Use OpenStreetMap zoom level

lib/convert-coordinates.js

@@ -1,5 +1,7 @@
 const utm = require('utm')
 
+const degToRad = require('./deg-to-rad')
+
 /**
  * Convert OMSI coordinates to LGLN coordinates
  * @param {Object} options Convert options
@@ -21,14 +23,20 @@ function convertCoordinates (options) {
     options.z = parseFloat(options.z)
   }
 
-  const { easting, northing } = utm.fromLatLon(options.x, options.y)
-  const zoom = (100 * options.z) / 2
+  const {
+    easting: centerX,
+    northing: centerY
+  } = utm.fromLatLon(options.x, options.y)
+
+  const C = 40075016.686
+  const tileWidth = C * Math.cos(degToRad(options.x)) / 2 ** options.z
+  const halfTileWidth = tileWidth / 2
 
   return {
-    x1: easting - zoom,
-    y1: northing - zoom,
-    x2: easting + zoom,
-    y2: northing + zoom
+    x1: centerX - halfTileWidth,
+    y1: centerY - halfTileWidth,
+    x2: centerX + halfTileWidth,
+    y2: centerY + halfTileWidth
   }
 }
 

lib/deg-to-rad.js

@@ -0,0 +1,10 @@
+/**
+ * Convert degree to radian
+ * @param {number} angle Degree
+ * @returns {number} Radian
+ */
+function degToRad (angle) {
+  return angle * (Math.PI / 180)
+}
+
+module.exports = degToRad

package.json

@@ -1,6 +1,6 @@
 {
   "name": "omsi-to-lgln",
-  "version": "0.1.0-alpha.2",
+  "version": "0.1.0-alpha.3",
   "description": "Convert OMSI URLs to LGLN URLs",
   "main": "index.js",
   "bin": "bin/omsi-to-lgln.js",