Isocontour Transform
The isocontour transform ≥ 5.8 models a spatial distribution of data values using a set of discrete levels. Each contour line is an isoline of constant value. A common use case is to convey density estimates for 2D point data, as these can provide a more scalable representation for large numbers of data points.
The isocontour transform take as input one or more raster grids (matrices) of numerical values and generates a stream of GeoJSON geometry data as output. These shapes can then be visualized using either the geoshape or geopath transform. For a complete example, see the contour plot example visualization.
Transform Parameters
| Property | Type | Description |
|---|---|---|
| field | Field | The field with raster grid data. If unspecified, the data object itself is interpreted as a raster grid. |
| thresholds | Number[ ] | A threshold array of explicit contour level values. If specified, the levels, nice, resolve, and zero parameters are ignored. |
| levels | Number | The desired number of equally-spaced contours. This parameter is ignored if the thresholds parameter is provided. |
| nice | Boolean | A boolean flag (default false) indicating if the contour threshold values should be automatically aligned to “nice”, human-friendly values. Setting this flag may cause the number of thresholds to deviate from the exact levels value. |
| resolve | String | The method for resolving thresholds across multiple input grids. If 'independent' (the default), threshold calculation will be performed separately for each grid. If 'shared', a single set of threshold values will be used for all input grids. |
| zero | Boolean | A boolean flag (default true) indicating if the contour threshold values should include zero as a baseline value. |
| smooth | Boolean | A boolean flag (default true) indicating if the contour polygons should be smoothed using linear interpolation. This parameter is ignored when using kernel density estimation. |
| scale | Number | Number[ ] | A numerical value or two-element [sx, sy] number array by which to scale the output isocontour coordinates. This parameter can be useful to scale the contours to match a desired coordinate space. Support for array input in versions ≥ 5.9. |
| translate | Number[ ] | A two-element [dx, dy] number array by which to translate output isocontour coordinates. This parameter can be useful to map the contours to match a desired coordinate space. ≥ 5.9 |
| as | String | The output field at which to write a generated isocontour. The default is "contour". |
Usage
This example generates 10 levels of contours for the 2D kernel density estimate of a source data stream. The backing kde2d transform draws pre-computed pixel (x, y) coordinates from the x_value and y_value data fields, and uses the width and height signals to configure the area over which densities, and subsequent contours, should be computed.
{
"type": "kde2d",
"x": "x_value",
"y": "y_value",
"size": [{"signal": "width"}, {"signal": "height"}],
"as": "grid"
},
{
"type": "isocontour",
"field": "grid",
"levels": 10
}