Identify raster cell

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Sample viewer app

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

screenshot

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 identifyLayerCompleted 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

To set up the sample's offline data, see the Use offline data in the samples section of the Qt Samples repository overview.

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

IdentifyRasterCell.cppIdentifyRasterCell.cppIdentifyRasterCell.hIdentifyRasterCell.qml
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// [WriteFile Name=IdentifyRasterCell, Category=Layers]
// [Legal]
// Copyright 2020 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 "IdentifyRasterCell.h"

#include "CalloutData.h"
#include "Map.h"
#include "MapQuickView.h"
#include "Raster.h"
#include "RasterCell.h"
#include "RasterLayer.h"

#include <QDir>
#include <QString>
#include <memory>

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

IdentifyRasterCell::IdentifyRasterCell(QObject* parent /* = nullptr */):
  QObject(parent),
  m_map(new Map(BasemapStyle::ArcGISOceans, this))
{
  // initialize the raster layer
  const QString filepath = defaultDataPath() + "/ArcGIS/Runtime/Data/raster/SA_EVI_8Day_03May20/SA_EVI_8Day_03May20.tif";
  Raster* raster = new Raster(filepath, this);
  m_rasterLayer = new RasterLayer(raster, this);

  m_map->operationalLayers()->append(m_rasterLayer);
}

IdentifyRasterCell::~IdentifyRasterCell() = default;

void IdentifyRasterCell::init()
{
  // Register the map view for QML
  qmlRegisterType<MapQuickView>("Esri.Samples", 1, 0, "MapView");
  qmlRegisterType<IdentifyRasterCell>("Esri.Samples", 1, 0, "IdentifyRasterCellSample");
}

MapQuickView* IdentifyRasterCell::mapView() const
{
  return m_mapView;
}

// Set the view (created in QML)
void IdentifyRasterCell::setMapView(MapQuickView* mapView)
{
  if (!mapView || mapView == m_mapView)
    return;

  m_mapView = mapView;
  m_mapView->setMap(m_map);

  connectSignals();

  emit mapViewChanged();
}

void IdentifyRasterCell::connectSignals()
{
  connect(m_rasterLayer, &Layer::doneLoading, this, [this](Error error)
  {
    if (!error.isEmpty())
    {
      qDebug() << error.message() << error.additionalMessage();
      return;
    }

    m_mapView->setViewpointGeometry(m_rasterLayer->fullExtent());
  });

  connect(m_mapView, &MapQuickView::identifyLayerCompleted, this, [this](QUuid, IdentifyLayerResult* rawIdentifyResult)
  {
    const auto identifyResult = std::unique_ptr<IdentifyLayerResult>(rawIdentifyResult);
    const auto elements = identifyResult->geoElements();
    for (GeoElement* geoElement : elements)
    {
      if (RasterCell* rasterCell = dynamic_cast<RasterCell*>(geoElement))
      {
        QString calloutString;
        const AttributeListModel* attributes = rasterCell->attributes();
        const QStringList attributeNames = rasterCell->attributes()->attributeNames();

        for (int i = 0; i < attributeNames.size(); ++i)
        {
          const QString value = QVariant((*attributes)[attributeNames[i]]).toString();
          calloutString.append(attributeNames[i] + ": " + value + "\n");
        }

        const double xPoint = rasterCell->geometry().extent().xMin();
        const double yPoint = rasterCell->geometry().extent().yMin();

        calloutString.append("X: " + QString::number(xPoint, 'f', 2) + " Y: " + QString::number(yPoint, 'f', 2));

        m_mapView->calloutData()->setLocation(m_clickedPoint);
        m_mapView->calloutData()->setDetail(calloutString);
        m_mapView->calloutData()->setVisible(true);
        m_calloutData = m_mapView->calloutData();
        emit calloutDataChanged();
      }
    }
  });

  connect(m_mapView, &MapQuickView::mouseClicked, this, [this](const QMouseEvent& e)
  {
    m_mapView->identifyLayer(m_rasterLayer, e.x(), e.y(), 10, false, 1);
    m_clickedPoint = m_mapView->screenToLocation(e.x(), e.y());
  });

  connect(m_mapView, &MapQuickView::mousePressedAndHeld, this, [this](const QMouseEvent& e)
  {
    m_mapView->identifyLayer(m_rasterLayer, e.x(), e.y(), 10, false, 1);
    m_clickedPoint = m_mapView->screenToLocation(e.x(), e.y());
    m_mousePressed = true;
  });

  connect(m_mapView, &MapQuickView::mouseReleased, this, [this](QMouseEvent)
  {
    m_mousePressed = false;
  });

  // if mouse is moved after press-and-hold, then update callout on-the-fly
  connect(m_mapView, &MapQuickView::mouseMoved, this, [this](const QMouseEvent& e)
  {
    if (m_mousePressed)
    {
      m_mapView->identifyLayer(m_rasterLayer, e.x(), e.y(), 10, false, 1);
      m_clickedPoint = m_mapView->screenToLocation(e.x(), e.y());
    }
  });
}

CalloutData* IdentifyRasterCell::calloutData() const
{
  return m_calloutData;
}

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