[Fix] Maidenhead grid special case on anti-meridian. Add meridian & anti-meridian unit test cases.

src/utils/geo.js

@@ -104,7 +104,10 @@ export const latLonToMaidenhead = ({ lat, lon }, precision = 6) => {
   if (lon < -180 || lon > 180) throw new Error('invalid longitude, it should be between -180 and 180');
 
   const latNorm = lat + 90;
-  const lonNorm = lon + 180;
+
+  // Handle case where longitude is given as +180, which should be treated as -180,
+  // by normalizing to 0-360 range first then applying modulus again to get back to -180..180 range.
+  const lonNorm = (((lon + 180) % 360) + 360) % 360;
 
   // Field (2 chars): 20° lon x 10° lat
   const field1 = String.fromCharCode(65 + Math.floor(lonNorm / 20)); // A-R

src/utils/geo.test.js

@@ -3,6 +3,9 @@ import { validateGridLocator, latLonToMaidenhead, maidenheadToLatLon, maidenhead
 
 describe('Maidenhead Grid tests', () => {
   const gridCases = [
+    // note that 'grid' fields entered in the test cases are all 8 characters long,
+    // as tests will also validate the first 2, 4, 6 and 8 characters.
+
     // location in San Diego, CA, USA
     {
       grid: 'DM12kv99',
@@ -15,8 +18,32 @@ describe('Maidenhead Grid tests', () => {
     {
       grid: 'QF56od55',
       actualLatLon: { lat: -33.8519, lon: 151.210886 },
-      latLonSWCornerGrid6: [-33.875, 151.1667],
-      latLonNECornerGrid6: [-33.8333, 151.25],
+    },
+
+    // location at equator / prime meridian
+    {
+      grid: 'JJ00aa00',
+      actualLatLon: { lat: 0, lon: 0 },
+    },
+
+    // location at equator just west of antimeridian
+    {
+      grid: 'RJ90XA90',
+      actualLatLon: { lat: 0, lon: 179.999 },
+    },
+
+    // location at equator on antimeridian
+    // note that this is not strictly valid as longitude should be given as -180 rather than +180,
+    // however some sources may expected +180 to be functional so it should be tested
+    {
+      grid: 'AJ00AA00',
+      actualLatLon: { lat: 0, lon: 180.0 },
+    },
+
+    // location at equator on antimeridian
+    {
+      grid: 'AJ00AA00',
+      actualLatLon: { lat: 0, lon: -180.0 },
     },
   ];
 
@@ -64,7 +91,9 @@ describe('Maidenhead Grid tests', () => {
     for (const size of sizes) {
       it("should convert Maidenhead Grid '" + grid.substring(0, size) + "' to Lat/Lon", () => {
         const { lat, lon } = maidenheadToLatLon(grid.substring(0, size));
-        const { lat: expectedLat, lon: expectedLon } = actualLatLon;
+        // handle case where longitude is given as +180 or -180, although +180 is strictly invalid it can sometimes be used so should be tested
+        const { lat: expectedLat, lon: rawLon } = actualLatLon,
+          expectedLon = ((rawLon + 180) % 360) - 180;
         let latBucketSize, lonBucketSize, latBucketStart, latBucketEnd, lonBucketStart, lonBucketEnd;
 
         switch (size) {
@@ -119,19 +148,21 @@ describe('Maidenhead Grid tests', () => {
       });
     }
 
-    it(
-      "should convert Maidenhead Grid '" + grid.substring(0, defaultSize) + "' to Lat/Lon bounding box coordinates",
-      () => {
-        const result = maidenheadToBoundingBox(grid.substring(0, defaultSize));
-        expect(result).toHaveLength(2);
-        expect(result[0]).toHaveLength(2);
-        expect(result[1]).toHaveLength(2);
-
-        expect(result[0][0]).toBeCloseTo(latLonSWCornerGrid6[0], 3);
-        expect(result[0][1]).toBeCloseTo(latLonSWCornerGrid6[1], 3);
-        expect(result[1][0]).toBeCloseTo(latLonNECornerGrid6[0], 3);
-        expect(result[1][1]).toBeCloseTo(latLonNECornerGrid6[1], 3);
-      },
-    );
+    if (latLonSWCornerGrid6 && latLonNECornerGrid6) {
+      it(
+        "should convert Maidenhead Grid '" + grid.substring(0, defaultSize) + "' to Lat/Lon bounding box coordinates",
+        () => {
+          const result = maidenheadToBoundingBox(grid.substring(0, defaultSize));
+          expect(result).toHaveLength(2);
+          expect(result[0]).toHaveLength(2);
+          expect(result[1]).toHaveLength(2);
+
+          expect(result[0][0]).toBeCloseTo(latLonSWCornerGrid6[0], 3);
+          expect(result[0][1]).toBeCloseTo(latLonSWCornerGrid6[1], 3);
+          expect(result[1][0]).toBeCloseTo(latLonNECornerGrid6[0], 3);
+          expect(result[1][1]).toBeCloseTo(latLonNECornerGrid6[1], 3);
+        },
+      );
+    }
   }
 });