AP_Terrain: use get_distance_NE instead of location_diff

libraries/AP_Terrain/TerrainIO.cpp

@@ -231,7 +231,7 @@ void AP_Terrain::seek_offset(void)
 
     // shift another two blocks east to ensure room is available
     loc2.offset(0, 2*grid_spacing*TERRAIN_GRID_BLOCK_SIZE_Y);
-    Vector2f offset = location_diff(loc1, loc2);
+    const Vector2f offset = loc1.get_distance_NE(loc2);
     uint16_t east_blocks = offset.y / (grid_spacing*TERRAIN_GRID_BLOCK_SIZE_Y);
 
     uint32_t file_offset = (east_blocks * block.grid_idx_x + 

libraries/AP_Terrain/TerrainUtil.cpp

@@ -79,7 +79,7 @@ void AP_Terrain::calculate_grid_info(const Location &loc, struct grid_info &info
     ref.lng = info.lon_degrees*10*1000*1000L;
 
     // find offset from reference
-    Vector2f offset = location_diff(ref, loc);
+    const Vector2f offset = ref.get_distance_NE(loc);
 
     // get indices in terms of grid_spacing elements
     uint32_t idx_x = offset.x / grid_spacing;