原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/scaleline-indiana-east.html

运行界面

演示了在客户端OpenStreetMap重新投影至NAD83 Indiana East，包括使用美国单位的比例尺控件。
我特意去瞅了下源码，这里截取了部分代码，来理解下图上mi这个单位：

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else if (units == 'us') {
  if (nominalCount < 0.9144) {
    suffix = 'in';
    pointResolution *= 39.37;
  } else if (nominalCount < 1609.344) {
    suffix = 'ft';
    pointResolution /= 0.30480061;
  } else {
    suffix = 'mi';
    pointResolution /= 1609.3472;
  }
}

而默认单位即公制单位（metric）源码为：

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else if (units == 'metric') {
  if (nominalCount < 0.001) {
    suffix = 'μm';
    pointResolution *= 1000000;
  } else if (nominalCount < 1) {
    suffix = 'mm';
    pointResolution *= 1000;
  } else if (nominalCount < 1000) {
    suffix = 'm';
  } else {
    suffix = 'km';
    pointResolution /= 1000;
  }
}

main.js代码如下：

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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 proj4 from 'proj4';
import {ScaleLine} from 'ol/control.js';
import {fromLonLat, transformExtent} from 'ol/proj.js';
import {register} from 'ol/proj/proj4.js';

proj4.defs(
  'Indiana-East',
  'PROJCS["IN83-EF",GEOGCS["LL83",DATUM["NAD83",' +
    'SPHEROID["GRS1980",6378137.000,298.25722210]],PRIMEM["Greenwich",0],' +
    'UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],' +
    'PARAMETER["false_easting",328083.333],' +
    'PARAMETER["false_northing",820208.333],' +
    'PARAMETER["scale_factor",0.999966666667],' +
    'PARAMETER["central_meridian",-85.66666666666670],' +
    'PARAMETER["latitude_of_origin",37.50000000000000],' +
    'UNIT["Foot_US",0.30480060960122]]'
);
register(proj4);

const map = new Map({
  layers: [
    new TileLayer({
      source: new OSM(),
    }),
  ],
  target: 'map',
  view: new View({
    projection: 'Indiana-East',
    center: fromLonLat([-85.685, 39.891], 'Indiana-East'),
    zoom: 7,
    extent: transformExtent(
      [-172.54, 23.81, -47.74, 86.46],
      'EPSG:4326',
      'Indiana-East'
    ),
    minZoom: 6,
  }),
});

map.addControl(new ScaleLine({units: 'us'}));

界面布局文件index.html代码如下：

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>OpenStreetMap Reprojection with ScaleLine Control</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>
    <!-- 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>