Spatial operations

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Find the union, intersection, or difference of two geometries.

Image of spatial operations

Use case

The different spatial operations (union, difference, symmetric difference, and intersection) can be used for a variety of spatial analyses. For example, government authorities may use the intersect operation to determine whether a proposed road cuts through a restricted piece of land such as a nature reserve or a private property.

When these operations are chained together, they become even more powerful. An analysis of food deserts within an urban area might begin by union-ing service areas of grocery stores, farmers markets, and food co-ops. Taking the difference between this single geometry of all services areas and that of a polygon delineating a neighborhood would reveal the areas within that neighborhood where access to healthy, whole foods may not exist.

How to use the sample

The sample provides an option to select a spatial operation. When an operation is selected, the resulting geometry is shown in red.

How it works

  1. Create an AGSGraphicsOverlay and add it to the AGSMapView.
  2. Create each polygon AGSGeometry using AGSPolygonBuilder.
  3. Add the overlapping polygons to the graphics overlay.
  4. Perform spatial relationships between the polygons by using the appropriate operation:

    • class AGSGeometryEngine.union(ofGeometry1:geometry2:) - This method returns the two geometries united together as one geometry.
    • class AGSGeometryEngine.difference(ofGeometry1:geometry2:) - This method returns the difference of Geometry1 from Geometry2.
    • class AGSGeometryEngine.symmetricDifference(ofGeometry1:geometry2:) - This method returns any part of Geometry1 or Geometry2 which do not intersect.
    • class AGSGeometryEngine.intersection(ofGeometry1:geometry2:) - This method returns the intersection of Geometry1 and Geometry2.
  5. Use the geometry that is returned from the method call to create a new AGSGraphic and add it to the graphics overlay for it to be displayed.

Relevant API

  • AGSGeometry
  • AGSGeometryEngine
  • AGSGraphic
  • AGSGraphicsOverlay
  • class AGSGeometryEngine.difference(ofGeometry1:geometry2:)
  • class AGSGeometryEngine.intersection(ofGeometry1:geometry2:)
  • class AGSGeometryEngine.symmetricDifference(ofGeometry1:geometry2:)
  • class AGSGeometryEngine.union(ofGeometry1:geometry2:)

Tags

analysis, combine, difference, geometry, intersection, merge, polygon, union

Sample Code

SpatialOperationsViewController.swift
                                                                                                                                                                           
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// Copyright 2017 Esri.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import UIKit
import ArcGIS

class SpatialOperationsViewController: UIViewController {
    // MARK: Storyboard view and properties

    /// The map view managed by the view controller.
    @IBOutlet var mapView: AGSMapView! {
        didSet {
            // Initialize map with basemap.
            mapView.map = AGSMap(basemapStyle: .arcGISTopographic)
            // Add the graphics overlay with two polygon graphics and the result graphic to map view.
            mapView.graphicsOverlays.add(makeGraphicsOverlay())
            // Set the map view's viewpoint.
            let center = AGSPoint(x: -13453, y: 6710127, spatialReference: .webMercator())
            mapView.setViewpointCenter(center, scale: 30000, completion: nil)
        }
    }

    /// The resulting graphic for the spatial operation.
    private var resultGraphic: AGSGraphic!

    private let polygon1: AGSGeometry = {
        // Create the polygon 1.
        let polygon = AGSPolygonBuilder(spatialReference: .webMercator())
        polygon.addPointWith(x: -13960, y: 6709400)
        polygon.addPointWith(x: -14660, y: 6710000)
        polygon.addPointWith(x: -13760, y: 6710730)
        polygon.addPointWith(x: -13300, y: 6710500)
        polygon.addPointWith(x: -13160, y: 6710100)
        return polygon.toGeometry()
    }()

    private let polygon2: AGSGeometry = {
        // The outer ring of polygon 2.
        let outerRing = AGSMutablePart(spatialReference: .webMercator())
        outerRing.addPointWith(x: -13060, y: 6711030)
        outerRing.addPointWith(x: -12160, y: 6710730)
        outerRing.addPointWith(x: -13160, y: 6709700)
        outerRing.addPointWith(x: -14560, y: 6710730)
        outerRing.addPointWith(x: -13060, y: 6711030)

        // The inner ring of polygon 2.
        let innerRing = AGSMutablePart(spatialReference: .webMercator())
        innerRing.addPointWith(x: -13060, y: 6710910)
        innerRing.addPointWith(x: -14160, y: 6710630)
        innerRing.addPointWith(x: -13160, y: 6709900)
        innerRing.addPointWith(x: -12450, y: 6710660)
        innerRing.addPointWith(x: -13060, y: 6710910)

        // Create polygon 2.
        let polygon = AGSPolygonBuilder(spatialReference: .webMercator())
        polygon.parts.add(outerRing)
        polygon.parts.add(innerRing)
        return polygon.toGeometry()
    }()

    /// An enum of spatial operations.
    private enum SpatialOperation: CaseIterable {
        case none, union, difference, symmetricDifference, intersection
        /// Human readable label strings for each spatial operation.
        var label: String {
            switch self {
            case .none: return "None"
            case .union: return "Union"
            case .difference: return "Difference"
            case .symmetricDifference: return "Symmetric Difference"
            case .intersection: return "Intersection"
            }
        }
    }
    /// The selected operation.
    private var selectedOperation = SpatialOperation.none

    // MARK: Methods

    func makeGraphicsOverlay() -> AGSGraphicsOverlay {
        // A black line symbol for borders of the graphics.
        let lineSymbol = AGSSimpleLineSymbol(style: .solid, color: .black, width: 1)
        // The blue fill symbol of polygon 1.
        let fillSymbol1 = AGSSimpleFillSymbol(style: .solid, color: .blue, outline: lineSymbol)
        // The graphic of polygon 1.
        let polygon1Graphic = AGSGraphic(geometry: polygon1, symbol: fillSymbol1)
        // The green fill symbol of polygon 2.
        let fillSymbol2 = AGSSimpleFillSymbol(style: .solid, color: .green, outline: lineSymbol)
        // The graphic of polygon 2.
        let polygon2Graphic = AGSGraphic(geometry: polygon2, symbol: fillSymbol2)

        // Using red fill symbol with black border for result graphic.
        let symbol = AGSSimpleFillSymbol(style: .solid, color: .red, outline: lineSymbol)
        let graphic = AGSGraphic(geometry: nil, symbol: symbol)
        resultGraphic = graphic

        // An overlay to display polygon graphics.
        let graphicsOverlay = AGSGraphicsOverlay()

        // Add graphics to graphics overlay.
        graphicsOverlay.graphics.addObjects(from: [polygon1Graphic, polygon2Graphic, graphic])
        return graphicsOverlay
    }

    private func performOperation(_ operation: SpatialOperation) {
        let resultGeometry: AGSGeometry?
        switch operation {
        case .none:
            resultGeometry = nil
        case .union:
            resultGeometry = AGSGeometryEngine.union(ofGeometry1: polygon1, geometry2: polygon2)!
        case .difference:
            resultGeometry = AGSGeometryEngine.difference(ofGeometry1: polygon1, geometry2: polygon2)!
        case .symmetricDifference:
            resultGeometry = AGSGeometryEngine.symmetricDifference(ofGeometry1: polygon1, geometry2: polygon2)!
        case .intersection:
            resultGeometry = AGSGeometryEngine.intersection(ofGeometry1: polygon1, geometry2: polygon2)!
        }
        // Update the geometry.
        resultGraphic.geometry = resultGeometry
    }

    @IBAction func chooseOperationBarButtonTapped(_ sender: UIBarButtonItem) {
        let selectedIndex = SpatialOperation.allCases.firstIndex(of: selectedOperation)

        let controller = OptionsTableViewController(labels: SpatialOperation.allCases.map { $0.label }, selectedIndex: selectedIndex) { [weak self] newIndex in
            guard let self = self else { return }
            let newOperation = SpatialOperation.allCases[newIndex]
            self.selectedOperation = newOperation
            // Perform the new spatial operation.
            self.performOperation(newOperation)
        }

        // Configure the options controller as a popover.
        controller.modalPresentationStyle = .popover
        controller.presentationController?.delegate = self
        controller.preferredContentSize = CGSize(width: 300, height: CGFloat(SpatialOperation.allCases.count) * 44)
        controller.popoverPresentationController?.barButtonItem = sender

        // Show the popover.
        present(controller, animated: true)
    }

    // MARK: UIViewController

    override func viewDidLoad() {
        super.viewDidLoad()
        // Add the source code button item to the right of navigation bar.
        (navigationItem.rightBarButtonItem as? SourceCodeBarButtonItem)?.filenames = ["SpatialOperationsViewController", "OptionsTableViewController"]
    }
}

// MARK: - UIAdaptivePresentationControllerDelegate

extension SpatialOperationsViewController: UIAdaptivePresentationControllerDelegate {
    func adaptivePresentationStyle(for controller: UIPresentationController, traitCollection: UITraitCollection) -> UIModalPresentationStyle {
        // Show presented controller as a popover even on small displays.
        return .none
    }
}

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