The U-shaped dotted line on the blueprint is the ceiling line. I think this illustration will clear up matters. Basically, the Omniputer is in a pit. The "Computer Banks and Ship's Automated Systems are under the main floor. The plans call for a "grille deck" and my guess is all of that would have been wild so they could get the camera in right around the Omniputer for closeups. Again, these were concepts not actual set construction plans, and had this ever been made I'm sure they would have discussed all the practical matters (like is there a step up out of the pit that doesn't require using the ladder tube).