List transformations by suitability

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Get a list of suitable transformations for projecting a geometry between two spatial references with different horizontal datums.

Image of list transformations by suitability

Use case

Transformations (sometimes known as datum or geographic transformations) are used when projecting data from one spatial reference to another when there is a difference in the underlying datum of the spatial references. Transformations can be mathematically defined by specific equations (equation-based transformations), or may rely on external supporting files (grid-based transformations). Choosing the most appropriate transformation for a situation can ensure the best possible accuracy for this operation. Some users familiar with transformations may wish to control which transformation is used in an operation.

How to use the sample

Select a transformation from the list to see the result of projecting the point from EPSG:27700 to EPSG:3857 using that transformation. The result is shown as a red cross; you can visually compare the original blue point with the projected red cross.

Select 'Consider current extent' to limit the transformations that are appropriate for the current extent.

If the selected transformation is not usable (has missing grid files) then an error is displayed.

How it works

  1. Pass the input and output spatial references to TransformationCatalog.GetTransformationsBySuitability for transformations based on the map's spatial reference OR additionally provide an extent argument to only return transformations suitable to the extent. This returns a list of ranked transformations.
  2. Use one of the DatumTransformation objects returned to project the input geometry to the output spatial reference.

Relevant API

  • DatumTransformation
  • GeographicTransformation
  • GeographicTransformationStep
  • GeometryEngine
  • GeometryEngine.Project
  • TransformationCatalog

Additional information

Some transformations aren't available until transformation data is provided.

This sample uses a GeographicTransformation, which extends the DatumTransformation class. As of 100.9, ArcGIS Runtime also includes a HorizontalVerticalTransformation, which also extends DatumTransformation. The HorizontalVerticalTransformation class is used to transform coordinates of z-aware geometries between spatial references that have different geographic and/or vertical coordinate systems.

This sample can be used with or without provisioning projection engine data to your device. If you do not provision data, a limited number of transformations will be available.

To download projection engine data to your device:

  1. Log in to the ArcGIS for Developers site using your Developer account.
  2. In the Dashboard page, tap 'Download APIs and SDKs'.
  3. Tap the download button next to Projection Engine Data to download projection engine data to your computer.
  4. Unzip the downloaded data on your computer.
  5. Create an ~/ArcGIS/Runtime/Data/PEDataRuntime directory on your device and copy the files to this directory.

About the data

The map starts out zoomed into the grounds of the Royal Observatory, Greenwich. The initial point is in the British National Grid spatial reference, which was created by the United Kingdom Ordnance Survey. The spatial reference after projection is in web mercator.

Tags

datum, geodesy, projection, spatial reference, transformation

Sample Code

ListTransformations.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.Collections.Generic;
using System.Drawing;
using Esri.ArcGISRuntime;
using Esri.ArcGISRuntime.Geometry;
using Esri.ArcGISRuntime.Mapping;
using Esri.ArcGISRuntime.Symbology;
using Esri.ArcGISRuntime.UI;
using Esri.ArcGISRuntime.UI.Controls;
using Foundation;
using UIKit;

namespace ArcGISRuntime.Samples.ListTransformations
{
    [Register("ListTransformations")]
    [ArcGISRuntime.Samples.Shared.Attributes.Sample(
        name: "List transformations by suitability",
        category: "Geometry",
        description: "Get a list of suitable transformations for projecting a geometry between two spatial references with different horizontal datums.",
        instructions: "Select a transformation from the list to see the result of projecting the point from EPSG:27700 to EPSG:3857 using that transformation. The result is shown as a red cross; you can visually compare the original blue point with the projected red cross.",
        tags: new[] { "datum", "geodesy", "projection", "spatial reference", "transformation" })]
    public class ListTransformations : UIViewController
    {
        // Hold references to UI controls.
        private UILabel _inWkidLabel;
        private UILabel _outWkidLabel;
        private UIPickerView _transformationsPicker;
        private UISwitch _useExtentSwitch;
        private MapView _myMapView;
        private UIStackView _transformToolsView;
        private UIStackView _outerStackView;

        // Point whose coordinates will be projected using a selected transform.
        private MapPoint _originalPoint;

        // Graphic representing the projected point.
        private Graphic _projectedPointGraphic;

        // GraphicsOverlay to hold the point graphics.
        private GraphicsOverlay _pointsOverlay;

        // Text view to display messages to the user (exceptions, etc.).
        private UITextView _messagesTextView;

        public ListTransformations()
        {
            Title = "List transformations by suitability";
        }

        private void Initialize()
        {
            // Create the map and add it to the map view control.
            Map myMap = new Map(BasemapStyle.ArcGISImagery);

            // Create a point in the Greenwich observatory courtyard in London, UK, the location of the prime meridian.
            _originalPoint = new MapPoint(538985.355, 177329.516, SpatialReference.Create(27700));

            // Set the initial extent to an extent centered on the point.
            myMap.InitialViewpoint = new Viewpoint(_originalPoint, 5000);

            // Handle the map loading to fill the UI controls.
            myMap.Loaded += MyMap_Loaded;

            // Add the map to the map view.
            _myMapView.Map = myMap;

            // Create a graphics overlay to hold the original and projected points.
            _pointsOverlay = new GraphicsOverlay();
            _myMapView.GraphicsOverlays.Add(_pointsOverlay);

            // Add the point as a graphic with a blue square.
            SimpleMarkerSymbol markerSymbol = new SimpleMarkerSymbol(SimpleMarkerSymbolStyle.Square, Color.Blue, 15);
            Graphic originalGraphic = new Graphic(_originalPoint, markerSymbol);
            _pointsOverlay.Graphics.Add(originalGraphic);

            // Get the path to the projection engine data (if it exists).
            string peFolderPath = GetProjectionDataPath();
            if (!String.IsNullOrEmpty(peFolderPath))
            {
                TransformationCatalog.ProjectionEngineDirectory = peFolderPath;
                _messagesTextView.Text = "Using projection data found at '" + peFolderPath + "'";
            }
            else
            {
                _messagesTextView.Text = "Projection engine data not found.";
            }
        }

        private void MyMap_Loaded(object sender, EventArgs e)
        {
            // Unsubscribe from event.
            ((Map) sender).Loaded -= MyMap_Loaded;

            // Get the map's spatial reference.
            SpatialReference mapSpatialReference = ((Map) sender).SpatialReference;

            // Run on the UI thread.
            InvokeOnMainThread(() =>
            {
                // Show the input and output spatial reference (WKID) in the labels.
                _inWkidLabel.Text = "In WKID = " + _originalPoint.SpatialReference.Wkid;
                _outWkidLabel.Text = "Out WKID = " + mapSpatialReference.Wkid;

                // Call a function to create a list of transformations to fill the picker.
                GetSuitableTransformations(_originalPoint.SpatialReference, mapSpatialReference, _useExtentSwitch.On);
            });
        }

        private void UseExtentSwitch_ValueChanged(object sender, EventArgs e)
        {
            // Recreate the contents of the datum transformations list box.
            GetSuitableTransformations(_originalPoint.SpatialReference, _myMapView.Map.SpatialReference, _useExtentSwitch.On);
        }

        // Function to get suitable datum transformations for the specified input and output spatial references.
        private void GetSuitableTransformations(SpatialReference inSpatialRef, SpatialReference outSpatialRef, bool considerExtent)
        {
            // Get suitable transformations. Use the current extent to evaluate suitability, if requested.
            IReadOnlyList<DatumTransformation> transformations;
            if (considerExtent)
            {
                Envelope currentExtent = _myMapView.GetCurrentViewpoint(ViewpointType.BoundingGeometry).TargetGeometry as Envelope;
                transformations = TransformationCatalog.GetTransformationsBySuitability(inSpatialRef, outSpatialRef, currentExtent);
            }
            else
            {
                transformations = TransformationCatalog.GetTransformationsBySuitability(inSpatialRef, outSpatialRef);
            }

            // Get the default transformation for the specified input and output spatial reference.
            DatumTransformation defaultTransform = TransformationCatalog.GetTransformation(inSpatialRef, outSpatialRef);

            // Create a picker model to display the updated transformations.
            TransformationsPickerModel pickerModel = new TransformationsPickerModel(transformations, defaultTransform);

            // Handle the selection event to work with the selected transformation, avoiding duplicate subscriptions.
            if (_transformationsPicker?.Model != null && _transformationsPicker.Model is TransformationsPickerModel tm)
            {
                tm.TransformationSelected -= TransformationsPicker_TransformationSelected;
            }

            pickerModel.TransformationSelected += TransformationsPicker_TransformationSelected;

            // Apply the model to the picker.
            _transformationsPicker.Model = pickerModel;
        }

        // Handle selection events in the transformation picker.
        private void TransformationsPicker_TransformationSelected(object sender, TransformationSelectionEventArgs e)
        {
            // Get the selected transform from the event arguments. Return if none is selected.
            DatumTransformation selectedTransform = e.Transformation;
            if (selectedTransform == null)
            {
                return;
            }

            try
            {
                // Project the original point using the selected transform.
                MapPoint projectedPoint = (MapPoint) GeometryEngine.Project(_originalPoint, _myMapView.SpatialReference, selectedTransform);

                // Update the projected graphic (if it already exists), create it otherwise.
                if (_projectedPointGraphic != null)
                {
                    _projectedPointGraphic.Geometry = projectedPoint;
                }
                else
                {
                    // Create a symbol to represent the projected point (a cross to ensure both markers are visible).
                    SimpleMarkerSymbol projectedPointMarker = new SimpleMarkerSymbol(SimpleMarkerSymbolStyle.Cross, Color.Red, 15);

                    // Create the point graphic and add it to the overlay.
                    _projectedPointGraphic = new Graphic(projectedPoint, projectedPointMarker);
                    _pointsOverlay.Graphics.Add(_projectedPointGraphic);
                }

                _messagesTextView.Text = "Projected point using transform: " + selectedTransform.Name;
            }
            catch (ArcGISRuntimeException ex)
            {
                // Exception if a transformation is missing grid files.
                _messagesTextView.Text = "Error using selected transformation: " + ex.Message;

                // Remove the projected point graphic (if it exists).
                if (_projectedPointGraphic != null && _pointsOverlay.Graphics.Contains(_projectedPointGraphic))
                {
                    _pointsOverlay.Graphics.Remove(_projectedPointGraphic);
                    _projectedPointGraphic = null;
                }
            }
        }

        private string GetProjectionDataPath()
        {
            // Return the projection data path; note that this is not valid by default.
            //You must manually download the projection engine data and update the path returned here.
            return "";
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();
            Initialize();
        }

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

            _outerStackView = new UIStackView();
            _outerStackView.TranslatesAutoresizingMaskIntoConstraints = false;
            _outerStackView.Axis = UILayoutConstraintAxis.Vertical;
            _outerStackView.Distribution = UIStackViewDistribution.FillEqually;

            _myMapView = new MapView();
            _myMapView.TranslatesAutoresizingMaskIntoConstraints = false;

            _transformToolsView = new UIStackView();
            _transformToolsView.TranslatesAutoresizingMaskIntoConstraints = false;
            _transformToolsView.Axis = UILayoutConstraintAxis.Vertical;
            _transformToolsView.Spacing = 8;
            _transformToolsView.LayoutMarginsRelativeArrangement = true;
            _transformToolsView.LayoutMargins = new UIEdgeInsets(8, 8, 8, 8);

            _inWkidLabel = new UILabel();
            _inWkidLabel.TranslatesAutoresizingMaskIntoConstraints = false;
            _outWkidLabel = new UILabel();
            _outWkidLabel.TranslatesAutoresizingMaskIntoConstraints = false;

            UIStackView labelsRow = new UIStackView(new[] {_inWkidLabel, _outWkidLabel});
            labelsRow.TranslatesAutoresizingMaskIntoConstraints = false;
            labelsRow.Axis = UILayoutConstraintAxis.Horizontal;
            labelsRow.Distribution = UIStackViewDistribution.FillEqually;
            _transformToolsView.AddArrangedSubview(labelsRow);

            _useExtentSwitch = new UISwitch();
            _useExtentSwitch.TranslatesAutoresizingMaskIntoConstraints = false;
            UILabel useExtentSwitchLabel = new UILabel();
            useExtentSwitchLabel.TranslatesAutoresizingMaskIntoConstraints = false;
            useExtentSwitchLabel.Text = "Use extent";

            UIStackView switchRow = new UIStackView(new UIView[] {_useExtentSwitch, useExtentSwitchLabel});
            switchRow.TranslatesAutoresizingMaskIntoConstraints = false;
            switchRow.Axis = UILayoutConstraintAxis.Horizontal;
            switchRow.Spacing = 8;
            _transformToolsView.AddArrangedSubview(switchRow);

            _transformationsPicker = new UIPickerView();
            _transformationsPicker.TranslatesAutoresizingMaskIntoConstraints = false;
            _transformationsPicker.SetContentCompressionResistancePriority((float) UILayoutPriority.DefaultLow, UILayoutConstraintAxis.Vertical);
            _transformToolsView.AddArrangedSubview(_transformationsPicker);

            _messagesTextView = new UITextView();
            _messagesTextView.TranslatesAutoresizingMaskIntoConstraints = false;
            _messagesTextView.SetContentCompressionResistancePriority((float) UILayoutPriority.Required, UILayoutConstraintAxis.Vertical);
            _messagesTextView.ScrollEnabled = false;
            _transformToolsView.AddArrangedSubview(_messagesTextView);

            _outerStackView.AddArrangedSubview(_myMapView);
            _outerStackView.AddArrangedSubview(_transformToolsView);

            // Add the views.
            View.AddSubviews(_outerStackView);

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

        public override void TraitCollectionDidChange(UITraitCollection previousTraitCollection)
        {
            base.TraitCollectionDidChange(previousTraitCollection);

            // Reset constraints.
            _outerStackView.RemoveFromSuperview();
            View.AddSubview(_outerStackView);

            if (View.TraitCollection.VerticalSizeClass == UIUserInterfaceSizeClass.Compact)
            {
                _outerStackView.Axis = UILayoutConstraintAxis.Horizontal;
                _outerStackView.TopAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TopAnchor).Active = true;
                _outerStackView.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor).Active = true;
                _outerStackView.TrailingAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TrailingAnchor).Active = true;
                _outerStackView.BottomAnchor.ConstraintEqualTo(View.BottomAnchor).Active = true;
                _transformToolsView.BottomAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.BottomAnchor).Active = true;
            }
            else
            {
                _outerStackView.Axis = UILayoutConstraintAxis.Vertical;
                _outerStackView.TopAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TopAnchor).Active = true;
                _outerStackView.LeadingAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.LeadingAnchor).Active = true;
                _outerStackView.TrailingAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.TrailingAnchor).Active = true;
                _outerStackView.BottomAnchor.ConstraintEqualTo(View.SafeAreaLayoutGuide.BottomAnchor).Active = true;
            }
        }

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

            // Subscribe to events.
            _useExtentSwitch.ValueChanged += UseExtentSwitch_ValueChanged;
        }

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

            // Unsubscribe from events, per best practice.
            _useExtentSwitch.ValueChanged -= UseExtentSwitch_ValueChanged;

            if (_transformationsPicker?.Model != null && _transformationsPicker.Model is TransformationsPickerModel tm)
            {
                tm.TransformationSelected -= TransformationsPicker_TransformationSelected;
            }
        }
    }

    // Class that defines a view model for showing available datum transformations in a picker control.
    public class TransformationsPickerModel : UIPickerViewModel
    {
        // Event raised when the selected transformation changes.
        public event EventHandler<TransformationSelectionEventArgs> TransformationSelected;

        // List of datum transformation values.
        private readonly IReadOnlyList<DatumTransformation> _datumTransformations;

        // Store the default transformation.
        private readonly DatumTransformation _defaultTransformation;

        // Store the selected transformation.
        private DatumTransformation _selectedTransformation;

        // Constructor that takes the datum transformations list to display.
        public TransformationsPickerModel(IReadOnlyList<DatumTransformation> transformationList, DatumTransformation defaultTransform)
        {
            _datumTransformations = transformationList;
            _defaultTransformation = defaultTransform;
        }

        // Return the number of picker components (just one).
        public override nint GetComponentCount(UIPickerView pickerView)
        {
            return 1;
        }

        // Return the number of rows in the section (the size of the transformations list).
        public override nint GetRowsInComponent(UIPickerView pickerView, nint component)
        {
            return _datumTransformations.Count;
        }

        // Get the title to display in the picker component.
        public override string GetTitle(UIPickerView pickerView, nint row, nint component)
        {
            return _datumTransformations[(int) row].Name;
        }

        // Handle the selection event for the picker.
        public override void Selected(UIPickerView pickerView, nint row, nint component)
        {
            // Get the selected datum transformation factor.
            _selectedTransformation = _datumTransformations[(int) pickerView.SelectedRowInComponent(0)];

            // Raise the selection event (with the new transformation) so listeners can handle it.
            EventHandler<TransformationSelectionEventArgs> selectionEventHandler = TransformationSelected;
            if (selectionEventHandler != null)
            {
                TransformationSelectionEventArgs args = new TransformationSelectionEventArgs(_selectedTransformation);
                selectionEventHandler(this, args);
            }
        }

        // Return the desired width for each component in the picker.
        public override nfloat GetComponentWidth(UIPickerView picker, nint component)
        {
            return 280f;
        }

        // Return the desired height for rows in the picker.
        public override nfloat GetRowHeight(UIPickerView picker, nint component)
        {
            return 30f;
        }

        // Override GetView to create different label colors for each type of transformation.
        public override UIView GetView(UIPickerView pickerView, nint row, nint component, UIView view)
        {
            // Get the transformation being displayed.
            DatumTransformation thisTransform = _datumTransformations[(int) row];

            // See if this is the default transformation and if it's available (has required PE files).
            bool isDefault = thisTransform.Name == _defaultTransformation.Name;
            bool isNotAvailable = thisTransform.IsMissingProjectionEngineFiles;

            // Create the correct color for the transform type (available=black, default=blue, or unavailable=gray).
            UIColor labelColor = ApplicationTheme.ForegroundColor;
            if (isNotAvailable)
            {
                labelColor = UIColor.LightGray;
            }

            if (isDefault)
            {
                labelColor = ApplicationTheme.ForegroundColor;
            }

            // Create a label to display the transform.
            return new UILabel(new RectangleF(0, 0, 260f, 30f))
            {
                TextColor = labelColor,
                Font = UIFont.SystemFontOfSize(16f),
                TextAlignment = UITextAlignment.Center,
                Text = thisTransform.Name
            };
        }
    }

    // Event arguments to return when a new datum transformation is selected in the picker.
    public class TransformationSelectionEventArgs : EventArgs
    {
        // Selected datum transformation.
        public DatumTransformation Transformation { get; }

        public TransformationSelectionEventArgs(DatumTransformation transformation)
        {
            Transformation = transformation;
        }
    }
}

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