// 获取小数部分
const fractional = function(x) {x = Math.abs(x);return x - Math.floor(x);
}
const formatInt = function(x, len) {let result = '' + x;len = len - result.length;while (len > 0) {result = '0' + result;len--;}return result;
}/*** 创建标准分幅网格* @param west,south,east,north 传入要创建的标准分幅网格的经纬度范围* @param scalem 表示比例尺的分母（例如 10000 表示 1:1万）* @returns 返回一个 geojson 对象*/
export function makeStandardMapGrids(west, south, east, north, scalem) {let lngDiff = 0;let latDiff = 0;let scaleCode = '';switch (scalem) {case 1000000:lngDiff = 6;latDiff = 4;break;case 500000:lngDiff = 3;latDiff = 2;scaleCode = 'B';break;case 250000:lngDiff = 1.5;latDiff = 1;scaleCode = 'C';break;case 100000:lngDiff = 0.5;latDiff = 1 / 3;scaleCode = 'D';break;case 50000:lngDiff = 0.25;latDiff = 1 / 6;scaleCode = 'E';break;case 25000:lngDiff = 0.125;latDiff = 1 / 12;scaleCode = 'F';break;case 10000:lngDiff = 0.0625;latDiff = 1 / 24;scaleCode = 'G';break;case 5000:lngDiff = 0.03125;latDiff = 1 / 48;scaleCode = 'H';case 2000:lngDiff = 37.5 / 3600.0;latDiff = 25.0 / 3600.0;scaleCode = 'I';break;case 1000:lngDiff = 18.75 / 3600.0;latDiff = 12.5 / 3600.0;scaleCode = 'J';break;case 500:lngDiff = 9.375 / 3600.0;latDiff = 6.25 / 3600.0;scaleCode = 'K';break;default:return null;}const GridX0 = -180;const GridX1 = 180;const GridY0 = -88;const GridY1 = 88;let x0 = Math.max(GridX0, west);let y0 = Math.max(GridY0, south);let x1 = Math.min(GridX1, east);let y1 = Math.min(GridY1, north);if (((x1 - x0) < lngDiff) || ((y1 - y0) < latDiff)) {return null;}let features = []; // 存储生成的面要素let coordinates = [] // 存储生成的面要素坐标对// 计算标准分幅网格行列范围const col0 = parseInt((x0 - GridX0) / lngDiff);const col1 = parseInt((x1 - GridX0) / lngDiff);const row0 = parseInt((y0 - GridY0) / latDiff);const row1 = parseInt((y1 - GridY0) / latDiff);const millionRowCode = 'ABCDEFGHIJKLMNOPQRSTUV';for (let row = row0; row <= row1; row++) {let gy0 = GridY0 + row * latDiff;let gy1 = gy0 + latDiff;let gcy = (gy0 + gy1) * 0.5; // 分幅中心点 y 坐标let millionRow = parseInt((gy0 - 0) / 4); // 1:100分幅行号let Hemisphere = ''; // 北半球标志if (millionRow < 0) {millionRow = -1 - millionRow;Hemisphere = 'S'; // 南半球标志}for (let col = col0; col <= col1; col++) {let gx0 = GridX0 + col * lngDiff;let gx1 = gx0 + lngDiff;let gcx = (gx0 + gx1) * 0.5; // 分幅中心点 x 坐标let millionCol = parseInt((gcx - GridX0) / 6) + 1; // 1:100分幅列号(从1开始)coordinates = [[[gx0, gy0],[gx1, gy0],[gx1, gy1],[gx0, gy1],[gx0, gy0]]];millionCol = (millionCol < 10) ? ('0' + millionCol) : millionCol;let gridID = Hemisphere + millionRowCode[millionRow] + millionCol;if (scaleCode != '') {// 计算当前分幅在 1:100万 分幅内的行列号(注意，这里行列号从左向右，从北向南，从1开始编号)let colID = parseInt((fractional((gcx - GridX0) / 6) * 6) / lngDiff) + 1;let rowID = parseInt((fractional((GridY1 - gcy) / 4) * 4) / latDiff) + 1;gridID += scaleCode + formatInt(rowID, 3) + formatInt(colID, 3);}// 生成矢量要素(几何信息+属性信息)let feature = {type: "Feature",geometry: {type: "Polygon",coordinates: coordinates},properties: {ID: gridID,extend: '西:' + gx0 + ' 东:' + gx1 + ' 南:' + gy0 + ' 北:' + gy1,lat: gcy.toFixed(6),lng: gcx.toFixed(6)},location: {latitude: Number(gcy.toFixed(6)),longitude: Number(gcx.toFixed(6)),}};features.push(feature);}}return {type: "FeatureCollection",features: features};
}

 我国把 1:1 万、1:2.5 万、1:5 万、1:10 万、1:25 万、1:50 万、1:100 万 7 种比例尺作为国家基本地图的比例尺系列。根据国家标准GB/13989-92 《国家基本比例尺地形图分幅和编号》规定，我国基本比例尺地形图均1:100 万地形图为基础，按规定的经差和纬差划分图幅。下表为地形图的经纬差、行列数及图幅数。