller lots cut the parent parcel along proposed lot lines. Each resulting sub-parcel inherits zoning, ownership, and other attributes from the parent.

Zoning District Changes

Municipal planners re-drawing a zoning district often cut existing polygons to reflect a new boundary. The cut tool keeps the geometry clean and produces a ready-to-edit result.

Agricultural Field Management

Farmers and agronomists divide fields for crop rotation, test plots, or different soil management zones. Cutting an existing field polygon is faster than re-drawing from scratch.

Forest Compartment Subdivision

Foresters subdivide compartments for silvicultural prescriptions, harvest planning, or fire management. Each sub-compartment needs its own attribute record.

Wildfire Evacuation Zones

Emergency managers carve evacuation zones out of larger administrative boundaries based on real-time fire behavior. Speed matters.

Right-of-Way Carve-Outs

Transportation engineers cut a road or railway right-of-way out of adjacent parcels. The cut tool produces both the ROW polygon and the modified parcel polygons.

Study Area Refinement

Researchers refine a study area by cutting it into strata (elevation bands, habitat types, treatment zones). Each stratum becomes a separate feature for analysis.

Step-by-Step Workflow in gis.tools

1. Open the Cut Polygon by Line tool.
2. Load a GeoJSON or Shapefile containing the polygon(s) to cut.
3. Click the polygon to select it.
4. Switch to draw-line mode and draw the cut line across the polygon. Make sure the line enters through one edge and exits through another.
5. Click Apply. The polygon is replaced by two (or more) new polygons.
6. Review the result on the map.
7. Edit attributes if needed.
8. Export the updated layer as GeoJSON, Shapefile, or KML.
9. Pair with Merge Features if you need to reverse a cut.

Worked Example

A county planning technician needs to subdivide a 40-acre agricultural parcel into two separate 20-acre tracts for a proposed sale. She loads the parcel shapefile, selects the parent polygon, and draws a straight line across the middle (snapping to existing parcel corners using the Feature Editor's snapping). She clicks Apply and sees the parent split into two child polygons. The Area Calculator confirms each child is 19.97 and 20.03 acres. She updates the parcel ID on each child and exports the result. Total time: three minutes. The confidential ownership data never left her browser.

Common Pitfalls and Gotchas

• Line doesn't fully cross: A cut line that starts or ends inside the polygon won't produce a valid split.
• Multiple crossings: A wavy cut line that crosses the boundary more than twice produces complex results. Consider using a straight line.
• Snap errors: Without snapping, the cut line may not precisely align with the polygon boundary.
• Self-intersecting cut: A cut line that crosses itself inside the polygon is ambiguous.
• Holes split across the cut: Holes that straddle the cut line are divided; review the result.
• Attribute duplication: Both children inherit the parent's attributes; numerical fields like "area" or "population" may need manual adjustment.
• Winding order: Cut pieces may have different winding orientation; run the GeoJSON Validator & Fixer afterward.
• Adjacent polygons: Cutting one polygon does not automatically affect neighbors; slivers may appear.

Tips for Best Results

• Enable snapping so the cut line aligns with existing vertices.
• Draw the cut line in straight segments unless curvature is required.
• Verify that the line crosses the polygon boundary exactly twice for simple splits.
• Review attribute inheritance before exporting.
• For cadastral work, use meter-based CRS (UTM, State Plane) for precise area calculations.
• After cutting, run Topology Helpers to clean up any minor artifacts.
• Keep a copy of the original polygon before cutting.

Comparison with Other GIS Approaches

QGIS's Split Features tool, ArcGIS Pro's Cut tool, and PostGIS ST_Split(geometry, blade) all provide the same functionality. Turf.js turf.polygonSlice (in some forks) handles the math. Our browser tool replicates the desktop UX: select a polygon, draw a line, click Apply. No install required.

Performance Considerations

Cutting is O(n) in the number of vertices per polygon. Even complex polygons cut in milliseconds. The UI is the limiting factor, not the math.

Data Privacy and Browser-Based Processing

Parcel data, confidential boundaries, and emergency response geometry all stay in your browser. No uploads, no logging.

Related GIS Concepts

• Ring topology: the fact that a polygon is defined by a closed ring of vertices.
• Split algorithm: the process of dividing a geometry along a cutting feature.
• Difference operation: related — erases part of a geometry overlapping another feature.
• Clip operation: similar — keeps only the part of a geometry inside a boundary.
• MultiPolygon: the container for multiple disjoint polygon parts, useful for representing cut results that should stay as a single feature.

Frequently Asked Questions

Can I cut with a curved line?

Yes — the cut line can be a LineString with many vertices. Just make sure it fully crosses the polygon.

What happens to attributes after cutting?

Both children inherit the parent's attributes. Numerical fields like area should be recalculated.

Can I cut multiple polygons with one line?

Yes — if the cut line crosses several polygons, each is split independently.

Does it handle holes?

Yes — holes that straddle the cut line are divided; holes entirely inside one piece stay with that piece.

Can I undo a cut?

Yes — the Undo/Redo Edit Stack records each cut operation.

Related Tools on gis.tools

• Feature Editor
• Draw Points/Lines/Polygons
• Merge Features
• Split Multipart Features
• Topology Helpers
• Geometry Repair
• GIS Clip Tool (Clip Layer by Polygon)
• Area Calculator