This is an interesting article: It seems that since we are in the middle of the galaxy it's very complex to reverse engineer what we're seeing — until now that is thanks to computer aided astronomy. And it seems that the results are changing the way we think about the shape of the our galaxy, it seems that the Milky Way is two large spiral arms rather than four:
"Astronomy concepts are always changing as new information comes to light. The latest theory to undergo revision is our concept of the structure of the Milky Way. As we reside in the thick of it, it's tough to see the proverbial forest for the trees. New findings may make a compelling case that the Milky Way has two major spiral arms, rather than four that have been theorized in the past.
A team led by Robert Benjamin of the University of Wisconsin used the recently completed Spitzer Space Telescope infrared survey of the Milky Way, spanning 120° of sky. Previous studies led astronomers to conclude the galaxy has a pronounced central bar and four major spiral arms named for the constellations we see in their directions: Sagittarius-Carina, Scutum-Crux, Perseus, and Norma-Cygnus."