Get a list of suitable transformations for projecting a geometry between two spatial references with different horizontal datums.
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.
If the selected transformation is not usable (has missing grid files) then an error is displayed.
How it works
- Pass the input and output spatial references to
TransformationCatalog::transformationsBySuitability
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. - 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
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.
Additional information
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.
Some transformations aren't available until transformation data is provided.
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.
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.
To download projection engine data to your device:
- Log in to the ArcGIS for Developers site using your Developer account.
- In the Dashboard page, click 'Download APIs and SDKs'.
- Click the download button next to
ArcGIS Runtime Coordinate System Data 100.x
to download projection engine data to your computer. - Unzip the downloaded data on your computer.
- Create an
~/ArcGIS/Runtime/Data/PEDataRuntime
directory on your device and copy the files to this directory.
Tags
datum, geodesy, projection, spatial reference, transformation
Sample Code
// [WriteFile Name=ListTransformations, Category=Geometry]
// [Legal]
// Copyright 2017 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.
// [Legal]
#ifdef PCH_BUILD
#include "pch.hpp"
#endif // PCH_BUILD
#include "ListTransformations.h"
#include "Map.h"
#include "MapQuickView.h"
#include "TransformationCatalog.h"
#include "GeometryEngine.h"
#include "GraphicsOverlay.h"
#include "Graphic.h"
#include "Point.h"
#include "SimpleMarkerSymbol.h"
#include "DatumTransformation.h"
#include "GeographicTransformationStep.h"
#include "GeographicTransformation.h"
#include <QDir>
#include <QtCore/qglobal.h>
#include <QUrl>
#include <QVariantMap>
#ifdef Q_OS_IOS
#include <QStandardPaths>
#endif // Q_OS_IOS
using namespace Esri::ArcGISRuntime;
// helper method to get cross platform data path
namespace
{
QString defaultDataPath()
{
QString dataPath;
#ifdef Q_OS_ANDROID
dataPath = "/sdcard";
#elif defined Q_OS_IOS
dataPath = QStandardPaths::writableLocation(QStandardPaths::DocumentsLocation);
#else
dataPath = QDir::homePath();
#endif
return dataPath;
}
} // namespace
ListTransformations::ListTransformations(QQuickItem* parent /* = nullptr */):
QQuickItem(parent)
{
}
void ListTransformations::init()
{
// Register the map view for QML
qmlRegisterType<MapQuickView>("Esri.Samples", 1, 0, "MapView");
qmlRegisterType<ListTransformations>("Esri.Samples", 1, 0, "ListTransformationsSample");
}
void ListTransformations::componentComplete()
{
QQuickItem::componentComplete();
// get data path
const QUrl dataPath = QUrl(defaultDataPath() + "/ArcGIS/Runtime/Data/PEDataRuntime");
// connect to TransformationCatalog error signal
connect(TransformationCatalog::instance(), &TransformationCatalog::errorOccurred, this, [this](Error e)
{
if (e.isEmpty())
return;
emit showStatusBar(QString("Error setting projection engine directory: %1. %2").arg(e.message(), e.additionalMessage()));
});
// Create a geometry located in the Greenwich observatory courtyard in London, UK, the location of the
// Greenwich prime meridian. This will be projected using the selected transformation.
const double x = 538985.355;
const double y = 177329.516;
m_originalPoint = Point(x, y, SpatialReference(27700));
// find QML MapView component
m_mapView = findChild<MapQuickView*>("mapView");
// Create a map using the light gray canvas basemap
m_map = new Map(BasemapStyle::ArcGISLightGray, this);
// Set initial viewpoint of the map
m_map->setInitialViewpoint(Viewpoint(m_originalPoint, 5000));
// connect to map loaded signal
connect(m_map, &Map::doneLoading, this, [this, dataPath]
{
// Set the TransformationCatalog path
TransformationCatalog::setProjectionEngineDirectory(dataPath.toString());
if (TransformationCatalog::projectionEngineDirectory().length() > 0)
emit showStatusBar(QString("Projection engine directory set: %1").arg(TransformationCatalog::instance()->projectionEngineDirectory()));
// get the initial list of transformations
refreshTransformationList(true);
});
// add graphics to the map view
addGraphics();
// set map to map view
m_mapView->setMap(m_map);
}
// Create a GraphicsOverlay with two Graphics - one to hold the default
// transformed geometry, and the other to use the selected transformation
void ListTransformations::addGraphics()
{
// Create the original graphic
SimpleMarkerSymbol* blueSquare = new SimpleMarkerSymbol(SimpleMarkerSymbolStyle::Square, QColor("blue"), 20.f, this);
m_originalGraphic = new Graphic(m_originalPoint, blueSquare, this);
// Create the projected graphic
SimpleMarkerSymbol* redCross = new SimpleMarkerSymbol(SimpleMarkerSymbolStyle::Cross, QColor("red"), 20.f, this);
m_projectedGraphic = new Graphic(m_originalPoint, redCross, this);
// Create the Overlay
GraphicsOverlay* overlay = new GraphicsOverlay(this);
m_mapView->graphicsOverlays()->append(overlay);
overlay->graphics()->append(m_originalGraphic);
overlay->graphics()->append(m_projectedGraphic);
}
void ListTransformations::refreshTransformationList(bool orderBySuitability)
{
// get the input/output spatial reference
SpatialReference inSpatialReference = m_originalGraphic->geometry().spatialReference();
SpatialReference outSpatialReference = m_map->spatialReference();
// request the transformations
if (orderBySuitability)
m_transformations = TransformationCatalog::transformationsBySuitability(inSpatialReference, outSpatialReference, m_mapView->visibleArea().extent());
else
m_transformations = TransformationCatalog::transformationsBySuitability(inSpatialReference, outSpatialReference);
// update the QML property list
m_transformationList.clear();
for (DatumTransformation* transformation : qAsConst(m_transformations))
{
QVariantMap transformationMap;
transformationMap["name"] = transformation->name();
transformationMap["isMissingProjectionEngineFiles"] = transformation->isMissingProjectionEngineFiles();
m_transformationList.append(transformationMap);
}
// update the property
emit transformationListChanged();
}
void ListTransformations::updateGraphicTransformation(int index)
{
GeographicTransformation* transform = static_cast<GeographicTransformation*>(m_transformations.at(index));
if (transform->isMissingProjectionEngineFiles())
{
QString missingFiles = "Missing grid files: ";
const QList<GeographicTransformationStep> steps = transform->steps();
for (const GeographicTransformationStep& step : steps)
{
const auto filenames = step.projectionEngineFilenames();
for (const QString& filename : filenames)
{
missingFiles += filename;
}
}
emit showStatusBar(missingFiles);
}
else
{
Geometry projectedPoint = GeometryEngine::project(m_originalPoint, m_map->spatialReference(), transform);
m_projectedGraphic->setGeometry(projectedPoint);
}
}