Add Maidenhead grid labels

qsomap.py

@@ -280,6 +280,15 @@ def render(ref_lat, ref_lon, output_stream):
                 stroke-width: 0.1px;
                 opacity: 0.5;
             }
+
+            .maidenhead_label {
+                font-family: sans-serif;
+                dominant-baseline: middle;
+                text-anchor: middle;
+                fill: red;
+                opacity: 0.25;
+            }
+
     """))
 
     doc.add(doc.circle(center=(R, R), r=R, fill='#ddeeff',
@@ -378,6 +387,24 @@ def render(ref_lat, ref_lon, output_stream):
 
         group.add(doc.polyline(points, **{'class': 'maidenhead_line'}))
 
+    for x in range(0, N):
+        for y in range(0, N):
+            sectorname = chr(ord('A') + (x + N//2) % N) \
+                         + chr(ord('A') + y)
+
+            lon = (x + 0.5) * 2 * np.pi / N
+            lat = (y + 0.5) * np.pi / N - np.pi/2
+
+            tx, ty = map_azimuthal_equidistant(lat, lon, ref_lat, ref_lon, R)
+
+            font_size = 10
+            if y == 0 or y == N-1:
+                font_size = 3
+
+            group.add(doc.text(sectorname, (tx + R, ty + R),
+                               **{'class': 'maidenhead_label',
+                                  'font-size': font_size}))
+
     doc.add(group)
 
     """