Cascade Mapping

Cascade Mapping is the practice of charting the propagation of a single cataloguing event through the floors, ledgers, and personnel of the Tower, on the assumption that no entry changes alone. Where Consequence Mapping records what followed a change and Procedural Mapping records who authorized it, Cascade Mapping records the path — the sequence by which one alteration produced the next, and that one the next. It is the oldest of the mapping disciplines and the least trusted, a circumstance its practitioners attribute to being correct in advance of events rather than after them.

Method

A cascade mapper begins with a single datum, called the origin mark, and works outward in both directions: forward through effects and backward through causes, until the chain either closes on itself or reaches a floor where records do not survive. The forward work is routine. The backward work is where cascades are usually found to have been running unnoticed for years, and where the practice acquires its reputation for unpleasantness.

Maps are drawn on linen-backed sheets in a convention fixed by Concordance 19-B, "On the Representation of Succession," which specifies that every node be marked with the number of the floor on which it occurred, even where that number is disputed. Sheets for disputed floors are backed in red. Clerks are instructed not to erase a red-backed sheet under any circumstance; corrections are made on a fresh overlay pinned to the original. The rule has held since the winter of 402, when a mapper at Floor 208 erased a disputed node and the cascade, deprived of its preferred path, took a longer one through the bindery.

Relation to the Cascade Family

Cascade Mapping is a member of the broader mapping family and is frequently confused with three of its relatives. Resonance Mapping follows amplitude; Cascade Mapping follows succession. Conjectural Mapping works on subjects that may exist; Cascade Mapping works on change that has already occurred, though a sizable minority of practitioners hold that the distinction is one of bookkeeping and not of substance. Dependency Mapping charts what a subject requires; Cascade Mapping charts what a subject has done to its neighbours, which is often the reverse and occasionally the same diagram.

The family resemblance is more than nominal. A cascade map that crosses twelve or more floors is routinely refiled as an instance of Contingency Cascade Analysis, a demotion that the Mapping Registry, Floor 44 has repeatedly protested without effect.

The Venn Dispute

The principal scholarly disagreement concerns whether a cascade may be mapped while it is still running. Librarian Venn held, in the Eighth Concordance, Annex 9, that "a cascade under observation is a cascade altered by the observation, and the map thereof records the observer's footfall and not the cascade." Venn's position, called the closed-chain doctrine, requires the mapper to wait until the chain has visibly terminated before beginning work, sometimes for decades.

The opposing view is set out in the Eleventh Concordance, Annex 2, which holds that a cascade never terminates, only attenuates, and that the closed-chain doctrine is therefore a rule for producing nothing. The Eleventh does not name Venn. It does not need to. The two annexes have never been reconciled, and a mapper citing either will be corrected by the next mapper along by citing the other.

"Where the chain is said to have ended, the record will be found to have merely changed hands."

— Eleventh Concordance, Annex 2

The Floor 300 Incident

The practice's modern standing dates from the incident of 14-Θ-411, when a junior clerk named Mirin Halloway mapped a small cascade originating in a mis-shelved folio on Floor 300 and traced it, over eleven months, through the reshelving of four adjacent cases, the transfer of a curator, the reassignment of a corridor, and the eventual quiet abandonment of an entire reading room on Floor 303. Her map ran to nine sheets and was initially rejected by the Registry as "a history, not a map."

The rejection was withdrawn after the reading room's abandonment was formally noticed — by Halloway, whose map had predicted it, and by no one else. Her closing annotation, now reproduced on the inside cover of every standard cascade sheet, reads: "The first node is where we looked. It is not where it began."

Status Under the Mandate

Cascade Mapping is not itself named in Clause 3, which governs Dimensional Cascades rather than documentary ones, but the Bureau of Dimensional Reconciliation has twice ruled that cascade maps are admissible as supporting evidence in Mandate proceedings. The rulings are not consistent with one another, and the Bureau's final report of 12-Θ-441 declined to settle the matter, observing only that the maps had been "of assistance in the reconstruction, if not in the conclusion."

Practitioners accordingly keep two copies of every map: one for the Registry and one for the Bureau. The second copy is traditionally drawn in a different hand, on the theory that the Bureau prefers corroboration to consistency.

Maps are stored flat. Nothing is to be stored on top of a cascade map, including other cascade maps.