Grid (Hex) Layer

"format": "grid"

Hex grid layers need to be prepared in a PostGIS database. It is recommended to create a series of grid layer for different zoom level. Grid layers are styled with a special bi-variate theme. The grid panel allows to control which field should be used to define the size of a grid cell and a second field is applied to colour grid cells.

Grid layers do not have locations and grid cells can not be selected.

Grid geometry

"geom": "geom"

Grid cells require a PostGIS geometry column of the type GEOMETRY(POINT,4326). By default the geom value will be set to 'geom'.

Grid fields

The fields available for the styling of grid layers must be defined in the grid_fields entry.

"grid_fields": {
  "Population 2001": "pop__01",
  "Population 2011": "pop__11",
  "Female population": "gen_female__11",
  "Male population": "gen_male__11"
}

It is possible to set in the layer entry which fields should be set as the default.

"grid_color": "pop__11"

The default field to be used for the color scale can be set as grid_color.

"grid_size": "pop__01"

The default field to be used for the size scale can be set as grid_size.

Value distribution

The distribution of values is linear between min, avg and max.

Following method is applied to determine the bounds between the colour bins.

A grid cell falls into a bin if it's value is smaller than the bin's colour value.

for (var i = 1; i < n; i++) {

  if (i < (n / 2)) {
    layer.colorBins.push(min + ((avg - min) / (n / 2) * i));
  }

  if (i === (n / 2)) {
    layer.colorBins.push(avg);
  }

  if (i > (n / 2)) {
    layer.colorBins.push(avg + ((max - avg) / (n / 2) * (i - (n / 2))));
  }
      
}

// with n = 4, min = 0, avg = 3000, max = 30000;
// yield 1500, 3000, 16500

// with n = 4, min = 1000, avg = 3000, max = 30000;
// yield 2000, 3000, 16500

// with n = 5, min = 0, avg = 3000, max = 30000;
// yield 1200, 2400, 8400, 19200

// with n = 5, min = 1000, avg = 3000, max = 30000;
// yield 1800, 2600, 8400, 19200