Atlas is an addon for World of Warcraft that includes detailed maps for every instance in the game. Originally, Blizzard neglected to include maps for instances. And that’s why Atlas was born. The 1st author – Daniel Gilbert created Atlas with the help from several artists and map creators together with several translators to provide the instance maps for player.
However, after a number of years Blizzard began to include official maps for some instances but not all. And then in around the build of 7.0, finally Blizzard decided to complete all the missing instance maps.
These official maps which Blizzard provided are in the same hand-drawn style as the built-in world maps. Conversely, the Atlas maps are based on data used by the minimap, which provides a more detailed, accurate, and colorful representation. You can think of this as the difference between a street map and a satellite image.