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.
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
- Set the location of projection engine data on the device with
TransformationCatalog.projectionEngineDirectory. - Pass the input and output spatial references to
TransformationCatalog.transformationsBySuitabilityfor 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
DatumTransformationobjects 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
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, the ArcGIS Maps SDK for Qt 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.
- On the Dashboard page, click the 'Downloads' tab and select 'Projection Engine Data' from the navigation column.
- Download the applicable release version of Projection Engine Data.
- Unzip the downloaded data on your computer.
- Create an
~/ArcGIS/Runtime/Data/PEDataRuntimedirectory 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]
import QtQuick
import QtQuick.Controls
import Esri.ArcGISRuntime
import Esri.ArcGISExtras
Rectangle {
id: rootRectangle
clip: true
width: 800
height: 600
property bool peDataSet: true
readonly property string dataPath: {
Qt.platform.os === "ios" ?
System.writableLocation(System.StandardPathsDocumentsLocation) + "/ArcGIS/Runtime/Data/PEDataRuntime" :
System.writableLocation(System.StandardPathsHomeLocation) + "/ArcGIS/Runtime/Data/PEDataRuntime"
}
Component.onCompleted: TransformationCatalog.projectionEngineDirectory = dataPath
Connections {
target: TransformationCatalog
function onErrorChanged() {
peDataSet = false;
}
}
MapView {
id: mapView
anchors {
left: parent.left
right: parent.right
top: parent.top
}
height: parent.height / 2
Component.onCompleted: {
// Set the focus on MapView to initially enable keyboard navigation
forceActiveFocus();
}
Map {
id: map
Basemap {
initStyle: Enums.BasemapStyleArcGISLightGray
}
ViewpointCenter {
center: originalGeometry
targetScale: 5000
}
onLoadStatusChanged: {
if (loadStatus !== Enums.LoadStatusLoaded)
return;
getTransformations();
statusBar.expand();
}
}
// Create a GraphicsOverlay with two Graphics - one to hold the default
// transformed geometry, and the other to use the selected transformation
GraphicsOverlay {
Graphic {
geometry: originalGeometry
SimpleMarkerSymbol {
style: Enums.SimpleMarkerSymbolStyleSquare
color: "blue"
size: 20
}
}
Graphic {
id: projectedGraphic
geometry: originalGeometry
SimpleMarkerSymbol {
style: Enums.SimpleMarkerSymbolStyleCross
color: "red"
size: 20
}
}
}
// 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.
Point {
id: originalGeometry
x: 538985.355
y: 177329.516
SpatialReference { wkid: 27700 }
}
}
Rectangle {
id: transformationView
anchors {
left: parent.left
right: parent.right
top: mapView.bottom
}
height: parent.height / 2
color: "#f7f7f7"
CheckBox {
id: orderCheckbox
anchors {
left: parent.left
top: parent.top
margins: 10
}
text: "Order by suitability for map extent"
onCheckedChanged: {
getTransformations();
}
}
ListView {
id: transformationList
anchors {
left: parent.left
right: parent.right
top: orderCheckbox.bottom
bottom: parent.bottom
margins: 10
}
clip: true
delegate: Item {
id: itemDelegate
height: 45
width: transformationList.width
clip: true
// show the DatumTransformation name
Text {
id: label
anchors {
verticalCenter: parent.verticalCenter
left: parent.left
right: parent.right
}
width: parent.width
text: model.modelData.missingProjectionEngineFiles ? "%1 <font color=red><b>Missing grid files</b></font>".arg(model.modelData.name) : model.modelData.name
textFormat: Text.RichText
wrapMode: Text.WrapAnywhere
maximumLineCount: 2
font.pixelSize: 12
}
MouseArea {
id: itemMouseArea
anchors.fill: parent
onClicked: {
transformationList.currentIndex = index;
const transform = transformationList.model[index];
if (transform.missingProjectionEngineFiles) {
let missingFiles = "Missing grid files: ";
const steps = transform.steps;
for (let i = 0; i < steps.length; i++) {
for (let j = 0; j < steps[i].projectionEngineFilenames.length; j++) {
missingFiles += steps[i].projectionEngineFilenames[j];
}
}
statusText.text = missingFiles + " ";
if (statusBar.isExpanded)
timer.restart();
else
statusBar.expand();
} else {
projectedGraphic.geometry = GeometryEngine.projectWithDatumTransformation(originalGeometry, map.spatialReference, transform);
}
}
}
}
highlightMoveDuration: 1
highlightFollowsCurrentItem: true
highlight: Rectangle {
color: "#d6d6d6"
radius: 4
}
}
}
Rectangle {
id: statusBar
property int expanded: parent.height - height
property int hidden: parent.height
property bool isExpanded: y === expanded
anchors {
left: parent.left
right: parent.right
}
height: 45
color: "black"
y: hidden
Text {
id: statusText
anchors {
left: parent.left
right: parent.right
verticalCenter: parent.verticalCenter
margins: 10
}
color: "white"
wrapMode: Text.WrapAnywhere
text: peDataSet ?
"Projection engine directory set %1".arg(dataPath) :
"Error setting projection engine directory: %1. %2".arg(TransformationCatalog.error.message).arg(TransformationCatalog.error.additionalMessage)
}
Timer {
id: timer
interval: 5000
onTriggered: statusBar.animate();
}
onYChanged: {
if (y === expanded)
timer.restart();
}
NumberAnimation {
id: statusAnimation
target: statusBar
properties: "y"
duration: 500
easing.type: Easing.OutQuad
}
function expand() {
statusAnimation.from = statusBar.hidden;
statusAnimation.to = statusBar.expanded;
statusAnimation.start();
}
function hide() {
statusAnimation.from = statusBar.expanded;
statusAnimation.to = statusBar.hidden;
statusAnimation.start();
}
function animate() {
if (statusBar.y === statusBar.hidden)
expand();
else
hide();
}
}
function getTransformations() {
if (orderCheckbox.checked)
transformationList.model = TransformationCatalog.transformationsBySuitabilityWithAreaOfInterest(originalGeometry.spatialReference, map.spatialReference, mapView.visibleArea.extent);
else
transformationList.model = TransformationCatalog.transformationsBySuitability(originalGeometry.spatialReference, map.spatialReference);
}
}