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ArcGIS Runtime SDK for Qt

Identify raster cell

Sample Viewer View Sample on GitHub

Get the cell value of a local raster at the tapped location and display the result in a callout.

Use case

You may want to identify a raster layer to get its exact cell value in case the approximate value conveyed by its symbology is not sufficient. The information available for the raster cell depends on the type of raster layer being identified. For example, a 3-band satellite or aerial image might provide 8-bit RGB values, whereas a digital elevation model (DEM) would provide floating point z values. By identifying a raster cell of a DEM, you can retrieve the precise elevation of a location.

How to use the sample

Tap on the raster or press on the raster, hold, and move around the raster to identify it and see the raster cell attributes information displayed in a callout.

How it works

  1. Connect to signals emmited by a mouseClicked, mousePressedAndHeld, and mouseMoved on the MapView.
  2. On tapped or pressed, held, and dragged:
    • Call identifyLayer(...) passing in the raster layer, screen point, tolerance, whether to return popups only, and maximum number of results per layer.
    • Connect to the identifyLayerStatusChanged. If the identifyLayerStatus is Enums.TaskStatusCompleted then use the identifyLayerResult to get the result of the identify and then get the GeoElement from the layer result and get any RasterCells from them.
    • Create a callout at the calculated map point and populate the callout content with text from the RasterCell attributes.
    • Show the callout.

Relevant API

  • GeoView.identifyLayer(...)
  • IdentifyLayerResult
  • RasterCell
  • RasterCell.attributes
  • RasterLayer

Offline data

Read more about how to set up the sample's offline data here.

Link Local Location
South Africa data <userhome>/ArcGIS/Runtime/Data/raster/SA_EVI_8Day_03May20

About the data

The data shown is an NDVI classification derived from MODIS imagery between 27 Apr 2020 and 4 May 2020. It comes from the NASA Worldview application. In a normalized difference vegetation index, or NDVI, values range between -1 and +1 with the positive end of the spectrum showing green vegetation.

Tags

band, cell, cell value, continuous, discrete, identify, pixel, pixel value, raster

Sample Code

import QtQuick 2.6
import Esri.ArcGISRuntime 100.9
import Esri.ArcGISExtras 1.1
import Esri.ArcGISRuntime.Toolkit.Controls 100.9 // needed to use Callout in QML

Rectangle {
    id: rootRectangle
    clip: true
    width: 800
    height: 600

    readonly property url dataPath: System.userHomePath + "/ArcGIS/Runtime/Data/raster/SA_EVI_8Day_03May20/"
    property Point clickedPoint: null
    property string calloutText: ""
    property bool pressedMouse: false

    MapView {
        id: mapView
        anchors.fill: parent

        Callout {
            id: callout
            calloutData: parent.calloutData
            autoAdjustWidth: false
            calloutWidth: 300
            accessoryButtonHidden: true
            calloutContent: customComponent
            leaderHeight: 30
        }

        Component {
              id: customComponent

              Text {
                  id: componentText
                  text: calloutText
                  Binding {
                      target: callout
                      value: contentHeight + callout.calloutFramePadding
                      property: "calloutHeight"
                  }

                  wrapMode: Text.WordWrap
              }
          }

        Map {
            BasemapOceans {}
            RasterLayer {
                id: rasterLayer
                Raster {
                    path: dataPath + "SA_EVI_8Day_03May20.tif"
                }

                onErrorChanged: {
                 console.warn(loadError.message);
                }

                onLoadStatusChanged: {
                    if (loadStatus !== Enums.LoadStatusLoaded)
                        return;

                    mapView.setViewpointGeometry(fullExtent);
                }
            }
        }

        onMouseClicked: {
            clickedPoint = screenToLocation(mouse.x, mouse.y);
            if (identifyLayerStatus !== Enums.TaskStatusInProgress) {
                identifyLayer(rasterLayer, mouse.x, mouse.y, 10, false, 1);
            }
        }

        onMousePressedAndHeld: {
            pressedMouse = true;
            clickedPoint = screenToLocation(mouse.x, mouse.y);
            if (identifyLayerStatus !== Enums.TaskStatusInProgress) {
                identifyLayer(rasterLayer, mouse.x, mouse.y, 10, false, 1);
            }
        }
        onMouseReleased: pressedMouse = false;
        onMousePositionChanged: {
            if (pressedMouse) {
                clickedPoint = screenToLocation(mouse.x, mouse.y);
                if (identifyLayerStatus !== Enums.TaskStatusInProgress) {
                    identifyLayer(rasterLayer, mouse.x, mouse.y, 10, false, 1);
                }
            }
        }

        onIdentifyLayerStatusChanged: {
            if (identifyLayerStatus !== Enums.TaskStatusCompleted)
                return;

            for (let i = 0; i < identifyLayerResult.geoElements.length; i++) {
                calloutText = "";
                let geoElement = identifyLayerResult.geoElements[i];
                const attributes = geoElement.attributes;
                const attributeNames = attributes.attributeNames;

                for (let j = 0; j < attributeNames.length; j++) {
                    calloutText = calloutText + attributeNames[j] + ": " + attributes.attributeValue(attributeNames[j]) + "\n";
                }

                const xPoint = geoElement.geometry.extent.xMin;
                const yPoint = geoElement.geometry.extent.yMin;

                calloutText = calloutText + "X: " + xPoint.toFixed(2) + " Y: " + yPoint.toFixed(2);

                callout.calloutData.location = clickedPoint;
                callout.showCallout();
            }
        }
    }
}