Alexis Huet’s map posted to Reddit is based on spacecraft data. Most of our information about what lies beneath the dense clouds of Venus was obtained by the Soviet space probe missions Venera 15 and 16 and by the American Pioneer Venus and Magellan spacecraft during the period 1978 to 1994. Today we have good information about 98% of the surface of Venus, according to this page from ESO. This map comes from NOAA’s Science on a Sphere. It’s a compilation of Venus radar data, showing Venus’ topography as it’s known today. NOAA wrote: “Most of Venus appears to be covered with gently rolling plains. Two areas rise up above the rest of the surface and are referred to as ‘continents.’”