Densify and generalize

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A multipart geometry can be densified by adding interpolated points at regular intervals. Generalizing multipart geometry simplifies it while preserving its general shape. Densifying a multipart geometry adds more vertices at regular intervals.

Image of densify and generalize

Use case

The sample shows a polyline representing a ship's location at irregular intervals. The density of vertices along the ship's route is appropriate to represent the path of the ship at the sample map view's initial scale. However, that level of detail may be too great if you wanted to show a polyline of the ship's movement down the whole of the Willamette river. Then, you might consider generalizing the polyline to still faithfully represent the ship's passage on the river without having an overly complicated geometry.

Densifying a multipart geometry can be used to more accurately represent curved lines or to add more regularity to the vertices making up a multipart geometry.

How to use the sample

Tap the button to open the settings. Use the sliders to control the parameters of the densify and generalize methods. You can deselect the checkboxes for either method to remove its effect from the result polyline.

How it works

  1. Use the static method class AGSGeometryEngine.densifyGeometry(_:maxSegmentLength:) to densify the polyline object. The resulting polyline object will have more points along the line, so that there are no points greater than maxSegmentLength from the next point.
  2. Use the static method class AGSGeometryEngine.generalizeGeometry(_:maxDeviation:removeDegenerateParts:) to generalize the polyline object. The resulting polyline object will have points shifted from the original line to simplify the shape. None of these points can deviate farther from the original line than maxDeviation. The last parameter, removeDegenerateParts, will clean up extraneous parts of a multipart geometry. This will have no effect in this sample as the polyline does not contain extraneous parts.
  3. Note that maxSegmentLength and maxDeviation are in the units of the geometry's coordinate system. In this example, a cartesian coordinate system is used and at a small enough scale that geodesic distances are not required.

Relevant API

  • AGSGeometryEngine
  • AGSMultipoint
  • AGSPoint
  • AGSPointCollection
  • AGSPolyline
  • AGSSimpleLineSymbol
  • AGSSpatialReference

Tags

densify, edit and manage data, generalize, simplify

Sample Code

DensifyAndGeneralizeViewController.swiftGeneralizeSettingsViewController.swift
                                                                                                                                                                            
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// Copyright 2018 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 DensifyAndGeneralizeViewController: UIViewController {
    @IBOutlet var mapView: AGSMapView!

    /// The overlay for which to display the graphics.
    private let graphicsOverlay = AGSGraphicsOverlay()
    /// The base geometry that is densified and generalized.
    private let originalPolyline: AGSPolyline

    /// The graphics for displaying the points of the resultant geometry.
    private let resultPointsGraphic: AGSGraphic = {
        let graphic = AGSGraphic()
        graphic.symbol = AGSSimpleMarkerSymbol(style: .circle, color: .magenta, size: 7)
        return graphic
    }()

    /// The graphic for displaying the lines of the resultant geometry.
    private let resultPolylineGraphic: AGSGraphic = {
        let graphic = AGSGraphic()
        graphic.symbol = AGSSimpleLineSymbol(style: .solid, color: .magenta, width: 3)
        return graphic
    }()

    private var shouldGeneralize = false
    private var maxDeviation = 10.0
    private var shouldDensify = false
    private var maxSegmentLength = 100.0

    required init?(coder aDecoder: NSCoder) {
        let spatialReference = AGSSpatialReference(wkid: 32126)
        /// The base collection of points from which the original polyline and mulitpoint
        /// geometries are made.
        let collection = AGSMutablePointCollection(spatialReference: spatialReference)
        collection.addPointWith(x: 2330611.130549, y: 202360.002957)
        collection.addPointWith(x: 2330583.834672, y: 202525.984012)
        collection.addPointWith(x: 2330574.164902, y: 202691.488009)
        collection.addPointWith(x: 2330689.292623, y: 203170.045888)
        collection.addPointWith(x: 2330696.773344, y: 203317.495798)
        collection.addPointWith(x: 2330691.419723, y: 203380.917080)
        collection.addPointWith(x: 2330435.065296, y: 203816.662457)
        collection.addPointWith(x: 2330369.500800, y: 204329.861789)
        collection.addPointWith(x: 2330400.929891, y: 204712.129673)
        collection.addPointWith(x: 2330484.300447, y: 204927.797132)
        collection.addPointWith(x: 2330514.469919, y: 205000.792463)
        collection.addPointWith(x: 2330638.099138, y: 205271.601116)
        collection.addPointWith(x: 2330725.315888, y: 205631.231308)
        collection.addPointWith(x: 2330755.640702, y: 206433.354860)
        collection.addPointWith(x: 2330680.644719, y: 206660.240923)
        collection.addPointWith(x: 2330386.957926, y: 207340.947204)
        collection.addPointWith(x: 2330485.861737, y: 207742.298501)

        // create the base geometries
        originalPolyline = AGSPolyline(points: collection.array())
        let multipoint = AGSMultipoint(points: collection.array())

        // create graphics for displaying the base points and lines
        let originalPolylineGraphic = AGSGraphic(geometry: originalPolyline, symbol: AGSSimpleLineSymbol(style: .dot, color: .red, width: 3))
        let originalPointsGraphic = AGSGraphic(geometry: multipoint, symbol: AGSSimpleMarkerSymbol(style: .circle, color: .red, size: 7))

        // add the graphics in the order we want them to appear, back to front
        graphicsOverlay.graphics.addObjects(from: [
            originalPointsGraphic,
            originalPolylineGraphic,
            resultPointsGraphic,
            resultPolylineGraphic
        ])

        super.init(coder: aDecoder)
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        // add the source code button item to the right of navigation bar
        (navigationItem.rightBarButtonItem as! SourceCodeBarButtonItem).filenames = [
            "DensifyAndGeneralizeViewController",
            "GeneralizeSettingsViewController"
        ]

        // initialize map with basemap
        let map = AGSMap(basemapStyle: .arcGISStreetsNight)
        // assign map to map view
        mapView.map = map

        // add the graphics overlay to the map view
        mapView.graphicsOverlays.add(graphicsOverlay)

        // set the initial viewpoint to show the extent of the graphics
        mapView.setViewpointGeometry(originalPolyline.extent, padding: 100)
    }

    private func updateGraphicsForDensifyAndGeneralize() {
        var resultPolyline = originalPolyline

        if shouldGeneralize {
            // generalize the polyline with the specified max deviation
            resultPolyline = AGSGeometryEngine.generalizeGeometry(
                resultPolyline,
                maxDeviation: maxDeviation,
                removeDegenerateParts: true
            ) as! AGSPolyline
        }
        if shouldDensify {
            // densify the points of the polyline with the specified max segment length
            resultPolyline = AGSGeometryEngine.densifyGeometry(
                resultPolyline,
                maxSegmentLength: maxSegmentLength
            ) as! AGSPolyline
        }

        // get the result points in an array
        let points = resultPolyline.parts.array().flatMap { $0.points.array() }
        // create a multipoint geometry from the result points
        let resultMultipoint = AGSMultipoint(points: points)

        // update the result graphics with the result geometries
        resultPolylineGraphic.geometry = resultPolyline
        resultPointsGraphic.geometry = resultMultipoint
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let controller = segue.destination as? GeneralizeSettingsViewController {
            // fill the model values in the settings controller
            controller.shouldGeneralize = shouldGeneralize
            controller.shouldDensify = shouldDensify
            controller.maxSegmentLength = maxSegmentLength
            controller.maxDeviation = maxDeviation

            // register to receive value change callbacks
            controller.delegate = self

            // setup the controller to display as a popover
            controller.presentationController?.delegate = self
            controller.preferredContentSize = CGSize(width: 300, height: 250)
        }
    }
}

extension DensifyAndGeneralizeViewController: GeneralizeSettingsViewControllerDelegate {
    func generalizeSettingsViewControllerDidUpdate(_ generalizeSettingsViewController: GeneralizeSettingsViewController) {
        // update the model with the new values
        shouldDensify = generalizeSettingsViewController.shouldDensify
        shouldGeneralize = generalizeSettingsViewController.shouldGeneralize
        maxSegmentLength = generalizeSettingsViewController.maxSegmentLength
        maxDeviation = generalizeSettingsViewController.maxDeviation

        // update the graphics for the new values
        updateGraphicsForDensifyAndGeneralize()
    }
}

extension DensifyAndGeneralizeViewController: UIAdaptivePresentationControllerDelegate {
    func adaptivePresentationStyle(for controller: UIPresentationController, traitCollection: UITraitCollection) -> UIModalPresentationStyle {
        return .none
    }
}

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