Clamp latitude to 85 degrees (#123)

Author: CelliesProjects

README.md

@@ -99,11 +99,11 @@ bool fetchMap(LGFX_Sprite &map, double longitude, double latitude, uint8_t zoom,
 ```
 
 - Overflowing `longitude` are wrapped and normalized to +-180°.
-- Overflowing `latitude` are clamped to +-90°.
+- Overflowing `latitude` are clamped to +-85°.
 - Valid range for the `zoom` level is from `getMinZoom()` to `getMaxZoom()`.  
 - `timeoutMS` can be used to throttle the amount of downloaded tiles per call.  
 Setting it to anything other than `0` sets a timeout. Sane values start around ~100ms.  
-**Note:** No more tiles will be downloaded after the timeout expires, but tiles that are downloading will be finished.  
+**Note:** No more tile downloads will be started after the timeout expires, but tiles that are downloading will be finished.  
 **Note:** You might end up with missing map tiles. Or no map at all if you set the timeout too short.
 
 ### Free the psram memory used by the tile cache

src/OpenStreetMap-esp32.cpp

@@ -325,8 +325,13 @@ bool OpenStreetMap::fetchMap(LGFX_Sprite &mapSprite, double longitude, double la
         return false;
     }
 
+    // Web Mercator projection only supports latitudes up to ~85.0511°.
+    // See https://en.wikipedia.org/wiki/Web_Mercator_projection#Formulas
+    // We use 85.0° as a safe and simple boundary.
+    constexpr double MAX_MERCATOR_LAT = 85.0;
+
     longitude = fmod(longitude + 180.0, 360.0) - 180.0;
-    latitude = std::clamp(latitude, -90.0, 90.0);
+    latitude = std::clamp(latitude, -MAX_MERCATOR_LAT, MAX_MERCATOR_LAT);
 
     tileList requiredTiles;
     computeRequiredTiles(longitude, latitude, zoom, requiredTiles);