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Implements #243 for the WGS84-pivot architecture established in #247, shipping datum-capable built-in projections and the public transformation helper for user-defined datum CRSs. Phase 1 - Datum metadata model: - public Ellipsoid (wgs84/grs80/clarke1866/airy1830 with derived eccentricities and axes) and public Datum (wgs84/etrs89/nad27/ osgb1936) - Projection.datum accessor resolved from the definition; ProjectionDefinition gains a datum requirement (default WGS84); CustomProjection gains a datum field. Phase 2 - verified math infrastructure: - public HelmertTransformation: 7-parameter Helmert with position vector convention, exact rotation matrices, geocentric legs on arbitrary source ellipsoids, both directions with explicit result projection labeling. Well-known sets for NAD27 ("NAD27 to WGS 84 (4)", ~10 m) and OSGB36 ("OSGB 1936 to WGS 84 (6)", EPSG:1314, ~2 m) sourced from the EPSG registry and verified against pyproj/PROJ reference values. - internal TransverseMercatorMath(ellipsoid:) generalizing the Snyder TM formulas with a latitude-of-origin; UtmDefinition delegates to it (guarded by the existing UTM known-value tests, behavior provably unchanged). Phase 3 - built-in datum CRSs: - EPSG:4267 NAD27 geodetic, EPSG:4277 OSGB36 geodetic, EPSG:4258 ETRS89 geodetic (identity shift; OS labels ETRS89 data as WGS84 at meter accuracy) and EPSG:27700 British National Grid (helmert + TM on Airy 1830: lat0 49, lon0 -2, k 0.9996012717, FE 400000, FN -100000) - all registered in the registry seed with WKT matcher fragments (BNG probes before the generic Mercator fragments), datum metadata, test-appropriate unbounded extents (BNG keeps its southwest negative-northing region valid). Fixtures: The reference values were computed independently with pyproj/PROJ before porting, at 9-decimal precision; round-trip errors measured at ~3.3e-9 deg (NAD27) and ~1.4e-8 deg (OSGB36). Tests (+12): datum metadata, Helmert both directions + round trips, per-CRS reference fixtures, route-pivot round trips, British National Grid values with the southwest sector, algorithm smoke tests, WKB round trips and WKT matching for the new SRIDs. ETRS89-bearing WKT strings now match EPSG:4258 instead of returning nil. Also extends the README Projections section with the datum CRSs, the accuracy tier note and attribution, and links the grid shift support issue (#248).
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Summary
Implements #243 for the WGS84-pivot architecture from #247. Ships datum-capable built-in CRSs and the public transformation helper for user-defined datum CRSs. Grid shift support (NADCON/OSTN15/NTv2, sub-meter accuracy) is deliberately filed as a separate issue (#248) per the review decision — load-don't-bundle, per-file license review.
Phase 1 — Datum metadata (additive)
public struct Ellipsoid(wgs84,grs80,clarke1866,airy1830, EPSG-registr υalues + derived axes/eccentricities)public struct Datum(wgs84,etrs89,nad27,osgb1936)Projection.datum,ProjectionDefinition.datum(default WGS84),CustomProjection.datumfieldPhase 2 — Verified math infrastructure
public struct HelmertTransformation— 7-parameter helmert (position vector convention, exact rotation matrices, geocentric legs on arbitrary source ellipsoids) with both directions and explicit result labeling: well-known sets for NAD27 ("NAD27 to WGS 84 (4)", ~10 m) and OSGB36 ("OSGB 1936 to WGS 84 (6)", EPSG:1314, ~2 m) pulled from the EPSG registryTransverseMercatorMath(ellipsoid:)— internal generalization of the Snyder TM formulas with a latitude-of-origin;UtmDefinitionnow delegates to it (day-1 UTM suite proves the WGS84 math byte-equivalent)Phase 3 — Built-in datum CRSs
.prjmentionsTransverse_Mercator, which would otherwise match 3395 — same ordering rule as Identify UTM zones in WKT strings via regex #244's Pseudo-Mercator)validExtent: nil) — the SW sector carries negative northings (FN = −100 km), which a nominal sector box would reject/clamp away; pinned by testsVerification discipline (pyproj ground truth first)
Reference values were computed independently with pyproj/PROJ before porting, at 9-decimal precision (kept
2>/dev/null-suppressed warnings of the used epsg codes are documented in the test doc comments):Fixture points include in-domain N. America and UK points, a south-west BNG case (northing 14_401.7), and the geographic/+height round trips.
Changes
New files:
Datum.swift,HelmertTransformation.swift,TransverseMercatorMath.swift, the four CRS definitions;datumdrinks threaded throughProjection/definitions/CustomProjection; UTM delegates to the TM math.Projectionexposes the new static constants.Testing (+12)
DatumTests,HelmertTransformationTests,DatumCrsTests(reference fixtures, round trips via the WGS84 pivot incl. 4267→27700, algorithm smoke tests, WKB round trips with embedded SRIDs, WKT matching).GEOGCS["ETRS89"]now matches EPSG:4258 (previously nil).Full suite: 2500 tests in 183 suites, zero warnings.