VisualPointClustering

The VisualPointClustering is a clustering operator for point collections that removes clutter and preserves the spatial structure of the input. The output is a point collection with a count and radius attribute. The operator utilizes the input resolution of the query to determine when points, being displayed as circles, would overlap. Moreover, it allows aggregating non-geo attributes to preserve the other columns of the input. For more information on the algorithm, cf. the paper Beilschmidt, C. et al.: A Linear-Time Algorithm for the Aggregation and Visualization of Big Spatial Point Data. SIGSPATIAL/GIS 2017: 73:1-73:4.

An exemplary use case for this operator is the visualization of point data in an online map application. There, you can use this operator as the final step of the workflow to cluster the points and display them as circles. These circles then pose a decluttered view of the data, e.g., via a WFS endpoint.

Parameters

ParameterTypeDescriptionExample Value
minRadiusPxnumberMinimum circle radius in screen pixels
10
deltaPxnumberMinimum circle to circle distance in screen pixels input
1
radiusColumnstringThe new column name to store radius information in screen pixels
"__radius"
countColumnstringThe new column name to store the number of points represented by each circle
"__count"
columnAggregatesMap from string to aggregate definition (one of MeanNumber, StringSample or Null)Specify how miscellaneous columns should be aggregated. You can optionally set a new Measurement. Otherwise, the Measurement is taken from the source column.
{
  "foo": {
    "columnName": "numericColumn",
    "aggregateType": "MeanNumber",
    "measurement": { "type": "unitless" }
  },
  "bar": {
    "columnName": "textColumn",
    "aggregateType": "StringSample"
  }
}

Inputs

The VisualPointClustering operator expects exactly one vector input that must be a point collection.

ParameterType
vectorSingleVectorSource

Errors

If the source value vector is not a point collection, an error is thrown.

If multiple columns in columnAggregates have the same names, an error is thrown.

Example JSON

{
  "type": "VisualPointClustering",
  "params": {
    "minRadiusPx": 8.0,
    "deltaPx": 1.0,
    "radiusColumn": "__radius",
    "countColumn": "__count",
    "columnAggregates": {
      "mean_population": {
        "columnName": "population",
        "aggregateType": "MeanNumber",
        "measurement": { "type": "unitless" }
      },
      "sample_names": {
        "columnName": "name",
        "aggregateType": "StringSample"
      }
    }
  },
  "sources": {
    "vector": {
      "type": "OgrSource",
      "params": {
        "data": {
          "type": "internal",
          "datasetId": "e977b123-ca47-4c5b-aace-481119826aaf"
        },
        "attributeProjection": ["name", "population"]
      }
    }
  }
}