Contour

Generate iso-lines (or iso-polygons) from a value column on a point or cell layer.

advanced

A contour step takes a layer of points (or fishnet cells) with a numeric value and produces iso-lines (or iso-polygons) at specified value intervals. The output represents lines of equal value, like contour lines on a topographic map.

Use for elevation contours from a point cloud, signal-strength isobars from a measurement grid, "minutes from the nearest hospital" service zones.

Inputs

  • Source layer of points or fishnet cells.
  • Value field: the numeric attribute to contour on.
  • Mode: - Iso-lines. Output is line geometry at each level. - Iso-polygons. Output is filled polygon bands between levels.
  • Levels: a list of values (100, 200, 300, ...) or an interval (every 50).
  • Smoothing: how much to smooth the result. 0 = strict mathematical contours from the input grid; higher values produce visually smoother but slightly less precise lines.

Output

A new layer of line (or polygon) geometry with attributes:

  • level: the value of the contour line / lower edge of the polygon band.
  • level_top: the upper edge (iso-polygons only).
  • Aggregates from the original layer (mean, count) per band.

How to use it on its own

  1. Build a value layer first: typically points-with-a-value or a fishnet-binned summary.
  2. New-item wizard → Derived layer.
  3. Pick the value layer as Source.
  4. Add a Contour step.
  5. Pick the value field, mode, levels, and smoothing.
  6. Save.

Example: drive-time service zones

  1. Start with a point grid (a fishnet's cell centroids).
  2. For each cell, calculate drive time to the nearest hospital (an external routing tool, or a built-in approximation).
  3. Contour on drive_time_minutes, iso-polygons, levels at 5, 10, 15, 20, 30. Output is a polygon layer with bands you can render as a heatmap-style service-area map.

Notes

  • Input density matters. Coarse input grids produce jagged contours; fine grids produce smooth contours at higher cost. For elevation data, expect to use a point cloud or DEM-derived fishnet at meter-level cell size.
  • Edges are interpolated linearly between adjacent input values. The contours are continuous within the input's coverage and stop at the input's bounding box.
  • Run cost. Contour is expensive on large inputs. Run as a derived layer with scheduled refresh (nightly), not as an on-the-fly step.