Query statistics by geometry

Note: Support for 3D on mobile devices may vary, view the system requirements for more information.

This sample demonstrates how to query for statistics in a FeatureLayerView by geometry and display the results of the query in a chart.

This application displays 2010 population density by census tracts. The population is queried by age and gender among census tracts intersecting a buffer, and is displayed in a population pyramid chart.

How it works

The population pyramid chart is updated as the user drags the center point of the buffer or resizes the buffer by moving its edge handler point. As the user moves one of the two points, a geodesic buffer is recalculated and used to query the statistics of all features in the layer view that intersect the buffer. The center and edge handler points of the buffer graphic are updated by listening to the move events on SketchViewModel.

// Listen to sketch view model's move events when user clicks
// on the center or edge graphic. When users move these graphics
// run a client side stats query to return total number of population
// by gender for the census tracts that intersect the buffer
sketchViewModel.on("move-start", onMove);
sketchViewModel.on("move", onMove);
sketchViewModel.on("move-complete", onMove);

/*********************************************************************
 * Edge or center graphics are being moved. Recalculate the buffer with
 * updated geometry information and run the query stats again.
 *********************************************************************/
function onMove(event) {
  // User is moving the 'center' graphic.
  // Adjust other graphics based on the movement of the center graphic
  if (activeGraphic === centerGraphic) {
    // Update the reference graphic's geometry
    centerGraphic.geometry = event.geometry;

    // Calculate location of edgeGraphic
    // dx/dy values are in pixels
    const edgeScreenPoint = view.toScreen(edgeGraphic.geometry);
    const edgeX = edgeScreenPoint.x + event.dx;
    const edgeY = edgeScreenPoint.y + event.dy;

    edgeGraphic.geometry = view.toMap(edgeX, edgeY);
  }
  // User is moving on the 'edge' graphic.
  // Resize the polyline graphic and recalculate the buffer polygon
  else if (activeGraphic === edgeGraphic) {
    // Update the reference graphic's geometry
    edgeGraphic.geometry = event.geometry;
  }

  const vertices = [
    [centerGraphic.geometry.x, centerGraphic.geometry.y],
    [edgeGraphic.geometry.x, edgeGraphic.geometry.y]
  ];

  // client-side stats query of features that intersect the buffer
  calculateBuffer(vertices);
}

The updated buffer polygon is then used to query statistics to get total population belonging to different age categories by gender among census tracts. Once the statistics query results are returned, the information is displayed in the population pyramid chart.

/*********************************************************************
 * Spatial query the census tracts feature layer view for statistics
 * using the updated buffer polygon.
*********************************************************************/
function queryLayerViewAgeStats(buffer) {
  // Data storage for the chart
  let femaleAgeData = [],
      maleAgeData = [];

  // Client-side spatial query:
  // Get a sum of age groups for census tracts that intersect the polygon buffer
  const query = featureLayerView.layer.createQuery();
  query.outStatistics = statDefinitions;
  query.geometry = buffer;

  // Query the features on the client using FeatureLayerView.queryFeatures
  return featureLayerView.queryFeatures(query).then(function(results) {
      // Statistics query returns a feature with 'stats' as attributes
      const attributes = results.features[0].attributes;
      // Loop through attributes and save the values for use in the population pyramid.
      for (var key in attributes) {
        if (key.includes("FEM")) {
          femaleAgeData.push(attributes[key]);
        } else {
          // Make 'all male age group population' total negative so that
          // data will be displayed to the left of female age group
          maleAgeData.push(-Math.abs(attributes[key]));
        }
      }
      // Return information, seperated by gender
      return [femaleAgeData, maleAgeData];
    })
    .catch(function(error) {
      console.log(error);
    });
}