Draw two or more polygons on the map, or upload a GeoJSON file or a zipped Shapefile, then pick any layers you want and merge them into one combined shape. Pick a color for the result and save it as a GeoJSON file — fast, in your browser, with no extra program needed. Open in full screen ↗
What Is a Polygon Union, and Why You Need One
A polygon union, also called a merge or a dissolve, takes two or more shapes and combines them into a single outline that covers the same total ground as all of them together. This is one of the most common tasks in GIS work: joining two neighboring land parcels into one property, combining several small service areas into one coverage zone, or merging a set of small survey plots into one boundary.
Common Questions
Does the union keep the original properties from each polygon?
The merged shape is treated as one new feature, so it does not automatically carry over every property from each source polygon. If you need to keep specific details, note them down before merging, or keep the original layers as a separate export alongside the union result.
How big a file can I load?
Files up to 50 MB are accepted for upload. Very large files with thousands of complex polygons may take a moment to merge, since everything runs in your browser rather than on a server.
Why does my file get rejected?
This happens when the file has no Polygon or MultiPolygon shapes at all — for example, a file with only points or only lines. Check that your source file really has closed area shapes, not just points or paths.
Can I run the tool again on my own result?
Yes. A union result is just another polygon layer, so you can check it again in "Select Layers to Merge" along with more shapes for a further run. This lets you build up a merged shape in several rounds.
Can I change the color of a layer after I make it?
Yes. Every row in the Layers list has its own color box. Click it and pick a new color — the change shows on the map right away, and it does not touch any other layer.
What is the difference between "union", "merge" and "dissolve" in GIS?
They are mostly the same idea under different common names. Desktop GIS programs often use "Merge" or "Dissolve", while spatial libraries like Turf.js and PostGIS call it "Union". This tool uses all three words in its content so it is easy to find, whichever term you search for.
What happens if the polygons I pick do not touch each other?
You still get one result layer, but its geometry becomes a MultiPolygon — one layer holding each separate piece, so nothing about their shape or position is lost or joined incorrectly.
What does "polygon union" mean, in simple words?
It means taking two or more shapes and combining them into one shape that covers all of the same ground. Think of it as gluing shapes together into a single outline.
What does a zipped Shapefile need to have inside?
One .zip file, with at least the .shp, .dbf and .shx parts of the Shapefile inside it. If the zip has more than one layer inside, each one loads as its own separate layer in the tool.
Is my map data sent to a server?
No. Drawing, file reading, and the union math all run in your own browser, using the Turf.js library. Nothing is sent to or kept on a server, so it is safe to use with private or sensitive location data.
Can I merge more than two polygons at once?
Yes. Check as many layers as you like in "Select Layers to Merge" — two, three, or more — and "Run Union" combines every polygon from every checked layer into one result in a single step.
Can I use points or lines as input?
No. This tool only works with Polygon and MultiPolygon shapes, since only a closed shape has an inside area that can be combined. If your file has points or lines mixed in with polygons, those extra shapes are left out, and you get a message about how many were left out.
How the Union Is Worked Out
Check the box next to every layer you want to combine — two, three, or as many as you have. The tool gathers every polygon from those layers and merges them into one shape. If the pieces you picked touch or overlap, you get back one single, connected outline. If some of the pieces do not touch at all, the result becomes a MultiPolygon: one layer that still remembers each separate piece, so nothing is lost.
Only Polygons Work Here
This tool only accepts closed shapes: Polygon and MultiPolygon. Points and lines do not have an inside area, so they cannot be merged this way. If your file has other shapes mixed in with polygons, the tool keeps only the polygons and tells you how many other shapes it did not use.
Real-World Use Cases
🏘️
Neighborhood and Community Mapping
Local governments and community groups combine several small neighborhood boundaries into one larger ward or community area for elections, planning, or funding applications.
🏙️
Urban Planning and District Boundaries
City planners dissolve several small zoning areas or census blocks into one larger district outline, to simplify a map or prepare it for a public report.
🚨
Disaster Response and Risk Mapping
Emergency planners merge several separate flood-risk, fire-risk, or evacuation-zone polygons into one combined risk area, to brief responders with a single, simple boundary instead of many overlapping ones.
🌿
Environmental and Conservation Planning
Conservation groups combine several separate protected areas or habitat patches into one boundary, to report total protected land area or plan a connected wildlife corridor.
🚚
Retail and Delivery Zone Planning
Retail chains and delivery companies merge the service areas of several nearby stores or depots into one combined coverage zone for regional planning and staffing decisions.
🗺️
Land Parcels and Property Boundaries
Land surveyors and title companies merge two or more neighboring parcels into one combined property boundary after a sale, a land swap, or a family land division.
🌾
Farm Field Consolidation
Farm management teams merge several small, adjoining fields into one larger management zone, to simplify planting plans, spraying records, and yield reporting.
📡
Telecom Coverage Analysis
Network engineers merge the coverage circles of several nearby towers into one combined coverage footprint, to see the total ground served by a cluster of sites at a glance.
Layer Colors and Result Style
Two color settings help you tell layers apart on the map:
- Layer color: each polygon layer gets its own color when you add it. You can change this color any time from the layer row.
- Result Color: sets the color of the new union layer before you click "Run Union". Change it again after, from the layer row, if you want a different look.
The new union layer is drawn with a bolder outline and a stronger fill than the source layers, so the result always stands out on the map.
Step by Step
1Step 1
Open the tool
The map opens on a world view with no layers yet. The toolbar at the top has Draw Polygon, Upload, Sample Polygons and a basemap switch. The sidebar has the union tool, the layer list, and the export button.
2Step 2
Add your first polygon
Click "Draw Polygon" and click the map for each corner, or upload a GeoJSON file or a zipped Shapefile.
3Step 3
Add a second polygon, and more if you want
Draw or upload more shapes — they can overlap or sit apart. Or click "Sample Polygons" to add three ready-made overlapping shapes.
4Step 4
Check the layers to merge
In "Select Layers to Merge", check the box next to every layer you want in the result. Pick two, three, or as many as you have loaded.
5Step 5
Choose a result color
Click the color box next to "Result Color" and pick a color for the new union layer.
6Step 6
Click Run Union
The tool combines every polygon in the checked layers into one shape and adds it as a new layer. The map zooms to fit the result.
7Step 7
Check and manage your layers
In the Layers list, turn any layer on or off, change its color, zoom to it, or delete it. Union layers are marked differently from source polygon layers, so you always know which is which.
8Step 8
Merge again if you want
Check a union result together with another layer in "Select Layers to Merge" to combine even more shapes into one, one round at a time.
9Step 9
Save your results
Click the download icon on a layer row to save just that layer, or click "Download All Layers" to save every layer — sources and results — in one GeoJSON file.
Two Ways to Add Your Polygons
Add the polygons you want to work with using one of these methods, or use both together in the same session:
- Draw on the map: Click "Draw Polygon", then click the map to add each corner. Finish the shape with a double-click, the Enter key, or the "Finish" button. Draw two or more polygons so there is something to merge.
- Upload a file: Click "Upload" or drag a file onto the map — a GeoJSON/.json file, or a zipped Shapefile (.zip with .shp, .dbf and .shx inside). Each file becomes its own layer.
No file ready? Click "Sample Polygons" to add three overlapping test shapes right away. Once you have at least two polygon layers, check the boxes next to the ones you want in "Select Layers to Merge", choose a result color, and click "Run Union".
Working with Your Layers
Each polygon layer and each union layer you make shows up as one row in the Layers list. From that row, you can turn the layer on or off, change its color, zoom the map to it, save it as GeoJSON, or delete it. None of these actions change any other layer. A union result can even be checked again in "Select Layers to Merge" for a next run, so you can combine several rounds of merging together.
Saving Your Union Result
Two ways to save your work are always ready in the sidebar:
- Save one layer: click the download icon next to any layer row to save only that layer as a GeoJSON file.
- Download All Layers: puts every layer on the map — sources and union results together — into one GeoJSON file, with each feature marked with its layer name.
GeoJSON opens without problems in Leaflet, Mapbox GL JS, QGIS and ArcGIS, and it even shows as a live map on GitHub.
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Map tiles © OpenStreetMap contributors, drawn with Protomaps (hosted by KONAKTIVE), with CARTO as the fallback. Satellite imagery: Esri. Geometry calculations: Turf.js. Shapefile reading: shpjs. Map rendering: Leaflet. All calculations run in your browser, and nothing you enter is stored on a server.
