system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know what the Milky Way Galaxy looks like if we're stuck on the inside looking out?

While watching Cosmos last night, it dawned on me that all the pictures we see of the Milky Way must be artistic representations, since it would be impossible to view ourselves from a third person perspective.  How do we know for sure that we are living in a spiral galaxy or things such as the number of spiral arms?
gpt
Think about it like classical map making; you measure the direction one object is from another, then measure the distance. So what we have are many thousands of measurements, each plotting the direction and distance of a single point - a star.  That is enough to build a comprehensive map (or in this case 3D model) which gives you a top down view (or in the case of a full 3D model, any view you want to make).

Classical mapmaking would also use triangulation, but obviously we only have on measuring point.