M&D Spoiler question p. 242

[Juan Cires Martinez]

Spoiler:












On page 242:

 "He must, if one day call'd upon, produce an overhead view of a World
  that never was, in truth-like detail, one he'd begun in silence to
  contrive,-- a Map entirely within his mind, of a World he could escape
  to, if he had to.  If he had to, he would enter it entirely but never
  get lost, for he would have this Map, and in it, spread below, would
  lie ev'rything,-- Mountains of Glass, Sea of Sand, miraculous Springs,
  Volcanoes, Sacred Cities, mile-deep Chasm, Serpent's Cave, endless
  Prairie... another chapbook-Fancy with each Deviation and Dip of the
  Needle."

Could someone explain the expression "each Deviation and Dip of the
Needle"?

Saludos, Juan.