The World Hydrography Day (WHD) is observed every year on June 21 to publicise the work of hydrographers and to create awareness about hydrography which is an essential science before undertaking any activity on or under the sea. The 2017 theme is “Mapping our seas, oceans and waterways – more important than ever”.
2, 4, 9 have two similar elements intersecting each other. 6, 7, 8 have two different elements one placed inside the other. 1, 3, 5 have two different elements intersecting each other.