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 Android.App;
using Android.OS;
using Android.Widget;
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 System;
using System.Timers;
using ArcGISRuntime.Samples.Managers;
using Android.Views;

namespace ArcGISRuntime.Samples.LineOfSightGeoElement
{
    [Activity (ConfigurationChanges=Android.Content.PM.ConfigChanges.Orientation | Android.Content.PM.ConfigChanges.ScreenSize)]
	[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 : Activity
    {
        // Hold a reference to the SceneView
        private SceneView _mySceneView;

        // Hold the label that will show the analysis status
        private TextView _myStatusLabel;

        // Hold the slider that will control observer height
        private SeekBar _myHeightSlider;

        // 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 builidng 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 readonly int _frameMax = 150;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            Title = "Line of sight (GeoElement)";

            // Create the UI, setup the control references and execute initialization
            CreateLayout();
            Initialize();
        }

        private async void Initialize()
        {
            // Create scene
            Scene myScene = new Scene(Basemap.CreateImageryWithLabels())
            {
                InitialViewpoint = new Viewpoint(_observerPoint, 1600)
            };
            // Set initial viewpoint
            // 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
                // Create the model symbol for the taxi
                ModelSceneSymbol taxiSymbol = await ModelSceneSymbol.CreateAsync(new Uri(GetModelUri()));
                // 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)
                {
                    TargetOffsetZ = 2
                };
                // Apply an offset to the target. This helps avoid some false negatives
                // 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
                Timer timer = new Timer(60);
                // Move the taxi every time the timer expires
                timer.Elapsed += AnimationTimer_Elapsed;
                // Keep the timer running continuously
                timer.AutoReset = true;
                // Start the timer
                timer.Start();

                // Subscribe to TargetVisible events; allows for updating the UI and selecting the taxi when it is visible
                _geoLine.TargetVisibilityChanged += Geoline_TargetVisibilityChanged;

                // Add the scene to the view
                _mySceneView.Scene = myScene;
            }
            catch (Exception e)
            {
                new AlertDialog.Builder(this).SetMessage(e.ToString()).SetTitle("Error").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 travelled
            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 travelling from
            MapPoint starting = _points[_pointIndex];
            // Get the point the taxi is travelling 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
            MapPoint intermediatePoint = InterpolatedPoint(starting, ending, progress);
            // Update the taxi geometry
            _taxiGraphic.Geometry = intermediatePoint;

            // 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 travelling {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
            RunOnUiThread(UpdateUiAndSelection);
        }

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

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

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

        private static string GetModelUri()
        {
            // Returns the taxi model
            return DataManager.GetDataFolder("3af5cfec0fd24dac8d88aea679027cb9", "dolmus.3ds");
        }

        private void CreateLayout()
        {
            // Create a new vertical layout for the app
            LinearLayout layout = new LinearLayout(this) { Orientation = Orientation.Vertical };

            // Create the controls
            _myHeightSlider = new SeekBar(this) { Max = 100 };
            _myStatusLabel = new TextView(this) { Text = "Status: " };

            // Subscribe to height slider changes
            _myHeightSlider.ProgressChanged += MyHeightSlider_ProgressChanged;

            // Add the views to the layout
            layout.AddView(_myStatusLabel);
            layout.AddView(_myHeightSlider);
            _mySceneView = new SceneView(this);
            layout.AddView(_mySceneView);

            // Show the layout in the app
            SetContentView(layout);
        }

        private void MyHeightSlider_ProgressChanged(object sender, SeekBar.ProgressChangedEventArgs e)
        {
            // Update the height of the observer based on the slider value

            // Constrain the min and max to 20 and 150 units
            double minHeight = 20;
            double maxHeight = 150;

            // Scale the slider value; its default range is 0-10
            double value = e.Progress / 100.0;

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

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

            // Apply the updated geometry to the observer point
            _observerGraphic.Geometry = newPoint;
        }

        protected override void OnDestroy()
        {
            base.OnDestroy();

            // Remove the sceneview
            (_mySceneView.Parent as ViewGroup).RemoveView(_mySceneView);
            _mySceneView.Dispose();
            _mySceneView = null;
        }
    }
}

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