simplified getLetterDesignator

proj4js · May 6, 2019 · 80d0548 · 80d0548
1 parent c30067f
commit 80d0548
Showing 1 changed file with 15 additions and 65 deletions.
diff --git a/mgrs.js b/mgrs.js
@@ -297,72 +297,22 @@ function UTMtoLL(utm) {
  *     for.
  * @return {char} The letter designator.
  */
-function getLetterDesignator(lat) {
-  //This is here as an error flag to show that the Latitude is
-  //outside MGRS limits
-  let LetterDesignator = 'Z';
-
-  if ((84 >= lat) && (lat >= 72)) {
-    LetterDesignator = 'X';
-  }
-  else if ((72 > lat) && (lat >= 64)) {
-    LetterDesignator = 'W';
-  }
-  else if ((64 > lat) && (lat >= 56)) {
-    LetterDesignator = 'V';
-  }
-  else if ((56 > lat) && (lat >= 48)) {
-    LetterDesignator = 'U';
-  }
-  else if ((48 > lat) && (lat >= 40)) {
-    LetterDesignator = 'T';
-  }
-  else if ((40 > lat) && (lat >= 32)) {
-    LetterDesignator = 'S';
-  }
-  else if ((32 > lat) && (lat >= 24)) {
-    LetterDesignator = 'R';
-  }
-  else if ((24 > lat) && (lat >= 16)) {
-    LetterDesignator = 'Q';
-  }
-  else if ((16 > lat) && (lat >= 8)) {
-    LetterDesignator = 'P';
-  }
-  else if ((8 > lat) && (lat >= 0)) {
-    LetterDesignator = 'N';
-  }
-  else if ((0 > lat) && (lat >= -8)) {
-    LetterDesignator = 'M';
-  }
-  else if ((-8 > lat) && (lat >= -16)) {
-    LetterDesignator = 'L';
-  }
-  else if ((-16 > lat) && (lat >= -24)) {
-    LetterDesignator = 'K';
-  }
-  else if ((-24 > lat) && (lat >= -32)) {
-    LetterDesignator = 'J';
-  }
-  else if ((-32 > lat) && (lat >= -40)) {
-    LetterDesignator = 'H';
-  }
-  else if ((-40 > lat) && (lat >= -48)) {
-    LetterDesignator = 'G';
-  }
-  else if ((-48 > lat) && (lat >= -56)) {
-    LetterDesignator = 'F';
-  }
-  else if ((-56 > lat) && (lat >= -64)) {
-    LetterDesignator = 'E';
-  }
-  else if ((-64 > lat) && (lat >= -72)) {
-    LetterDesignator = 'D';
-  }
-  else if ((-72 > lat) && (lat >= -80)) {
-    LetterDesignator = 'C';
+function getLetterDesignator(latitude) {
+  if (latitude <= 84 && latitude >= 72) {
+    // the X band is 12 degrees high
+    return 'X';
+  } else if (latitude < 72 || latitude >= -80) {
+    // Latitude bands are lettered C through X, excluding I and O
+    const bandLetters = 'CDEFGHJKLMNPQRSTUVWX';
+    const bandHeight = 8;
+    const minLatitude = -80;
+    const index = Math.floor((latitude - minLatitude) / bandHeight);
+    return bandLetters[index];
+  } else if (latitude > 84 || latitude < -80) {
+    //This is here as an error flag to show that the Latitude is
+    //outside MGRS limits
+    return 'Z';
   }
-  return LetterDesignator;
 }
 
 /**