Dissolve

Merge adjacent polygons that share an attribute value into one multipolygon per shared value.

basic

A dissolve step merges polygons that share the same value of a key field into one polygon (or multipolygon) per distinct value. Boundaries between same-key polygons disappear; boundaries between different-key polygons remain.

Use to clean up tile boundaries, simplify a parcel layer to a land-use layer, or visualize "regions" computed from a finer input.

Inputs

  • Source layer (must be polygon or multipolygon).
  • Key field (optional). If set, polygons are dissolved per value. If omitted, every polygon dissolves into a single multipolygon.
  • Attributes to preserve. By default the key field. You can add summary aggregates the same way as a group-by step (sum of an area field, count of input polygons per group).

Output

One polygon (or multipolygon) per distinct key value. Schema is the key field plus any preserved aggregates.

How to use it on its own

  1. New-item wizard → Derived layer.
  2. Pick a polygon Source.
  3. Add a Dissolve step.
  4. Pick the Key field.
  5. Save.

Examples

  • Parcels to land-use polygons. Source: parcel layer with a land_use field. Key: land_use. Output: one multipolygon per land-use category (one shape for "residential", one for "commercial", etc.).
  • Tile-boundary cleanup. Source: a layer ingested from tiled-by-quad data where adjacent same-value cells should be one. Key: the cell value field. Output: clean polygons with no internal seams.
  • Single overall extent. Source: any polygon layer. Key: omitted. Output: one multipolygon that's the union of every input. Useful as a quick "what's the total extent" reference.

Dissolve vs. group-by with dissolve geometry strategy

Both can produce the same output. The differences:

  • Dissolve is the simpler path; pick this when you only want the merged geometry plus the key.
  • Group-by with geometry strategy: dissolve is the right path when you also want non-trivial aggregates (count, sum, average) computed alongside the dissolved geometry.

Notes

  • Non-adjacent same-key polygons become multipolygons. A layer where "residential" parcels are scattered across town produces a single multipolygon feature containing all of them.
  • Internal holes survive. If the source polygons have rings that surround other-key polygons (a residential block around a commercial lot), the dissolve preserves the hole.
  • Slow on huge inputs. Dissolving millions of polygons can take minutes; consider filtering first or running off-hours.