Fishnet

A fishnet step builds a regular square grid over the bounding box of a polygon source, then keeps (and clips) only cells that intersect the source geometry. Pair it with a spatial join and group-by for heatmap-style "count per cell" summaries.

Hex cells, a separate geo-boundary / manual extent picker, and a separate "clip polygon" control are not options on this path. The extent and clip come from the pipeline source itself.

Inputs

• Source layer of polygon (or multipolygon) features. The grid covers each feature's bounding box and is clipped with ST_Intersection to that feature.
• Cell size: edge length in meters, feet, or miles (converted to degrees for PostGIS ST_SquareGrid on EPSG:4326 storage).
• Output mode: - Polygons: one polygon row per intersecting cell (partial cells are clipped to the source edge). - Lines: grid-line edges clipped to the source (transect / sampling-line style).

Output

One row per grid cell (or line segment set) that intersects the source. Source attributes are dropped. Schema:

• cell_row, cell_col: integer grid indices from ST_SquareGrid.

There is no area_m2 or bbox property on this path.

How to use it on its own

1. New-item wizard → Derived layer.
2. Pick a polygon Source (city boundary, study area, etc.).
3. Add a Fishnet step.
4. Set cell size, unit, and polygons vs lines.
5. Save.

Expect a short server job. Very small cell sizes on large extents are refused at save time (safety cap about one million cells).

From a data layer Analyze tab

On a polygon data layer's Analyze tab, Fishnet for this layer opens cell size, unit, and polygons vs lines controls and creates a private derived layer with a single fishnet step (same PostGIS ST_SquareGrid path as above). The chip is hidden for point and line layers. Expect a short server job; fine grids on large extents may hit the save-time cell cap.

Example: incident density by 500 m cell

1. Fishnet over a city boundary polygon, square cells, 500 m edge, output polygons.
2. Spatial join: target = fishnet, join = incidents, predicate = contains (or intersects), as appropriate.
3. Group by: keys = cell_row, cell_col. Aggregate = count(*) as incident count. Geometry strategy = first (keep the cell polygon).
4. Symbology: graduated on the count.

Notes

• Square only. Hexagonal fishnets are not shipped.
• Cell counts grow fast. A fine grid over a county-scale polygon can hit the save-time safety cap; increase cell size or use a smaller source.
• Cells outside the source drop. Partial cells at the edge are clipped; cells that only touch at a point can become empty and disappear from useful results.
• Meter sizes are approximate in geographic CRS. The portal converts cell size with a constant meters-per-degree factor on EPSG:4326. At high latitudes cells are not perfectly square on the ground; city-to-state workflows are the intended scale.
• Point or line sources are a poor fit; the SQL intersects against the source geometry and can yield empty results.