Deferred: normative chainage for line groups (v1.1 candidate) #10

Status: DEFERRED — v1.1 candidate. Do not implement for the v1.0 freeze. This issue records the decision and context.

Decision (July 2026 community review)

Chainage (along-line distance) is not a normative v1.0 column of the groups table (#1). A normative field may be considered in v1.1 if real bundles demonstrate a need.

What v1.0 already allows (no spec change needed)

• Derived chainage: when every relevant line member has a sequence, consumers may derive an along-line coordinate as cumulative straight-line distance through station positions in sequence order. TX and RX chainages are derived independently because their sequence spaces are independent.
• Producer-supplied chainage: a designed or surveyed chainage (e.g. stake distance along a planned path) may be stored in ext_chainage_m. Such a value is provenance and may differ from chainage derived from as-built positions.

Open input needed before promoting to normative

• Scott MacInnes (Zonge): does delivered chainage matter to their tooling, or does the membership model plus consumer-side derivation cover Zonge line workflows? (Question queued as part of the v1.0 release follow-ups.)
• Evidence from real bundles using ext_chainage_m.

If promoted in v1.1, decide

• Column name and unit (chainage_m?), placement (groups table membership rows vs. elsewhere);
• semantics vs. derived chainage (designed vs. as-built; datum/zero point; behavior on gaps in sequence);
• validation rules (monotonicity along a line? consistency with sequence ordering?).