The official boundaries of the city of Antananarivo encompass an urban area of approximately . It was founded  above sea level at the apex of three hill ranges that converge in a Y form,  above the surrounding Betsimitatatra paddy fields and the grassy plains beyond. The city gradually spread out from this central point to cover the hillsides; by the late 19th century it had expanding to the flat terrain at the base of the hills. These plains are susceptible to flooding during the rainy season; they are drained by the Ikopa River, which skirts the capital to the south and west. The western slopes and plains, being best protected from cyclone winds originating over the Indian Ocean, were settled before those to the east.