According to the Livius site, Strabo's Geographies were partly based on autopsy (personal visits), and partly on other sources. According to Strabo himself, his books on India and Persia (books 15 and 16) were based on the (sometimes conflicting) written accounts of three high ranking officers in Alexander's army and navy, and his description of Persia follows the trail Alexander took through it to India. He supported and checked these accounts against the work of four librarian-scholars. It is not always clear which source he is using at any time, and this is especially true of his description of Babylon and the Hanging Gardens, which occurs in Book 16.
From the Lacus Curtius site:
Babylon, too, lies in a plain; and the circuit of its wall is three hundred and eighty-five stadia. The thickness of its wall is thirty-two feet; the height thereof between the towers is fifty cubits; that of the towers is sixty cubits; and the passage on top of the wall is such that four-horse chariots can easily pass one another; and it is on this account that this and the hanging garden are called one of the Seven Wonders of the World. The garden is quadrangular in shape, and each side is four plethra in length. It consists of arched vaults, which are situated, one after another, on checkered, cube-like foundations. The checkered foundations, which are hollowed out, are covered so deep with earth that they admit of the largest of trees, having been constructed of baked brick and asphalt—the foundations themselves and the vaults and the arches. The ascent to the uppermost terrace-roofs is made by a stairway; and alongside these stairs there were screws, through which the water was continually conducted up into the garden from the Euphrates by those appointed for this purpose. For the river, a stadium in width, flows through the middle of the city; and the garden is on the bank of the river.
—Geographies, Book 16, ch 1, § 5
Taken from The Geography of Strabo, Loeb Classical Library (8 vols), 1932