Children of the Lens by E.E. Smith

The "Big Brass" -- socially the _Directrix_, technically the _Z9M9Z_ -- floated through space at the center of a hollow sphere of maulers packed almost screen to screen. She carried the Brains. She had been built around the seventeen million cubic feet of unobstructed space which comprised her "tank" -- the three-dimensional chart in which vari-colored lights, stationary and moving, represented the positions and motions of solar systems, ships, loose planets, negaspheres, and all other objects and items in which Grand Fleet Operations was, or might become, interested. Completely encircling the tank's more than two thousand feet of circumference was the Rigellian-manned, multimillion-plug board; a crew and a board capable of handling efficiently more than a million combat units.

In the "reducer," the comparatively tiny ten-foot tank set into an alcove, there were condensed the continuously-changing major features of the main chart, so that one man could comprehend and direct the broad strategy of the engagement.