测量【译】

原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/measure.html

getLength()和getArea()函数可以计算几何图形的球面长度和面积。长度是通过假设几何坐标之间的大圆段(great circle segments)来计算的，而面积的计算就好像多边形的边缘是大圆段一样。

注意：geometry.getLength()和geometry.getArea()方法返回投影(平面)几何图形的测量(结果)。在某些情况下，如使用Web Mercator投影的北纬和南纬地区，在进行实地测量会有很大不同，为了获得更好的结果，请使用ol/sphere模块中的函数。

main.js代码如下：

import Draw from 'ol/interaction/Draw.js';
import Map from 'ol/Map.js';
import Overlay from 'ol/Overlay.js';
import View from 'ol/View.js';
import {Circle as CircleStyle, Fill, Stroke, Style} from 'ol/style.js';
import {LineString, Polygon} from 'ol/geom.js';
import {OSM, Vector as VectorSource} from 'ol/source.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
import {getArea, getLength} from 'ol/sphere.js';
import {unByKey} from 'ol/Observable.js';

const raster = new TileLayer({
  source: new OSM(),
});

const source = new VectorSource();

const vector = new VectorLayer({
  source: source,
  style: {
    'fill-color': 'rgba(255, 255, 255, 0.2)',
    'stroke-color': '#ffcc33',
    'stroke-width': 2,
    'circle-radius': 7,
    'circle-fill-color': '#ffcc33',
  },
});

/**
 * Currently drawn feature.
 * @type {import("../src/ol/Feature.js").default}
 */
let sketch;

/**
 * The help tooltip element.
 * @type {HTMLElement}
 */
let helpTooltipElement;

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

/**
 * The measure tooltip element.
 * @type {HTMLElement}
 */
let measureTooltipElement;

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

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

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

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

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

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

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

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

map.on('pointermove', pointerMoveHandler);

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

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

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

/**
 * Format length output.
 * @param {LineString} line The line.
 * @return {string} The formatted length.
 */
const formatLength = function (line) {
  const length = getLength(line);
  let 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 {Polygon} polygon The polygon.
 * @return {string} Formatted area.
 */
const formatArea = function (polygon) {
  const area = getArea(polygon);
  let 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() {
  const type = typeSelect.value == 'area' ? 'Polygon' : 'LineString';
  draw = new Draw({
    source: source,
    type: type,
    style: new Style({
      fill: new Fill({
        color: 'rgba(255, 255, 255, 0.2)',
      }),
      stroke: new Stroke({
        color: 'rgba(0, 0, 0, 0.5)',
        lineDash: [10, 10],
        width: 2,
      }),
      image: new CircleStyle({
        radius: 5,
        stroke: new Stroke({
          color: 'rgba(0, 0, 0, 0.7)',
        }),
        fill: new Fill({
          color: 'rgba(255, 255, 255, 0.2)',
        }),
      }),
    }),
  });
  map.addInteraction(draw);

  createMeasureTooltip();
  createHelpTooltip();

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

    /** @type {import("../src/ol/coordinate.js").Coordinate|undefined} */
    let tooltipCoord = evt.coordinate;

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

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

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

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

addInteraction();

界面布局文件index.html代码如下：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Measure</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      .ol-tooltip {
        position: relative;
        background: rgba(0, 0, 0, 0.5);
        border-radius: 4px;
        color: white;
        padding: 4px 8px;
        opacity: 0.7;
        white-space: nowrap;
        font-size: 12px;
        cursor: default;
        user-select: none;
      }
      .ol-tooltip-measure {
        opacity: 1;
        font-weight: bold;
      }
      .ol-tooltip-static {
        background-color: #ffcc33;
        color: black;
        border: 1px solid white;
      }
      .ol-tooltip-measure:before,
      .ol-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%;
      }
      .ol-tooltip-static:before {
        border-top-color: #ffcc33;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <form>
      <label for="type">Measurement type &nbsp;</label>
      <select id="type">
        <option value="length">Length (LineString)</option>
        <option value="area">Area (Polygon)</option>
      </select>
    </form>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>