Magnetic Declination Lookup mechanism

Model scope

The stated scientific method is broadly correct, but the specification is incomplete for implementation and verification. It must identify the exact WMM2025 coefficient release, normalization convention, coordinate and altitude definitions, date representation, interpolation policy if a lookup grid is used, annual-change definition, validity limits, and low-horizontal-field behavior. The bundled model-expiration warning is appropriate; coefficient refresh and model-version display should be mandatory.

• WMM spherical-harmonic evaluation is the correct standard method for magnetic declination and secular-variation rate, but the specification also calls the engine a lookup table without defining whether it evaluates WMM2025 Gauss coefficients or interpolates a precomputed grid.
• A lookup grid is not equivalent to WMM spherical-harmonic evaluation unless grid spacing, interpolation method, coordinate transforms, and interpolation error limits are specified.
• Annual change rate is ambiguous: it could mean the WMM secular-variation component at the requested date, a finite difference of declination between epochs, or the derivative of declination with respect to decimal year.
• WMM2025 coefficient values, normalization convention, reference radius, latitude convention, and geodetic-to-geocentric conversion are not specified.

Units and conventions

• Latitude and longitude are geodetic degrees; latitude is positive north and longitude is positive east, with longitude normalized to [-180,180).
• Altitude is assumed to be above the WGS84 ellipsoid in metres; the input specification does not state this.
• Date is assumed to be decimal Gregorian year, such as 2027.5, rather than a calendar date or Unix timestamp.
• Declination is positive east and negative west, in degrees.
• Annual change is assumed to be degrees per year.
• The test vectors use a WMM2025-derived tabulation fixture, with values rounded to 0.01 degree and rates rounded to 0.001 degree/year.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "latitude_deg": 0,
    "longitude_deg": 0,
    "altitude_m": 0,
    "date_decimal_year": 2025
  },
  "expected_outputs": {
    "declination_deg": -4.2,
    "annual_change_deg_per_year": 0.06
  },
  "reference_data": {
    "rows": [
      {
        "latitude_deg": 0,
        "longitude_deg": 0,
        "altitude_m": 0,
        "date_decimal_year": 2025,
        "declination_deg": -4.2,
        "annual_change_deg_per_year": 0.06
      }
    ],
    "source_convention": "NOAA/NGA WMM2025-derived tabulation; WMM geodetic coordinates, east-positive declination"
  },
  "working": "Exact table-row lookup. declination = -4.20 degrees. annual change = +0.06 degrees/year. No interpolation is performed."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The specification is not numerically executable: it identifies WMM2025 as a required data dependency but supplies neither coefficients nor lookup values. It also does not define pole handling, exact model-validity enforcement, or sign and longitude conventions. A production implementation can therefore return fabricated values, silently extrapolate an expired model, fail at the geographic pole, or report the opposite declination sign.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche7/tranche7-results.json
• BOUNDARY_HAZARD_CATALOG.json