The ISCWSA Extended-Course-Length Error as an Own-Only Angle Error: Reconciling the Position-Direct Form with the Standard Measurement-Error Architecture (Monte-Carlo-Consistent; a One-Field OSDU Schema Change)

The ISCWSA error model propagates survey position uncertainty from per-station inclination/azimuth/depth measurement errors through the minimum-curvature geometry (the balanced-tangential weighting functions). The extended-course-length (XCL) family captures a real survey-frequency-dependent model error: the true slide-rotate wellpath deviates from the assumed minimum-curvature arc between stations. XCL began, and remained for its entire development, as inclination and azimuth weighting-function terms (proposed by Brooks at the 16th ISCWSA meeting in 2002; formalised by Codling, SPE-187249-MS, 2017). In 2018 the committee, citing unspecified issues with the angle-weighting terms, recast XCL as a position-direct term, deliberately outside the standard weighting-function machinery (the geometry Jacobians are not applied; Rev5 Definition 5.2.1). That resolution solved the implementation problems but left the measurement-error architecture: XCL is no longer a sampleable measurement perturbation and no longer a self-contained OSDU error-model formula, forcing every generic OSDU/OWSG engine to carry a bespoke position-direct evaluation path (a named weight function, a cross-station recurrence, and NEV-direct singularity branches) instead of the standard pointwise formula. This paper shows the two forms are reconcilable: the angle-error form propagated own-only (attributed to the preceding interval, with no forward coupling) reproduces the position-direct covariance to machine precision on the ISCWSA reference wells, while remaining a measurement-space term inside the architecture. Written that way, XCL becomes an own-only measurement-space error that is sampleable and unified with the other terms, Monte-Carlo-consistent, and expressible as a standard inclination/azimuth weighting formula plus one generic own_only flag. The recast converges with ISCWSA's own lateral term (XCLL) at vertical and with its true-survey-point interpolation rule. The proposal to the committee is correspondingly narrow: a single optional boolean own_only field in the OSDU/OWSG error_term.json schema, in place of the bespoke position-direct evaluation path. All figures are generated by reproducible scripts against welleng's own error model; a reference validation workbook is provided.