A labeled topographic map of the Moon centered on its North Pole (source: USGS)
. The near side of the Moon is on the bottom and the far side is on top.
Since the Moon has no air or water and hence no "sea level", zero altitude is set at the Moon's mean (average) radius of just over a thousand miles. Altitudes below that use bluer tones suggestive of oceanic regions on the Earth, and altitudes above that green and redder tones suggestive of land regions on the Earth (areas shown in gray have no reliable altitude data). As the result the lunar maria, once thought to be oceans, are shown as though they really were oceans, while the lunar terrae, once thought to be highlands, are shown as though they really were continents.