Measure

Using a draw interaction to measure lengths and areas.

The ol.Sphere.getLength() and ol.Sphere.getArea() functions calculate spherical lengths and areas for geometries. Lengths are calculated by assuming great circle segments between geometry coordinates. Areas are calculated as if edges of polygons were great circle segments.

Note that the geometry.getLength() and geometry.getArea() methods return measures of projected (planar) geometries. These can be very different than on-the-ground measures in certain situations — in northern and southern latitudes using Web Mercator for example. For better results, use the functions on ol.Sphere.

<!DOCTYPE html>
<html>
  <head>
    <title>Measure</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.4.2/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="https://openlayers.org/en/v4.4.2/build/ol.js"></script>
    <style>
      .tooltip {
        position: relative;
        background: rgba(0, 0, 0, 0.5);
        border-radius: 4px;
        color: white;
        padding: 4px 8px;
        opacity: 0.7;
        white-space: nowrap;
      }
      .tooltip-measure {
        opacity: 1;
        font-weight: bold;
      }
      .tooltip-static {
        background-color: #ffcc33;
        color: black;
        border: 1px solid white;
      }
      .tooltip-measure:before,
      .tooltip-static:before {
        border-top: 6px solid rgba(0, 0, 0, 0.5);
        border-right: 6px solid transparent;
        border-left: 6px solid transparent;
        content: "";
        position: absolute;
        bottom: -6px;
        margin-left: -7px;
        left: 50%;
      }
      .tooltip-static:before {
        border-top-color: #ffcc33;
      }    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <form class="form-inline">
      <label>Measurement type &nbsp;</label>
      <select id="type">
        <option value="length">Length (LineString)</option>
        <option value="area">Area (Polygon)</option>
      </select>
    </form>
    <script>
      var raster = new ol.layer.Tile({
        source: new ol.source.OSM()
      });

      var source = new ol.source.Vector();

      var vector = new ol.layer.Vector({
        source: source,
        style: new ol.style.Style({
          fill: new ol.style.Fill({
            color: 'rgba(255, 255, 255, 0.2)'
          }),
          stroke: new ol.style.Stroke({
            color: '#ffcc33',
            width: 2
          }),
          image: new ol.style.Circle({
            radius: 7,
            fill: new ol.style.Fill({
              color: '#ffcc33'
            })
          })
        })
      });


      /**
       * Currently drawn feature.
       * @type {ol.Feature}
       */
      var sketch;


      /**
       * The help tooltip element.
       * @type {Element}
       */
      var helpTooltipElement;


      /**
       * Overlay to show the help messages.
       * @type {ol.Overlay}
       */
      var helpTooltip;


      /**
       * The measure tooltip element.
       * @type {Element}
       */
      var measureTooltipElement;


      /**
       * Overlay to show the measurement.
       * @type {ol.Overlay}
       */
      var measureTooltip;


      /**
       * Message to show when the user is drawing a polygon.
       * @type {string}
       */
      var continuePolygonMsg = 'Click to continue drawing the polygon';


      /**
       * Message to show when the user is drawing a line.
       * @type {string}
       */
      var continueLineMsg = 'Click to continue drawing the line';


      /**
       * Handle pointer move.
       * @param {ol.MapBrowserEvent} evt The event.
       */
      var pointerMoveHandler = function(evt) {
        if (evt.dragging) {
          return;
        }
        /** @type {string} */
        var helpMsg = 'Click to start drawing';

        if (sketch) {
          var geom = (sketch.getGeometry());
          if (geom instanceof ol.geom.Polygon) {
            helpMsg = continuePolygonMsg;
          } else if (geom instanceof ol.geom.LineString) {
            helpMsg = continueLineMsg;
          }
        }

        helpTooltipElement.innerHTML = helpMsg;
        helpTooltip.setPosition(evt.coordinate);

        helpTooltipElement.classList.remove('hidden');
      };


      var map = new ol.Map({
        layers: [raster, vector],
        target: 'map',
        view: new ol.View({
          center: [-11000000, 4600000],
          zoom: 15
        })
      });

      map.on('pointermove', pointerMoveHandler);

      map.getViewport().addEventListener('mouseout', function() {
        helpTooltipElement.classList.add('hidden');
      });

      var typeSelect = document.getElementById('type');

      var draw; // global so we can remove it later


      /**
       * Format length output.
       * @param {ol.geom.LineString} line The line.
       * @return {string} The formatted length.
       */
      var formatLength = function(line) {
        var length = ol.Sphere.getLength(line);
        var output;
        if (length > 100) {
          output = (Math.round(length / 1000 * 100) / 100) +
              ' ' + 'km';
        } else {
          output = (Math.round(length * 100) / 100) +
              ' ' + 'm';
        }
        return output;
      };


      /**
       * Format area output.
       * @param {ol.geom.Polygon} polygon The polygon.
       * @return {string} Formatted area.
       */
      var formatArea = function(polygon) {
        var area = ol.Sphere.getArea(polygon);
        var output;
        if (area > 10000) {
          output = (Math.round(area / 1000000 * 100) / 100) +
              ' ' + 'km<sup>2</sup>';
        } else {
          output = (Math.round(area * 100) / 100) +
              ' ' + 'm<sup>2</sup>';
        }
        return output;
      };

      function addInteraction() {
        var type = (typeSelect.value == 'area' ? 'Polygon' : 'LineString');
        draw = new ol.interaction.Draw({
          source: source,
          type: /** @type {ol.geom.GeometryType} */ (type),
          style: new ol.style.Style({
            fill: new ol.style.Fill({
              color: 'rgba(255, 255, 255, 0.2)'
            }),
            stroke: new ol.style.Stroke({
              color: 'rgba(0, 0, 0, 0.5)',
              lineDash: [10, 10],
              width: 2
            }),
            image: new ol.style.Circle({
              radius: 5,
              stroke: new ol.style.Stroke({
                color: 'rgba(0, 0, 0, 0.7)'
              }),
              fill: new ol.style.Fill({
                color: 'rgba(255, 255, 255, 0.2)'
              })
            })
          })
        });
        map.addInteraction(draw);

        createMeasureTooltip();
        createHelpTooltip();

        var listener;
        draw.on('drawstart',
            function(evt) {
              // set sketch
              sketch = evt.feature;

              /** @type {ol.Coordinate|undefined} */
              var tooltipCoord = evt.coordinate;

              listener = sketch.getGeometry().on('change', function(evt) {
                var geom = evt.target;
                var output;
                if (geom instanceof ol.geom.Polygon) {
                  output = formatArea(geom);
                  tooltipCoord = geom.getInteriorPoint().getCoordinates();
                } else if (geom instanceof ol.geom.LineString) {
                  output = formatLength(geom);
                  tooltipCoord = geom.getLastCoordinate();
                }
                measureTooltipElement.innerHTML = output;
                measureTooltip.setPosition(tooltipCoord);
              });
            }, this);

        draw.on('drawend',
            function() {
              measureTooltipElement.className = 'tooltip tooltip-static';
              measureTooltip.setOffset([0, -7]);
              // unset sketch
              sketch = null;
              // unset tooltip so that a new one can be created
              measureTooltipElement = null;
              createMeasureTooltip();
              ol.Observable.unByKey(listener);
            }, this);
      }


      /**
       * Creates a new help tooltip
       */
      function createHelpTooltip() {
        if (helpTooltipElement) {
          helpTooltipElement.parentNode.removeChild(helpTooltipElement);
        }
        helpTooltipElement = document.createElement('div');
        helpTooltipElement.className = 'tooltip hidden';
        helpTooltip = new ol.Overlay({
          element: helpTooltipElement,
          offset: [15, 0],
          positioning: 'center-left'
        });
        map.addOverlay(helpTooltip);
      }


      /**
       * Creates a new measure tooltip
       */
      function createMeasureTooltip() {
        if (measureTooltipElement) {
          measureTooltipElement.parentNode.removeChild(measureTooltipElement);
        }
        measureTooltipElement = document.createElement('div');
        measureTooltipElement.className = 'tooltip tooltip-measure';
        measureTooltip = new ol.Overlay({
          element: measureTooltipElement,
          offset: [0, -15],
          positioning: 'bottom-center'
        });
        map.addOverlay(measureTooltip);
      }


      /**
       * Let user change the geometry type.
       */
      typeSelect.onchange = function() {
        map.removeInteraction(draw);
        addInteraction();
      };

      addInteraction();
    </script>
  </body>
</html>