投影和比例尺

View 投影

本示例演示改变投影坐标系的时候保持比例尺.

本示例演示改变投影坐标系的时候保持比例尺。 getPointResolution() 用于计算与旧投影对应的新投影的分辨率。

main.js

import Map from 'ol/Map.js';
import OSM from 'ol/source/OSM.js';
import TileLayer from 'ol/layer/Tile.js';
import View from 'ol/View.js';
import {ScaleLine, defaults as defaultControls} from 'ol/control.js';
import {
  getPointResolution,
  get as getProjection,
  transform,
} from 'ol/proj.js';

const viewProjSelect = document.getElementById('view-projection');
const projection = getProjection(viewProjSelect.value);

const scaleControl = new ScaleLine({
  units: 'metric',
  bar: true,
  steps: 4,
  text: true,
  minWidth: 140,
});

const map = new Map({
  controls: defaultControls().extend([scaleControl]),
  layers: [
    new TileLayer({
      source: new OSM(),
    }),
  ],
  target: 'map',
  view: new View({
    center: transform([0, 52], 'EPSG:4326', projection),
    zoom: 6,
    projection: projection,
  }),
});

function onChangeProjection() {
  const currentView = map.getView();
  const currentProjection = currentView.getProjection();
  const newProjection = getProjection(viewProjSelect.value);
  const currentResolution = currentView.getResolution();
  const currentCenter = currentView.getCenter();
  const currentRotation = currentView.getRotation();
  const newCenter = transform(currentCenter, currentProjection, newProjection);
  const currentMPU = currentProjection.getMetersPerUnit();
  const newMPU = newProjection.getMetersPerUnit();
  const currentPointResolution =
    getPointResolution(currentProjection, 1 / currentMPU, currentCenter, 'm') *
    currentMPU;
  const newPointResolution =
    getPointResolution(newProjection, 1 / newMPU, newCenter, 'm') * newMPU;
  const newResolution =
    (currentResolution * currentPointResolution) / newPointResolution;
  const newView = new View({
    center: newCenter,
    resolution: newResolution,
    rotation: currentRotation,
    projection: newProjection,
  });
  map.setView(newView);
}
viewProjSelect.addEventListener('change', onChangeProjection);

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Projection and Scale</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <div>
      <label for="view-projection">View projection</label>
      <select id="view-projection">
        <option value="EPSG:3857">Spherical Mercator (EPSG:3857)</option>
        <option value="EPSG:4326" selected>WGS 84 (EPSG:4326)</option>
      </select>
    </div>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="./resources/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "projection-and-scale",
  "dependencies": {
    "ol": "7.3.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}