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 Android.App;
using Android.OS;
using Android.Views;
using Android.Widget;
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 System;
using System.Collections.Generic;
using System.Drawing;

namespace ArcGISRuntime.Samples.ListTransformations
{
    [Activity (ConfigurationChanges=Android.Content.PM.ConfigChanges.Orientation | Android.Content.PM.ConfigChanges.ScreenSize)]
	[ArcGISRuntime.Samples.Shared.Attributes.OfflineData()]
    [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 : Activity
    {
        // Hold a reference to the map view.
        private MapView _myMapView;

        // 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 TextView _messagesTextView;

        // Labels to display the input/output spatial references (WKID).
        private TextView _inWkidLabel;
        private TextView _outWkidLabel;

        // Spinner to display the datum transformations suitable for the input/output spatial references.
        private Spinner _transformationsPicker;

        // Switch to toggle suitable transformations for the current extent.
        private Switch _useExtentSwitch;

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

            Title = "List transformations by suitability";

            // Create the UI.
            CreateLayout();

            // Create a new map, add a point graphic, and fill the datum transformations list.
            Initialize();
        }

        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.
            Viewpoint initialViewpoint = new Viewpoint(_originalPoint, 5000);
            myMap.InitialViewpoint = initialViewpoint;

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

            // Run on the UI thread.
            RunOnUiThread(() =>
            {
                // 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.Checked);
            });
        }

        private void CreateLayout()
        {
            // View for the input/output wkid labels.
            LinearLayout wkidLabelsStackView = new LinearLayout(this) { Orientation = Orientation.Horizontal };
            wkidLabelsStackView.SetPadding(10, 10, 0, 10);

            // Create a label for the input spatial reference.
            _inWkidLabel = new TextView(this)
            {
                Text = "In WKID = ",
                TextAlignment = TextAlignment.ViewStart
            };

            // Create a label for the output spatial reference.
            _outWkidLabel = new TextView(this)
            {
                Text = "Out WKID = ",
                TextAlignment = TextAlignment.ViewStart
            };

            // Create some horizontal space between the labels.
            Space space = new Space(this);
            space.SetMinimumWidth(30);

            // Add the Wkid labels to the stack view.
            wkidLabelsStackView.AddView(_inWkidLabel);
            wkidLabelsStackView.AddView(space);
            wkidLabelsStackView.AddView(_outWkidLabel);

            // Create the 'use extent' switch.
            _useExtentSwitch = new Switch(this)
            {
                Checked = false,
                Text = "Use extent"
            };

            // Handle the checked change event for the switch.
            _useExtentSwitch.CheckedChange += UseExtentSwitch_CheckedChange;

            // Create a picker (Spinner) for datum transformations.
            _transformationsPicker = new Spinner(this);
            _transformationsPicker.SetPadding(5, 10, 0, 10);

            // Handle the selection event to work with the selected transformation.
            _transformationsPicker.ItemSelected += TransformationsPicker_ItemSelected;

            // Create a text view to show messages.
            _messagesTextView = new TextView(this);

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

            // Create a layout for the app tools.
            LinearLayout toolsLayout = new LinearLayout(this) { Orientation = Orientation.Vertical };
            toolsLayout.SetPadding(10, 0, 0, 0);
            toolsLayout.SetMinimumHeight(320);

            // Add the transformation UI controls to the tools layout.
            toolsLayout.AddView(wkidLabelsStackView);
            toolsLayout.AddView(_useExtentSwitch);
            toolsLayout.AddView(_transformationsPicker);
            toolsLayout.AddView(_messagesTextView);

            // Add the tools layout and map view to the main layout.
            mainLayout.AddView(toolsLayout);
            _myMapView = new MapView(this);
            mainLayout.AddView(_myMapView);

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

        // 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 list of transformations.
            List<DatumTransformation> transformsList = new List<DatumTransformation>();
            foreach(DatumTransformation transformation in transformations)
            {
                transformsList.Add(transformation);
            }

            // Create an adapter for showing the spinner list.
            TransformationsAdapter transformationsAdapter = new TransformationsAdapter(this, transformsList)
            {
                DefaultTransformation = defaultTransform
            };

            // Apply the adapter to the spinner.
            _transformationsPicker.Adapter = transformationsAdapter;
        }

        private void TransformationsPicker_ItemSelected(object sender, AdapterView.ItemSelectedEventArgs e)
        {
            // Get the selected transform from the spinner. Return if none is selected.
            TransformationsAdapter adapter = (TransformationsAdapter)_transformationsPicker.Adapter;
            DatumTransformation selectedTransform = adapter[e.Position];
            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 void UseExtentSwitch_CheckedChange(object sender, CompoundButton.CheckedChangeEventArgs e)
        {
            // Call a function to create a list of transformations to fill the picker.
            GetSuitableTransformations(_originalPoint.SpatialReference, _myMapView.Map.SpatialReference, _useExtentSwitch.Checked);
        }

        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 "";
        }
    }

    // An Adapter class to provide a list of datum transformations for display in a Spinner control.
    public class TransformationsAdapter : BaseAdapter<DatumTransformation>
    {
        // Property to expose the default datum transformation (will be displayed with different text color).
        public DatumTransformation DefaultTransformation { get; set; }

        // Fields to store the list of transformations and the current context.
        private List<DatumTransformation> _transformations;
        private Activity _context;

        // Constructor for the adapter. Store the context and the list of transformations to display.
        public TransformationsAdapter(Activity context, List<DatumTransformation> items) : base()
        {
            _transformations = items;
            _context = context;
        }

        // Provide an ID for an item at a given position (just return the position).
        public override long GetItemId(int position)
        {
            return position;
        }

        // Provide the datum transformation at this position in the list.
        public override DatumTransformation this[int position]
        {
            get { return _transformations[position]; }
        }

        // Provide the number of items (datum transformations) in the list.
        public override int Count
        {
            get
            {
                return _transformations.Count;
            }
        }

        // Override the GetView method to provide a custom (formatted) text view for each transformation in the list.
        public override View GetView(int position, View convertView, ViewGroup parent)
        {
            // Create a new text view to display the transformation name with the proper formatting.
            TextView transformTextView = new TextView(_context);

            // Get the datum transformation being displayed.
            DatumTransformation thisTransform = _transformations[position];

            // Set the text with the transformation name.
            transformTextView.SetText(thisTransform.Name, TextView.BufferType.Normal);

            // Use white as the default text color (available transforms).
            transformTextView.SetTextColor(Android.Graphics.Color.White);
            transformTextView.SetBackgroundColor(Android.Graphics.Color.DarkGray);

            // See if the transform is missing required projection engine files. If so, display the text in gray.
            if (thisTransform.IsMissingProjectionEngineFiles)
            {
                transformTextView.SetTextColor(Android.Graphics.Color.Gray);
            }

            // If this is the default transformation, show it in blue.
            if(thisTransform.Name == DefaultTransformation.Name)
            {
                transformTextView.SetTextColor(Android.Graphics.Color.Blue);
            }

            // Pass back the text view.
            return transformTextView;
        }
    }
}

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