Fishnet

Generate a regular grid of square (or hex) polygons over an extent. The starting layer for any "binned-by-cell" analysis.

intermediate

A fishnet step generates a regular grid of polygon cells over an extent. Each cell is a polygon feature with row/column attributes. The output is a starting layer; pair it with a spatial-join and group-by to produce a heatmap-style summary "X per cell".

Inputs

  • Cell shape: square or hex (hexagonal).
  • Cell size: edge length in meters (or feet, or miles).
  • Extent: one of: - A Geo boundary item. - The bounding box of another layer. - A manual bounding box.
  • Optional clip to a polygon: instead of filling the bounding box, generate only cells that intersect a polygon. Produces a grid shaped like the polygon.

Output

A polygon layer with one row per cell. Schema:

  • row, col (or i, j for hex): cell coordinates.
  • area_m2: the cell's geometric area (in m^2).
  • bbox: the cell's bounding box.

How to use it on its own

  1. New-item wizard → Derived layer.
  2. Add a Fishnet step (no Source: fishnet creates its own input).
  3. Pick shape, size, extent.
  4. (Optional) Pick a clip polygon.
  5. Save.

Example: incident density by 500m cell

  1. Fishnet over the city's geo boundary, square cells, 500m edge.
  2. Spatial join: target = fishnet, join = incidents, predicate = contains, cardinality = one-to-many.
  3. Group by: keys = row, col. Aggregate = count(*) as incident_count. Geometry strategy = first (the fishnet's cell geometry).
  4. Symbology: Graduated on incident_count.

The result is a heatmap-style choropleth without the rendering expense of a true heatmap.

Hex vs. square

  • Square cells are easier to read off a printed legend and align with city-grid neighborhoods nicely.
  • Hex cells have the same neighbor distance in every direction (good for distance-decay analysis) and visually read smoother for density.

Use whichever fits the audience.

Notes

  • Cell counts grow fast. A 250m grid over a 50km x 50km county is 40,000 cells; a 100m grid is 250,000. Watch the estimated row count before saving.
  • Cells outside the clip polygon drop entirely. A hex fishnet clipped to an irregular boundary doesn't carry "edge cells" that are mostly outside; they're trimmed to nothing and dropped.
  • Cell size is in projected units. The portal computes in a local equal-area projection so "500m cells" are actually 500m on the ground.