webmercator: simplify equation

MIERUNE · Dec 4, 2023 · 062af7a · 062af7a
1 parent f8f5ecb
commit 062af7a
Showing 1 changed file with 10 additions and 10 deletions.
diff --git a/nusamai-mvt/src/webmercator.rs b/nusamai-mvt/src/webmercator.rs
@@ -1,21 +1,21 @@
 use std::f64::consts::FRAC_PI_2;
 
-/// Converts Geographic coordinate (lng, lat) to Web Mercator coordinate (mx, my).
+/// Converts geographic coordinate (lng, lat) to Web Mercator coordinate (mx, my).
 ///
 /// The range of (mx, my) is [0.0, 0.0]-[1.0, 1.0] (same as Mapbox/MapLibre API, etc.)
 pub fn lnglat_to_web_mercator(lng: f64, lat: f64) -> (f64, f64) {
-    let mx = (lng / 180.0 + 1.0) / 2.0;
+    let mx = (lng + 180.0) / 360.0;
     let my = ((90.0 + lat).to_radians() / 2.0).tan().ln().to_degrees();
-    let my = (-my / 180.0 + 1.0) / 2.0;
+    let my = (-my + 180.0) / 360.0;
     (mx, my)
 }
 
-/// Converts Web Mercator coordinate (mx, my) to Geographic coordinate (lng, lat).
+/// Converts Web Mercator coordinate (mx, my) to geographic coordinate (lng, lat).
 ///
 /// The range of (mx, my) is [0.0, 0.0]-[1.0, 1.0] (same as Mapbox/MapLibre API, etc.)
 pub fn web_mercator_to_lnglat(mx: f64, my: f64) -> (f64, f64) {
-    let lng = (mx * 2.0 - 1.0) * 180.0;
-    let lat = (my * 2.0 - 1.0) * 180.0;
+    let lng = mx * 360.0 - 180.0;
+    let lat = my * 360.0 - 180.0;
     let lat = -(2.0 * (lat.to_radians().exp()).atan() - FRAC_PI_2).to_degrees();
     (lng, lat)
 }
@@ -30,15 +30,15 @@ mod tests {
             let (lng, lat) = (136.08, 37.39);
             let (mx, my) = lnglat_to_web_mercator(lng, lat);
             let (lng2, lat2) = web_mercator_to_lnglat(mx, my);
-            assert!((lng - lng2).abs() < 1e-10);
-            assert!((lat - lat2).abs() < 1e-10);
+            assert!((lng - lng2).abs() < 1e-13);
+            assert!((lat - lat2).abs() < 1e-13);
         }
         {
             let (lng, lat) = (0.3, 0.2);
             let (mx, my) = lnglat_to_web_mercator(lng, lat);
             let (lng2, lat2) = web_mercator_to_lnglat(mx, my);
-            assert!((lng - lng2).abs() < 1e-10);
-            assert!((lat - lat2).abs() < 1e-10);
+            assert!((lng - lng2).abs() < 1e-13);
+            assert!((lat - lat2).abs() < 1e-13);
         }
     }