Line of sight (geoelement)

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Show a line of sight between two moving objects.

Image of line of sight geoelement

Use case

A line of sight between GeoElements (i.e. observer and target) will not remain constant whilst one or both are on the move.

A GeoElementLineOfSight is therefore useful in cases where visibility between two GeoElements requires monitoring over a period of time in a partially obstructed field of view (such as buildings in a city).

How to use the sample

A line of sight will display between a point on the Empire State Building (observer) and a taxi (target). The taxi will drive around a block and the line of sight should automatically update. The taxi will be highlighted when it is visible. You can change the observer height with the slider to see how it affects the target's visibility.

How it works

  1. Instantiate an AnalysisOverlay and add it to the SceneView's analysis overlays collection.
  2. Instantiate a GeoElementLineOfSight, passing in observer and target GeoElements (features or graphics). Add the line of sight to the analysis overlay's analyses collection.
  3. To get the target visibility when it changes, react to the target visibility changing on the GeoElementLineOfSight instance.

Relevant API

  • AnalysisOverlay
  • GeoElementLineOfSight
  • LineOfSight.TargetVisibility

Offline data

This sample downloads the following items from ArcGIS Online automatically:

Tags

3D, line of sight, visibility, visibility analysis

Sample Code

LineOfSightGeoElement.cs
                                                                                                                                                                                                                                                                                                                                     
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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.

using System;
using System.Timers;
using ArcGISRuntime.Samples.Managers;
using Esri.ArcGISRuntime.Geometry;
using Esri.ArcGISRuntime.Mapping;
using Esri.ArcGISRuntime.Symbology;
using Esri.ArcGISRuntime.UI;
using Esri.ArcGISRuntime.UI.Controls;
using Esri.ArcGISRuntime.UI.GeoAnalysis;
using Foundation;
using UIKit;

namespace ArcGISRuntime.Samples.LineOfSightGeoElement
{
    [Register("LineOfSightGeoElement")]
    [ArcGISRuntime.Samples.Shared.Attributes.OfflineData("3af5cfec0fd24dac8d88aea679027cb9")]
    [ArcGISRuntime.Samples.Shared.Attributes.Sample(
        name: "Line of sight (geoelement)",
        category: "Analysis",
        description: "Show a line of sight between two moving objects.",
        instructions: "A line of sight will display between a point on the Empire State Building (observer) and a taxi (target).",
        tags: new[] { "3D", "line of sight", "visibility", "visibility analysis" })]
    public class LineOfSightGeoElement : UIViewController
    {
        // Hold references to UI controls.
        private SceneView _mySceneView;
        private UILabel _statusLabel;
        private UISlider _heightSlider;

        // URL of the elevation service - provides elevation component of the scene.
        private readonly Uri _elevationUri = new Uri("https://elevation3d.arcgis.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer");

        // URL of the building service - provides building models.
        private readonly Uri _buildingsUri = new Uri("https://tiles.arcgis.com/tiles/z2tnIkrLQ2BRzr6P/arcgis/rest/services/New_York_LoD2_3D_Buildings/SceneServer/layers/0");

        // Starting point of the observation point.
        private readonly MapPoint _observerPoint = new MapPoint(-73.984988, 40.748131, 20, SpatialReferences.Wgs84);

        // Graphic to represent the observation point.
        private Graphic _observerGraphic;

        // Graphic to represent the observed target.
        private Graphic _taxiGraphic;

        // Line of Sight Analysis.
        private GeoElementLineOfSight _geoLine;

        // For taxi animation - four points in a loop.
        private readonly MapPoint[] _points =
        {
            new MapPoint(-73.984513, 40.748469, SpatialReferences.Wgs84),
            new MapPoint(-73.985068, 40.747786, SpatialReferences.Wgs84),
            new MapPoint(-73.983452, 40.747091, SpatialReferences.Wgs84),
            new MapPoint(-73.982961, 40.747762, SpatialReferences.Wgs84)
        };

        // For taxi animation - tracks animation state.
        private int _pointIndex = 0;
        private int _frameIndex = 0;
        private const int FrameMax = 150;

        // Timer to run the taxi animation.
        private Timer _animationTimer;

        public LineOfSightGeoElement()
        {
            Title = "Line of sight (GeoElement)";
        }

        private async void Initialize()
        {
            // Create scene.
            Scene myScene = new Scene(Basemap.CreateImageryWithLabels())
            {
                // Set initial viewpoint.
                InitialViewpoint = new Viewpoint(_observerPoint, 1600)
            };

            // Create the elevation source.
            ElevationSource myElevationSource = new ArcGISTiledElevationSource(_elevationUri);
            // Add the elevation source to the scene.
            myScene.BaseSurface.ElevationSources.Add(myElevationSource);
            // Create the building scene layer.
            ArcGISSceneLayer mySceneLayer = new ArcGISSceneLayer(_buildingsUri);
            // Add the building layer to the scene.
            myScene.OperationalLayers.Add(mySceneLayer);

            // Add the observer to the scene.
            // Create a graphics overlay with relative surface placement; relative surface placement allows the Z position of the observation point to be adjusted.
            GraphicsOverlay overlay = new GraphicsOverlay {SceneProperties = new LayerSceneProperties(SurfacePlacement.Relative)};
            // Create the symbol that will symbolize the observation point.
            SimpleMarkerSceneSymbol symbol = new SimpleMarkerSceneSymbol(SimpleMarkerSceneSymbolStyle.Sphere, System.Drawing.Color.Red, 10, 10, 10, SceneSymbolAnchorPosition.Bottom);
            // Create the observation point graphic from the point and symbol.
            _observerGraphic = new Graphic(_observerPoint, symbol);
            // Add the observer to the overlay.
            overlay.Graphics.Add(_observerGraphic);
            // Add the overlay to the scene.
            _mySceneView.GraphicsOverlays.Add(overlay);

            try
            {
                // Add the taxi to the scene.
                // Get the path to the model.
                string taxiModelPath = DataManager.GetDataFolder("3af5cfec0fd24dac8d88aea679027cb9", "dolmus.3ds");
                // Create the model symbol for the taxi.
                ModelSceneSymbol taxiSymbol = await ModelSceneSymbol.CreateAsync(new Uri(taxiModelPath));
                // Set the anchor position for the mode; ensures that the model appears above the ground.
                taxiSymbol.AnchorPosition = SceneSymbolAnchorPosition.Bottom;
                // Create the graphic from the taxi starting point and the symbol.
                _taxiGraphic = new Graphic(_points[0], taxiSymbol);
                // Add the taxi graphic to the overlay.
                overlay.Graphics.Add(_taxiGraphic);

                // Create GeoElement Line of sight analysis (taxi to building).
                // Create the analysis.
                _geoLine = new GeoElementLineOfSight(_observerGraphic, _taxiGraphic)
                {
                    // Apply an offset to the target. This helps avoid some false negatives.
                    TargetOffsetZ = 2
                };
                // Create the analysis overlay.
                AnalysisOverlay myAnalysisOverlay = new AnalysisOverlay();
                // Add the analysis to the overlay.
                myAnalysisOverlay.Analyses.Add(_geoLine);
                // Add the analysis overlay to the scene.
                _mySceneView.AnalysisOverlays.Add(myAnalysisOverlay);

                // Create a timer; this will enable animating the taxi.
                _animationTimer = new Timer(120);
                // Keep the timer running continuously.
                _animationTimer.AutoReset = true;

                // Add the scene to the view.
                _mySceneView.Scene = myScene;
            }
            catch (Exception e)
            {
                new UIAlertView("Error", e.ToString(), (IUIAlertViewDelegate) null, "OK", null).Show();
            }
        }

        private void AnimationTimer_Elapsed(object sender, EventArgs e)
        {
            // Note: the contents of this function are solely related to animating the taxi.

            // Increment the frame counter.
            _frameIndex++;

            // Reset the frame counter once one segment of the path has been traveled.
            if (_frameIndex == FrameMax)
            {
                _frameIndex = 0;

                // Start navigating toward the next point.
                _pointIndex++;

                // Restart if finished circuit.
                if (_pointIndex == _points.Length)
                {
                    _pointIndex = 0;
                }
            }

            // Get the point the taxi is traveling from.
            MapPoint starting = _points[_pointIndex];
            // Get the point the taxi is traveling to.
            MapPoint ending = _points[(_pointIndex + 1) % _points.Length];
            // Calculate the progress based on the current frame.
            double progress = _frameIndex / (double) FrameMax;
            // Calculate the position of the taxi when it is {progress}% of the way through.
            _taxiGraphic.Geometry = InterpolatedPoint(starting, ending, progress);

            // Update the taxi rotation.
            GeodeticDistanceResult distance = GeometryEngine.DistanceGeodetic(starting, ending, LinearUnits.Meters, AngularUnits.Degrees, GeodeticCurveType.Geodesic);
            ((ModelSceneSymbol)_taxiGraphic.Symbol).Heading = distance.Azimuth1;
        }

        private MapPoint InterpolatedPoint(MapPoint firstPoint, MapPoint secondPoint, double progress)
        {
            // This function returns a MapPoint that is the result of traveling {progress}% of the way from {firstPoint} to {secondPoint}.

            // Get the difference between the two points.
            MapPoint difference = new MapPoint(secondPoint.X - firstPoint.X, secondPoint.Y - firstPoint.Y, secondPoint.Z - firstPoint.Z, SpatialReferences.Wgs84);
            // Scale the difference by the progress towards the destination.
            MapPoint scaled = new MapPoint(difference.X * progress, difference.Y * progress, difference.Z * progress);
            // Add the scaled progress to the starting point.
            return new MapPoint(firstPoint.X + scaled.X, firstPoint.Y + scaled.Y, firstPoint.Z + scaled.Z);
        }

        private void Geoline_TargetVisibilityChanged(object sender, EventArgs e)
        {
            // This is needed because Runtime delivers notifications from a different thread that doesn't have access to UI controls.
            BeginInvokeOnMainThread(UpdateUiAndSelection);
        }

        private void UpdateUiAndSelection()
        {
            switch (_geoLine.TargetVisibility)
            {
                case LineOfSightTargetVisibility.Obstructed:
                    _statusLabel.Text = "Status: Obstructed";
                    _taxiGraphic.IsSelected = false;
                    break;

                case LineOfSightTargetVisibility.Visible:
                    _statusLabel.Text = "Status: Visible";
                    _taxiGraphic.IsSelected = true;
                    break;

                default:
                case LineOfSightTargetVisibility.Unknown:
                    _statusLabel.Text = "Status: Unknown";
                    _taxiGraphic.IsSelected = false;
                    break;
            }
        }

        private void MyHeightSlider_ValueChanged(object sender, EventArgs e)
        {
            // Constrain the min and max to 20 and 150 units.
            const double minHeight = 20;
            const double maxHeight = 150;

            // Scale the slider value; its default range is 0-10.
            double value = (sender as UISlider).Value;

            // Get the current point.
            MapPoint oldPoint = (MapPoint) _observerGraphic.Geometry;

            // Update geometry with a new point with the same (x,y) but updated z.
            _observerGraphic.Geometry = new MapPoint(oldPoint.X, oldPoint.Y, (maxHeight - minHeight) * value + minHeight);
        }

        public override void ViewDidLoad()
        {
            Initialize();

            base.ViewDidLoad();
        }

        public override void LoadView()
        {
            // Create the views.
            View = new UIView() { BackgroundColor = ApplicationTheme.BackgroundColor };

            _mySceneView = new SceneView();
            _mySceneView.TranslatesAutoresizingMaskIntoConstraints = false;

            _heightSlider = new UISlider();
            _heightSlider.TranslatesAutoresizingMaskIntoConstraints = false;

            UIBarButtonItem sliderWrapper = new UIBarButtonItem(_heightSlider);
            sliderWrapper.Width = 300;

            UIToolbar sliderToolbar = new UIToolbar();
            sliderToolbar.TranslatesAutoresizingMaskIntoConstraints = false;
            sliderToolbar.Items = new[]
            {
                new UIBarButtonItem(UIBarButtonSystemItem.FlexibleSpace),
                sliderWrapper,
                new UIBarButtonItem(UIBarButtonSystemItem.FlexibleSpace)
            };

            _statusLabel = new UILabel
            {
                BackgroundColor = UIColor.FromWhiteAlpha(0f, .6f),
                TextColor = UIColor.White,
                TextAlignment = UITextAlignment.Center,
                TranslatesAutoresizingMaskIntoConstraints = false
            };

            // Add the views.
            View.AddSubviews(_mySceneView, sliderToolbar, _statusLabel);

            // Lay out the views.
            NSLayoutConstraint.ActivateConstraints(new[]
            {
                sliderToolbar.BottomAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.BottomAnchor),
                sliderToolbar.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor),
                sliderToolbar.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor),

                _mySceneView.TopAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TopAnchor),
                _mySceneView.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor),
                _mySceneView.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor),
                _mySceneView.BottomAnchor.ConstraintEqualTo(sliderToolbar.TopAnchor),

                _statusLabel.TopAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TopAnchor),
                _statusLabel.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor),
                _statusLabel.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor),
                _statusLabel.HeightAnchor.ConstraintEqualTo(40)
            });
        }

        public override void ViewWillAppear(bool animated)
        {
            base.ViewWillAppear(animated);

            // Subscribe to events.
            _heightSlider.ValueChanged += MyHeightSlider_ValueChanged;
            _geoLine.TargetVisibilityChanged += Geoline_TargetVisibilityChanged;
            _animationTimer.Elapsed += AnimationTimer_Elapsed;
            _animationTimer.Start();
        }

        public override void ViewDidDisappear(bool animated)
        {
            base.ViewDidDisappear(animated);

            // Unsubscribe from events, per best practice.
            _geoLine.TargetVisibilityChanged -= Geoline_TargetVisibilityChanged;
            _animationTimer.Stop();
            _animationTimer.Elapsed -= AnimationTimer_Elapsed;
            _heightSlider.ValueChanged -= MyHeightSlider_ValueChanged;
        }
    }
}

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