fix(Manual GPS): Determine whether the user is using UTM zone letters…

… for latitude bands or hemisphere

src/frontend/screens/ManualGpsScreen.js

@@ -68,12 +68,57 @@ class ManualGpsScreen extends React.Component<Props, State> {
 
   toLatLon() {
     const { zoneNum, zoneLetter, easting, northing } = this.state;
+    const parsedNorthing = parseNumber(northing);
+    let northern;
+    // There are two conventions for UTM. One uses a letter to refer to latitude
+    // bands from C to X, excluding "I" and "O". The other uses "N" or "S" to
+    // refer to the northern or southern hemisphere. If the user enters "N" or
+    // "S" we do not know which convention they are using, so we guess. We try
+    // to use the latitude band if we can, since it is better for catching
+    // errors in coordinate entry.
+    if (zoneLetter === "S") {
+      // "S" could refer to grid zone "S" (in the northern hemisphere) or it
+      // could mean "Southern" - conventions differ in different places
+      const startOfZoneS = 3544369.909548157;
+      const startOfZoneT = 4432069.057005376;
+      if (
+        parsedNorthing !== undefined &&
+        parsedNorthing >= startOfZoneS &&
+        parsedNorthing < startOfZoneT
+      ) {
+        // Indeterminate, this could be latitude band S, or it could mean
+        // southern hemisphere. The only place in the southern hemisphere that
+        // matches these coordinates is the very southern tip of Chile and
+        // Argentina, so assume that in this case zoneLetter "S" refers to
+        // latitude band "S", in the northern hemisphere.
+        // TODO: Check with the user what they mean, or use last known location
+      } else {
+        // The northing is not within the range of grid zone "S", so we assume
+        // the user meant "Southern" with the letter "S"
+        northern = false;
+      }
+    } else if (zoneLetter === "N") {
+      const startOfZoneN = 0;
+      const startOfZoneP = 885503.7592863895;
+      if (
+        parsedNorthing &&
+        parsedNorthing >= startOfZoneN &&
+        parsedNorthing < startOfZoneP
+      ) {
+        // Definitely in latitude band N, just use the band letter
+      } else {
+        // Outside latitude band "N", so the user probably means "Northern"
+        northern = true;
+      }
+    }
     try {
       return toLatLon(
         parseNumber(easting),
         parseNumber(northing),
         parseNumber(zoneNum),
-        zoneLetter
+        // If northern defined, then don't use the zoneLetter.
+        northern !== undefined ? undefined : zoneLetter,
+        northern
       );
     } catch (err) {
       ToastAndroid.showWithGravity(
@@ -127,7 +172,7 @@ class ManualGpsScreen extends React.Component<Props, State> {
               placeholderTextColor="silver"
               underlineColorAndroid="transparent"
               onChangeText={zoneLetter =>
-                this.setState({ zoneLetter: zoneLetter.trim() })
+                this.setState({ zoneLetter: zoneLetter.trim().toUpperCase() })
               }
               maxLength={1}
               autoCapitalize="characters"