Contour

Generate iso-lines from a numeric field on a point layer via a PostGIS TIN.

advanced

A contour step builds a Delaunay TIN from point features that carry a numeric field, then emits iso-lines where that field crosses chosen levels (ST_DelaunayTriangles plus edge interpolation). Think elevation contours or signal-strength isobars — lines of equal value.

Iso-polygons (filled bands), smoothing controls, and per-band aggregates are not options on this path. Output is line geometry with a level property only.

Inputs

  • Source layer of points with a numeric attribute. Fishnet cell centroids can work if you have already turned cells into points with a value; the generator itself expects point geometry for the TIN.
  • Value field: the numeric attribute to contour on.
  • Levels mode: - Auto: pick an interval (and optional min / max). At save time the portal reads the source min/max, builds the level list, and caches it on the recipe. - Manual: supply an explicit list of levels.
  • Levels are capped (about 100) so a tiny interval on a huge range cannot explode the job.

Output

A line layer. Each feature is a contour segment with:

  • level: the contour value for that line.

No level_top, and no mean/count aggregates from the source.

How to use it on its own

  1. Start from a point layer that already has the numeric field (survey points, sample sites, or centroids you computed elsewhere).
  2. New-item wizard → Derived layer.
  3. Pick that layer as Source.
  4. Add a Contour step.
  5. Choose the value field, auto vs manual levels, and interval or level list.
  6. Save.

Expect a server job. Dense point sets are expensive; prefer a bbox-filtered source or coarser spacing when exploring.

From a data layer Analyze tab

On a point data layer's Analyze tab, Contour for this layer opens numeric field, auto interval, and optional min/max controls and creates a private derived layer with a single contour step (same PostGIS TIN path as above). Point layers only; line and polygon layers do not show the chip. Manual level-list authoring stays in Create → Derived layer. Expect a server job, not an instant browser result.

Example: elevation iso-lines

  1. Source: elevation sample points with an elev_m field.
  2. Contour on elev_m, mode auto, interval 10 (optional min/max to clip the range).
  3. Output: line features tagged with level at 10 m steps across the sampled area.

For filled "service zone" bands, build polygons another way (buffer, dissolve, or a dedicated service-area tool) — this step does not emit iso-polygons.

Notes

  • Input density matters. Sparse points make coarse TINs and jagged lines; very dense clouds are slow (nearest-neighbor lookup in the SQL path).
  • Edges are interpolated linearly along TIN triangle edges between adjacent sample values.
  • Coverage stops with the TIN. Contours do not invent values outside the convex hull of the input points.
  • No smoothing slider. What you see is the mathematical TIN intersection, not a cartographic smooth.