GIS Tools - Merge Features

What Is a Merge Features Operation?

Merge Features combines two or more vector features into a single feature. It is the inverse of cutting or splitting: where cut divides, merge unites. In GIS terminology this is sometimes called "union" (when the features may overlap), "dissolve" (when merging by attribute), or simply "merge" (when unioning selected features regardless of attribute). Our Merge Features tool focuses on the selection-based merge: you pick features, click Merge, and they become one.

Behind the scenes the operation computes the geometric union of the input features. For adjacent polygons that share a boundary this produces a clean single polygon. For disjoint features it produces a MultiPolygon, MultiLineString, or MultiPoint — a single feature with multiple parts. The attribute of the result is taken from the first selected feature by default, or you can specify a merge rule (keep first, sum, max, concatenate).

Merging is an everyday GIS task: combining fragmented parcels into a single ownership, unifying split zoning districts after a rule change, joining line segments into a through-route, grouping points of interest by category. Our browser-based merge runs client-side so confidential boundary data never leaves your machine.

How Feature Merging Works

Union of Geometries

The geometric union uses a planar sweep algorithm (typically the one in JTS/Turf.js) to combine input polygons, eliminating shared boundaries and keeping the outer edge as the new boundary. For lines, adjacent segments are joined into a continuous LineString. For points, all selected points become members of a MultiPoint.

Shared Boundary Removal

When two adjacent polygons are merged, the shared edge between them is removed. The result has only the outer boundary — the union's "hull" — as its new ring.

Multipart Handling

If the selected features don't touch, the result is a multi-part geometry (MultiPolygon, MultiLineString, or MultiPoint). The tool reports this so you can decide whether to keep the multi-part structure or split it into separate features with the Split Multipart Features tool.

Attribute Merging

The attribute merge rule controls what happens to the combined feature's properties. Default is "keep first." Options include concatenate (strings), sum/max/min/average (numbers), and "leave blank."

Key Parameters and Options

Selection

Multi-select features by clicking, shift-clicking, or drawing a selection rectangle.

Merge Rule

How to handle overlapping geometry: union (default), intersection only, symmetric difference.

Attribute Rule

How to merge properties: keep first, keep last, concatenate, sum, average, max, min.

Output Geometry Type

Preserve multi-part structure, or force single-part output (for adjacent features that merge cleanly into one).

Retain Original Features

Keep the source features in addition to the merged result, or replace them.

Practical Applications

Parcel Consolidation

When adjacent parcels are sold to the same owner, the GIS layer needs to reflect the consolidation. Merging the polygons produces the new parent parcel.

Zoning Simplification

After a zoning rule change, multiple small zones of the same type may need to be combined into one larger zone. Merging cleans up the layer.

Administrative Boundary Updates

When municipalities amalgamate, their boundaries are merged into a single new municipality.

Route Construction

Piecing together a continuous route from separate road or rail segments is a common GIS workflow. Merging lines unites them.

Habitat Patch Aggregation

Conservation biologists often aggregate nearby habitat patches into a single "habitat complex" for metapopulation modeling.

Duplicate Feature Consolidation

Before a database import, it is common to merge duplicates that came in as separate records from different sources.

Study Area Expansion

Researchers may merge adjacent study areas into a single larger area for aggregated analysis.

Step-by-Step Workflow in gis.tools

Open the Merge Features tool.
Load a GeoJSON, Shapefile, or KML file.
Select the features to merge by clicking or rectangle-drag.
Pick an attribute merge rule.
Click Merge. The selected features become one.
Review the result on the map.
Adjust attributes if needed.
Export as GeoJSON, Shapefile, or KML.
For attribute-based merging, see the GIS Dissolve Tool (Merge by Field).

Worked Example

A municipal cadastre analyst is consolidating three adjacent parcels (lots 12A, 12B, and 12C) that were acquired by a single landowner into one new parcel (lot 12). She selects all three, picks "keep first" for attributes, and clicks Merge. The three polygons become a single clean polygon (shared boundaries eliminated). She edits the parcel ID attribute to "12" and exports the updated layer. The whole operation takes 90 seconds, and the confidential owner information never leaves her browser.

Common Pitfalls and Gotchas

Disjoint features produce multipart: Merging features that don't touch creates a MultiPolygon rather than a single polygon. Verify this is what you want.
Attribute loss: Only one feature's attributes survive by default. Numeric values may need manual recalculation.
Winding order reversal: Some merged polygons flip their winding; run the GeoJSON Validator & Fixer afterward.
Slivers at shared boundaries: If the source features don't align precisely, merging can leave tiny sliver polygons.
Self-intersecting inputs: Merging a self-intersecting polygon with a valid one can produce invalid output.
Different geometry types: You can't merge points with polygons; the tool will refuse.
Topology errors downstream: Merged features sometimes still have topology issues that only show up in analysis. Run Topology Helpers.
Losing the original: Without the "retain originals" option, the source features are deleted.

Tips for Best Results

Always check that adjacent features actually share an edge before merging; slight misalignment causes slivers.
Enable snapping in the Feature Editor first to align boundaries.
Document the attribute merge rule so downstream consumers understand how values were combined.
Run Geometry Repair on the result if any input was suspect.
Keep a copy of the original features if there's any chance you'll need them.
For attribute-driven merging (all features of the same zoning type), use the GIS Dissolve Tool (Merge by Field) instead.

Comparison with Other GIS Approaches

QGIS's Merge Selected Features and ArcGIS Pro's Merge tool are the desktop equivalents. PostGIS ST_Union performs the same geometric operation. Turf.js turf.union covers polygons; there's no direct line equivalent, so merging lines usually requires custom code. Our browser tool wraps these into a simple selection-based workflow.

Performance Considerations

Merging a handful of features is instantaneous. Merging hundreds of complex polygons takes a few seconds. For bulk merging by attribute, the Dissolve tool is faster because it can spatial-index the inputs.

Data Privacy and Browser-Based Processing

All computation is local. Parcel owners, habitat locations, and other sensitive attributes stay in your browser.

Related GIS Concepts

Union: the geometric OR of two or more geometries.
Dissolve: merging features that share an attribute value.
Aggregation: combining attributes across merged features using a rule (sum, average).
Multipart geometry: a single feature with multiple disjoint parts.
Planar sweep: the algorithm used to compute geometric unions efficiently.

Frequently Asked Questions

What's the difference between merge and dissolve?

Merge combines selected features regardless of attribute. Dissolve merges features that share an attribute value across the entire layer.

Can I merge polygons with lines?

No — inputs must be the same geometry type.

What happens to attributes?

The result takes attributes from the first selected feature by default, or uses a rule you specify.

Can I undo a merge?

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

Does merging create a MultiPolygon?

If the inputs touch, the result is a single Polygon. If they don't, the result is a MultiPolygon with disjoint parts.

About Merge Features