Analysis

What it does

Analysis builds a new layer from one or two existing layers.

Available: clip, erase, intersect, union, select by location, spatial join, dissolve, centroid, convex hull, simplify, buffer, voronoi, delaunay, and cartogram. Plus statistics, which is a dissolve without the geometry.

Pick an operation, choose the input layers, set the options, and calculate. The result is added to the map as a new layer.

Worth knowing before you start:

• Clip and intersect give the same shapes, but a different number of features. Clip merges the second layer first, so one input feature stays one output feature with its own attributes. Intersect works per overlapping pair, so a feature that meets three others becomes three features, each carrying the attributes of the one it met.
• Simplify keeps shared borders identical. Simplifying each polygon on its own simplifies a shared border twice, in two different ways, and leaves gaps and slivers between neighbors. This operation builds a topology first, so both sides keep the same points.
• Summing and averaging attributes is a setting, not a separate operation. You can add sum, mean, min, max, count, and a list of values to dissolve and to statistics. total and average are hidden for text fields.

Everything runs inside your browser, using GDAL compiled to WebAssembly. Your data is not uploaded anywhere.

Using it

1. Choose an operation. Its inputs and settings appear below it.
2. Pick the layers, set the values, press Calculate.
3. The result is added as a new layer, and the panel reports how many features it used.

Only loaded data is used. A GeoJSON layer gives every feature. A tiled layer gives only the features in view. The panel warns you on inputs where that can change the result.

Three example pages show the operations on real data:

• Two-layer operations: five countries and one rectangle through clip, erase, intersect, union, select by location, and spatial join, with the shapes and the attributes that come out of each.
• One-layer operations: dissolve, statistics, centroid, label point, buffer, convex hull, simplify, cartogram, voronoi, and delaunay on the same five countries.
• Which geometry each operation accepts: points, lines, and polygons against every operation, tested one combination at a time, because not every operation accepts every geometry type.

A long calculation can be canceled.

In your own map

Add geoprocessing to a toolbar. It appears as Analysis and works on whatever layers are on the map.

The WebAssembly is only fetched when it is needed, so a visitor who never opens the panel never downloads it. Opening the panel starts that download in the background while you fill in the form.

Extending it

An operation is one registry entry: inputs, parameters, a diagram, and either an SQL template or a JavaScript function.

Operations run in EPSG:3857 instead of longitude/latitude because SpatiaLite works on a flat plane. Inputs are written into one indexed SpatiaLite database before the query runs. Keep the index: on 144×576 polygons it took a query from 19.8 s to 1.3 s without changing the algorithm.

Union is the slow operation to watch when testing performance. On 257 countries against 4000 regions, clip took 4 s, intersect 5 s, and union 38 s.