Labeled topographic map of Venus, created from Magellan images. On the left, the Northern hemisphere, and on the right, the Southern hemisphere. (NASA, USGS, Venera 15, Venera 16, Arecibo Imaging Radar, Planetary Photojournal)
     Since the Earth is the only planet with land and seas, zero altitude and colors showing altitudes relative to that altitude have nothing to do with sea level. The blues and purples in these maps, which would correspond to the depths of the oceans on the Earth, represent areas that are below the "mean" or average height relative to the center of the planet (in other words at a less than average distance from the center), while greens, yellows, oranges and reds represent areas above the mean height. Almost all the features, high and low, visible on the scale of these maps are volcanic structures -- basaltic floodplains in the lower regions, and shallow rises built up by many layers of volcanic activity in the higher regions. Unlike the Earth, where most of the surface is either very high (the continental regions) or very low (the ocean basins), different heights on Venus occur more or less at random; so the geological history of the planet must be very different from that of the Earth. This is undoubtedly partly due to the high surface temperatures, which allow lava to flow long distances before forming a crust too thick to flow any further, and the consequent lack of water, which means that there is no erosion and weathering of the sort we are used to.