Geodetic Report on the Median Line (Series 417/418)

Scope and provenance

Geodetic Report on the Median Line (Series 417/418) is a two-part survey document deposited with the Clerks of the Median Line and subsequently accessioned under a split shelfmark. Series 417 comprises the western and downward survey; Series 418 comprises the eastern and upward. The two were commissioned separately, written in different hands, and have never been bound together, though Concordance 11, Annex C refers to them jointly as "the Report" for the purposes of settling disputes of measure.

The Reports are the principal documentary basis for all subsequent work by the Clerks on Line Sigma, and are cited in Ordinance 881 as having been superseded only in respect of their treatment of the Compass Rose Atrium datum. All other findings of Series 417/418 remain in force.

Commissioning

Series 417 was commissioned in the winter of the Long Recension, following a period of intermittent disagreement between the Clerks and the Line Sigma Master Line over the placement of tram stops relative to the operational midpoint of the Sanctum. Series 418 was commissioned eleven months later, after it became apparent that the western survey had recorded no fewer than four points as being simultaneously at the midpoint — a result the drafting clerk attributed to "the line's disinclination to be crossed twice."

The Surveyor of Record for Series 417, one Vetch, is noted in the marginalia of Report on Atmospheric Anomalies (TY 7124) as having complained repeatedly that his instruments were "calibrated for a straighter building." His complaint was filed, amended twice, and ultimately closed without reply; the closed file is now held as the Bexley Series by analogy of procedure, though the two matters are unrelated.

Findings

Series 417 establishes the median of the Tower's north–south axis at a value that the Report itself declines to state in metres, giving instead a series of eleven reference points by comparison with the hematite disc of the Compass Rose Atrium. Series 418 corrects two of these reference points and adds a ninth and tenth that Series 417 had omitted for reasons recorded only as "not yet true."

The Reports jointly assert three propositions:

• That the Tower possesses a geodetic centre, and that this centre is stable to within one part in nine hundred over the surveyed interval.
• That the centre does not coincide with the Secondary (Plumb-Line) Measurement datum, and that the discrepancy of 3.4 metres is "structural rather than error."
• That the median is best maintained not by measurement but by recomputation, the results of which are to be recorded daily in the Plinth Report register.

The third proposition is the one most often cited and least often honoured; the Plinth Report register shows gaps of up to nineteen days in most recorded winters.

The Median Tone controversy

Series 418 contains an unnumbered appendix in which the drafting clerk records the opinion — apparently unsolicited — that the Tower's floor numbering, being non-sequential, is held in integrity by harmonic nodes rather than by any geodetic fact, and that a "Median Tone" must exist to balance the resonant anchors. The appendix was struck from the official recension but survives in circulation, and is generally credited with the later speculation of Hieronymous Selvage.

The Clerks of the Median Line have never formally adopted the Median Tone thesis. They have, however, twice commissioned acoustic surveys of individual floors — the first producing Report on the Acoustic Properties of Floor 331 — which critics within the Scriptorium of the Final Line regard as an admission by conduct.

A Concordance ruling of disputed date (given variously as Year 7119 and Year 7123 in the two recensions of Concordance 9) holds that "the Report's appendix binds no one, but may be cited by anyone." This has permitted both sides of the argument to proceed indefinitely.

Accession and shelfmark

Series 417 and 418 were accessioned under a single report number split across two shelves, an arrangement that has produced persistent confusion in interlibrary requests. Requests for "the Geodetic Report" return Series 417 by default. Readers requiring the eastern survey must specify Series 418 explicitly, and are advised that the two volumes are stored at opposite ends of the West Marginal Wing despite neither containing any material pertaining to that wing.

Visitors may consult both series on site. Removal from the reading floor is not permitted; the volumes are chained, and the chains are of a length that permits consultation by two readers but not by three.

Related holdings

See also Incident Report 88-Gamma, which concerns a survey line laid during the drafting of Series 418 that failed to terminate; Functionary of the Final Line, whose duties are defined by reference to the Report's second proposition; and Visceral Conduit, a later and unrelated survey whose authors nevertheless cite Series 417 twice, apparently by mistake.