Simplify

Drop vertices within a tolerance of a straight line via PostGIS ST_Simplify. Attributes are kept.

basic

A simplify step reduces geometry detail with PostGIS ST_Simplify (Douglas–Peucker). Vertices that lie within the tolerance of a straight line between their neighbors are dropped. Attributes are copied from the source row.

Use when you want lighter polygons or lines for display, export, or a later analysis step — not when you need topology preserved (holes and shared edges can break; that stricter path is not offered here).

Inputs

  • Source layer. Any geometry type. Null geometries are skipped.
  • Tolerance. A number greater than zero.
  • Unit. Meters, feet, kilometers, or miles. The server converts to degrees of latitude before calling ST_Simplify (geometries are stored in EPSG:4326). Soft ceiling: 100 km.

Output

One row per non-null input geometry:

  • Geometry: ST_Simplify of the input (may be empty if the tolerance collapses the shape).
  • Properties: copied from the source row.

How to use it on its own

From Analyze (guided)

  1. Open a data layer and switch to the Analyze tab.
  2. Click Simplify this layer.
  3. Enter the tolerance and unit.
  4. Optionally edit the title.
  5. Click Create derived layer.

The portal creates a private derived layer and opens it. Expect a short server job, not an instant browser result. Meter-honest tolerance runs on PostGIS — Analyze does not invent an in-browser simplify.

From the new-item wizard

  1. New-item wizard → Derived layer.
  2. Pick a Source.
  3. Add a Simplify step.
  4. Enter tolerance and unit.
  5. Save.

Examples

  • Lighter parcel outlines. Source: parcels. Tolerance: 2 meters. Output: the same parcels with fewer vertices for faster map draw.
  • Export-friendly roads. Source: detailed centerlines. Tolerance: 10 meters. Output: smoother lines for a PDF or GeoJSON share.

Notes

  • Not topology-preserving. ST_Simplify (not ST_SimplifyPreserveTopology) is the fast path. Adjacent polygons can develop gaps or overlaps after a large tolerance.
  • Empty results are possible when the tolerance is large relative to the feature size.
  • Degree conversion is approximate. Tolerance uses ~111,320 m per degree of latitude; at high latitudes longitude degrees are shorter, so simplification can be slightly more aggressive.
  • Soft max 100 km. Same ceiling as Buffer — wide enough for any real “smooth this” use, narrow enough to refuse a pathological misclick.