Nearest vertex

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Find the closest vertex and coordinate of a geometry to a point.

Nearest vertex

Use case

Determine the shortest distance between a location and the boundary of an area. For example, developers can snap imprecise user taps to a geometry if the tap is within a certain distance of the geometry.

How to use the sample

Tap anywhere on the map. An orange cross will show at that location. A blue circle will show the polygon's nearest vertex to the point that was tapped. A red diamond will appear at the coordinate on the geometry that is nearest to the point that was tapped. If tapped inside the geometry, the red and orange markers will overlap. Tap again to dismiss the callout. The information callout showing distance between the tapped point and the nearest vertex/coordinate will be updated with every new location tapped.

How it works

  1. Get an AGSGeometry and an AGSPoint to check the nearest vertex against.
  2. Call class AGSGeometryEngine.nearestVertex(in:to:).
  3. Use the returned AGSProximityResult to get the AGSPoint representing the polygon vertex, and to determine the distance between that vertex and the tapped point.
  4. Call class AGSGeometryEngine.nearestCoordinate(in:to:).
  5. Use the returned AGSProximityResult to get the AGSPoint representing the coordinate on the polygon, and to determine the distance between that coordinate and the tapped point.

Relevant API

  • AGSGeometry
  • AGSProximityResult
  • class AGSGeometryEngine.nearestCoordinate(in:to:)
  • class AGSGeometryEngine.nearestVertex(in:to:)
  • class AGSGeometryEngine.normalizeCentralMeridian(of:)

Additional information

The value of AGSProximityResult.distance is planar (Euclidean) distance. Planar distances are only accurate for geometries that have a defined projected coordinate system, which maintain the desired level of accuracy. The example polygon in this sample is defined in California State Plane Coordinate System - Zone 5 (WKID 2229), which maintains accuracy near Southern California. Accuracy declines outside the state plane zone.

Tags

analysis, coordinate, geometry, nearest, proximity, vertex

Sample Code

NearestVertexViewController.swift
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// Copyright 2020 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 NearestVertexViewController: UIViewController {
    // MARK: Properties

    /// The map view managed by the view controller.
    @IBOutlet var mapView: AGSMapView! {
        didSet {
            mapView.map = makeMap()
            mapView.graphicsOverlays.add(makeGraphicsOverlay())
            mapView.setViewpointCenter(polygon.extent.center, scale: 8e6)
            mapView.touchDelegate = self
            mapView.callout.isAccessoryButtonHidden = true
        }
    }

    /// The example polygon geometry near San Bernardino County, California.
    let polygon: AGSPolygon = {
        let polygonBuilder = AGSPolygonBuilder(spatialReference: .statePlaneCaliforniaZone5)
        polygonBuilder.addPointWith(x: 6627416.41469281, y: 1804532.53233782)
        polygonBuilder.addPointWith(x: 6669147.89779046, y: 2479145.16609522)
        polygonBuilder.addPointWith(x: 7265673.02678292, y: 2484254.50442408)
        polygonBuilder.addPointWith(x: 7676192.55880379, y: 2001458.66365744)
        polygonBuilder.addPointWith(x: 7175695.94143837, y: 1840722.34474458)
        return polygonBuilder.toGeometry()
    }()

    /// The graphic for the tapped location point.
    let tappedLocationGraphic: AGSGraphic = {
        let symbol = AGSSimpleMarkerSymbol(style: .X, color: .orange, size: 15)
        return AGSGraphic(geometry: nil, symbol: symbol)
    }()
    /// The graphic for the nearest coordinate point.
    let nearestCoordinateGraphic: AGSGraphic = {
        let symbol = AGSSimpleMarkerSymbol(style: .diamond, color: .red, size: 10)
        return AGSGraphic(geometry: nil, symbol: symbol)
    }()
    /// The graphic for the nearest vertex point.
    let nearestVertexGraphic: AGSGraphic = {
        let symbol = AGSSimpleMarkerSymbol(style: .circle, color: .blue, size: 15)
        return AGSGraphic(geometry: nil, symbol: symbol)
    }()

    /// A distance formatter to format distance measurements and units.
    let distanceFormatter: MeasurementFormatter = {
        let formatter = MeasurementFormatter()
        formatter.numberFormatter.maximumFractionDigits = 1
        formatter.numberFormatter.minimumFractionDigits = 1
        return formatter
    }()

    // MARK: Methods

    /// Create a map.
    /// - Returns: A new `AGSMap` object.
    func makeMap() -> AGSMap {
        let map = AGSMap(spatialReference: .statePlaneCaliforniaZone5)
        let usStatesGeneralizedLayer = AGSFeatureLayer(
            item: AGSPortalItem(
                portal: .arcGISOnline(withLoginRequired: false),
                itemID: "99fd67933e754a1181cc755146be21ca"),
            layerID: 0
        )
        map.basemap.baseLayers.add(usStatesGeneralizedLayer)
        return map
    }

    func makeGraphicsOverlay() -> AGSGraphicsOverlay {
        let polygonFillSymbol = AGSSimpleFillSymbol(
            style: .forwardDiagonal,
            color: .green,
            outline: AGSSimpleLineSymbol(style: .solid, color: .green, width: 2)
        )
        // The graphic for the polygon.
        let polygonGraphic = AGSGraphic(geometry: polygon, symbol: polygonFillSymbol)

        let graphicsOverlay = AGSGraphicsOverlay()
        graphicsOverlay.graphics.addObjects(from: [
            polygonGraphic,
            nearestCoordinateGraphic,
            tappedLocationGraphic,
            nearestVertexGraphic
        ])
        return graphicsOverlay
    }

    // MARK: UIViewController

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

private extension AGSSpatialReference {
    /// California zone 5 (ftUS) state plane coordinate system.
    static let statePlaneCaliforniaZone5 = AGSSpatialReference(wkid: 2229)!
}

// MARK: - AGSGeoViewTouchDelegate

extension NearestVertexViewController: AGSGeoViewTouchDelegate {
    func showCallout(at mapPoint: AGSPoint) {
        // Get nearest vertex and nearest coordinate results.
        let nearestVertexResult = AGSGeometryEngine.nearestVertex(in: polygon, to: mapPoint)!
        let nearestCoordinateResult = AGSGeometryEngine.nearestCoordinate(in: polygon, to: mapPoint)!

        // Set the geometries for the tapped, nearest coordinate and
        // nearest vertex point graphics.
        nearestVertexGraphic.geometry = nearestVertexResult.point
        nearestCoordinateGraphic.geometry = nearestCoordinateResult.point

        // Get the distance to the nearest vertex in the polygon.
        let distanceVertex = Measurement(
            value: nearestVertexResult.distance,
            unit: UnitLength.feet
        )
        // Get the distance to the nearest coordinate in the polygon.
        let distanceCoordinate = Measurement(
            value: nearestCoordinateResult.distance,
            unit: UnitLength.feet
        )

        // Display the results in a callout at tapped location.
        mapView.callout.title = "Proximity result"
        mapView.callout.detail = String(
            format: "Vertex dist: %@; Point dist: %@",
            distanceFormatter.string(from: distanceVertex),
            distanceFormatter.string(from: distanceCoordinate)
        )
        mapView.callout.show(for: tappedLocationGraphic, tapLocation: mapPoint, animated: true)
    }

    func geoView(_ geoView: AGSGeoView, didTapAtScreenPoint screenPoint: CGPoint, mapPoint: AGSPoint) {
        if mapView.callout.isHidden {
            // If the callout is hidden, show it at the normalized map point.
            guard let normalizedMapPoint = AGSGeometryEngine.normalizeCentralMeridian(of: mapPoint) as? AGSPoint else { return }
            tappedLocationGraphic.geometry = normalizedMapPoint
            showCallout(at: normalizedMapPoint)
        } else {
            // Dismiss the callout and reset geometries.
            mapView.callout.dismiss()
            tappedLocationGraphic.geometry = nil
            nearestVertexGraphic.geometry = nil
            nearestCoordinateGraphic.geometry = nil
        }
    }
}

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