geoid
Enable an agent to identify a geoid realization, assess its suitability as a height reference, and apply it only with compatible geodetic conventions and documented uncertainty.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
recalled by Codex without web access - no source was read
Researched by: Codex
Purpose and description
Enable an agent to identify a geoid realization, assess its suitability as a height reference, and apply it only with compatible geodetic conventions and documented uncertainty.
The geoid is an equipotential surface of Earth's gravity field, including centrifugal effects from rotation, chosen to approximate global mean sea level and continued beneath the continents.
It can be Classify a claimed geoid surface and identify its defining conventions.; Evaluate geoid undulation within documented coverage using a specified realization.; Convert compatible ellipsoidal heights to orthometric heights with an explicit error budget.; Compare realizations after reconciling reference systems and conventions.; Flag locations or applications where validation, coverage, or metadata are inadequate..
Distinguishing features
A geoid is selected by constant gravity potential W = W0; constant gravitational acceleration is not its defining test. NGS describes the sea-level relationship in its [geoid definition](https://www.ngs.noaa.gov/GEOID/geoid_def.html).
A reference ellipsoid is a geometric reference surface; geoid undulation describes the geoid's signed separation from a specified ellipsoid.
A quasigeoid is a different height reference surface and is not generally equipotential; its height anomaly must not be silently substituted for geoid undulation. See [NGS modeling calculations](https://www.ngs.noaa.gov/research/geopotential-datums/modeling-calculations.shtml).
Observed mean sea level helps motivate geoid selection but is not an exact, universally interchangeable realization of the geoid.
A product called a geoid model can incorporate fitting to an existing vertical datum and therefore need not represent a pure equipotential surface. See [NGS technical notes](https://www.ngs.noaa.gov/GEOID/tech2.html).
Scope
+ Definition of the reference equipotential surface and selection of its potential value W0
+ Relationship to mean sea level, reference ellipsoids, and orthometric heights
+ Gravimetric realizations and identification of datum-fitted hybrid products
+ Reference frame, permanent-tide convention, epoch, and geographic applicability
+ Geoid undulation, validation evidence, uncertainty, and permitted height conversions
- Complete models of Earth's gravity field and interior mass distribution
- Reference ellipsoid geometry and terrestrial reference-frame maintenance
- Quasigeoid construction and normal-height systems as independent subjects
- Ocean circulation, sea-surface forecasting, and tidal-datum determination
- Terrain elevation models and land-surface morphology
- Survey instrument operation and national vertical-datum administration
Characteristics
- Reference potential
- W0 in m²/s², with adopting authority and convention Identifies which member of the family of equipotential surfaces is intended.
- Realization class
- Gravimetric geoid approximation; hybrid datum-fitted surface; insufficiently documented Determines whether values approximate a physical equipotential surface or support a particular datum conversion.
- Geoid undulation
- N in metres at a specified position, relative to a named ellipsoid and sign convention Provides the separation needed for compatible ellipsoidal-to-orthometric height conversion.
- Geodetic reference compatibility
- Named ellipsoid, terrestrial reference frame, coordinate epoch, and target height datum Prevents combining coordinates and corrections that refer to different systems.
- Permanent-tide convention
- Mean-tide; zero-tide; tide-free; unspecified Unreconciled tidal conventions can introduce systematic discrepancies.
- Spatial support
- Coverage boundary and grid spacing or spherical-harmonic degree and order Describes where the realization can be evaluated and the spatial detail represented.
- Temporal applicability
- Static at documented epoch; time-dependent with documented corrections; unspecified Determines whether temporal changes need treatment for the intended use.
- Validation uncertainty
- Metres, with statistic, confidence interpretation, region, and validation method Supports a defensible decision about whether the realization meets a height-accuracy requirement.
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 6 bundles · 11 layers · 17 findings · 28 questions.
Equipotential identity Establishes which geoid is intended and how its sea-level interpretation is justified.
An agent must distinguish a defined physical reference surface from an ambiguous use of the word geoid.
Potential selection
Records the criterion and authority selecting the reference surface.
Reference potential choice
Record W0, its provenance, and whether the definition selects it conventionally or through a stated sea-level fit; do not assume all geoid definitions choose the same surface.
- What W0 and definition identify this surface, and who adopted them? definition
- Which source explains how the potential value was selected? provenance
Sea-level interpretation
Separates the reference surface from observed ocean and tidal surfaces.
Mean sea-level relationship
Identify whether mean sea level is used as a conceptual motivation, observational fitting target, or datum reference; NGS gives a least-squares global sea-level fit as its stated definition. See [NGS geoid definition](https://www.ngs.noaa.gov/GEOID/geoid_def.html).
- What role does mean sea level play in this definition or realization? definition
- If sea-level observations were used, what averaging period and treatment of oceanographic departures were documented? measurement
Height surface boundaries Identifies neighboring reference surfaces and the height quantities they support.
Confusing an ellipsoid, geoid, quasigeoid, or datum correction can produce a plausible but incorrect elevation.
Ellipsoid separation
Defines the geometric reference for geoid undulation.
Undulation reference
A reported N requires a named ellipsoid, position convention, and sign; an undulation value alone does not identify an absolute height reference.
- Relative to which ellipsoid and coordinates is N evaluated? measurement
- How is positive separation defined, and is that convention preserved in the consuming software? action
Quasigeoid and datum separation
Distinguishes physical geoid approximations from other height correction surfaces.
Correction surface classification
Determine whether supplied values are geoid undulations, quasigeoid height anomalies, or hybrid datum corrections; hybrid fitting can break the equipotential interpretation. See [NGS technical notes](https://www.ngs.noaa.gov/GEOID/tech2.html).
- Does the product return geoid undulation, height anomaly, or a correction fitted to a named vertical datum? boundary
- What documentation establishes the corresponding orthometric or normal height interpretation? provenance
Realization evidence Records how a numerical approximation was constructed and where its values are supported.
The ideal surface cannot be assessed through a product name or grid resolution alone.
Gravity and fitting inputs
Traces the observations and assumptions behind the realization.
Construction lineage
Record gravity observations, satellite contributions, terrain treatment, and any GNSS/leveling fit actually documented for the selected release.
- Which observations and construction methods support this release? provenance
- Which terrain, density, or datum-fitting assumptions materially affect its values? measurement
Numerical support
Makes the evaluation domain and representation explicit.
Coverage and evaluation
Record geographic masks, grid or harmonic representation, interpolation rules, and missing-data behavior; numerical spacing does not establish accuracy.
- Where is evaluation supported, including coastlines, islands, and grid edges? boundary
- Which representation and evaluation method must an agent use to reproduce a value? action
Reference conventions Captures the reference-system and temporal conditions needed to combine geoid values with coordinates.
Convention mismatches can masquerade as geoid error or real elevation change.
Frame and tide alignment
Checks compatibility of the realization, input coordinates, and desired heights.
Compatibility contract
Record the supported frame, ellipsoid, tide convention, and output datum. Product compatibility can be highly specific, as illustrated by [GEOID18 technical details](https://ngs.noaa.gov/GEOID/GEOID18/geoid18_tech_details.shtml).
- Which frame, ellipsoid, tide convention, and height datum does this realization support? definition
- What documented transformations are required before these input coordinates are compatible? action
Epoch and release
Separates software release identity from the physical epoch represented.
Temporal reference
Record release identifier, reference epoch, and any prescribed temporal corrections; publication date alone does not define temporal applicability.
- Which epoch does the realization represent, and what temporal behavior is documented? measurement
- Does the intended observation epoch require a correction or a different realization? action
Height use and confidence Connects geoid evaluation to justified height conversion and validation.
An agent needs evidence that the resulting height is meaningful and accurate enough for its intended use.
Orthometric conversion
Controls how ellipsoidal height and geoid undulation produce an orthometric-height estimate.
Conversion conditions
Use H ≈ h − N only with compatible quantities and an accuracy-appropriate treatment of geometry and datum effects; NGS illustrates this relation in its [geoid survey overview](https://www.ngs.noaa.gov/GEOID/GSVS14/GSVS14SurveyOverview.pdf).
- Are h, N, and the intended H compatible in frame, datum, tide convention, and epoch? boundary
- What conversion method and propagated uncertainty meet the application's height tolerance? action
Independent validation
Assesses local performance and separates validation evidence from calibration evidence.
Regional error evidence
Record independent checks, regional residuals, systematic offsets, and correlated errors; distinguish uncertainty in the realization from uncertainty in coordinates and comparison heights.
- Which checks are independent of the observations used to construct or fit the realization? provenance
- What local bias, dispersion, and spatial correlation are supported by those checks? measurement
- Does the combined uncertainty permit the intended use, or require another reference or additional surveying? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Check these first
Recalled without web access and unsourced; every item is a lead to verify.
- The listed kinds distinguish modelling approaches, not separate fundamental types of equipotential surface.
- Precise applications must specify the reference potential, reference ellipsoid and permanent-tide convention.
- This is recalled knowledge; no sources were consulted, and numerical extrema should be checked against the selected model.
- Which of these check these first hold for the sense of geoid this model covers, and on what evidence? provenance
Kinds and varieties
Recalled without web access and unsourced; every item is a lead to verify.
- Gravimetric geoid models
- Hybrid geoid models fitted to terrestrial height control
- Which of these kinds and varieties hold for the sense of geoid this model covers, and on what evidence? provenance
Real-world use
Recalled without web access and unsourced; every item is a lead to verify.
- Converting satellite-derived ellipsoidal heights into approximate orthometric heights.
- Providing a reference surface for physical height systems used in surveying and mapping.
- Separating ocean surface topography from the gravity reference surface in oceanography.
- Studying Earth's gravity field and internal mass distribution.
- Which of these real-world use hold for the sense of geoid this model covers, and on what evidence? provenance
Typical measurements
Recalled without web access and unsourced; every item is a lead to verify.
- Geoid undulation, the separation of the geoid from a specified reference ellipsoid - Approximately −110 to +90 globally relative to commonly used geocentric reference ellipsoids; exact extrema depend on the model and ellipsoid. - m
- Which of these typical measurements hold for the sense of geoid this model covers, and on what evidence? provenance
Failure modes and hazards
Recalled without web access and unsourced; every item is a lead to verify.
- Treating observed mean sea level as exactly coincident with the geoid despite currents, atmospheric pressure and other oceanographic effects.
- Mixing ellipsoidal and orthometric heights, causing potentially large elevation errors.
- Using a geoid model with incompatible reference-frame, vertical-datum or permanent-tide conventions.
- Confusing geoid undulation with quasigeoid height anomaly.
- Assuming a regional model retains its accuracy outside its coverage or in areas with sparse gravity observations.
- Which of these failure modes and hazards hold for the sense of geoid this model covers, and on what evidence? provenance
Regional variation
Recalled without web access and unsourced; every item is a lead to verify.
- National vertical datums can have different offsets from a common global geoid reference.
- Some countries use orthometric heights associated with the geoid; others use normal heights associated with the quasigeoid.
- Regional models differ in resolution and accuracy because gravity observations and terrestrial height control vary geographically.
- Which of these regional variation hold for the sense of geoid this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Recalled without web access and unsourced; every item is a lead to verify.
- Reference ellipsoid - A smooth mathematical surface defined geometrically; the geoid is defined by gravity potential and has irregular departures from that ellipsoid.
- Mean sea surface - An average of the actual ocean surface, which can depart from the geoid because of ocean dynamics and other effects.
- Quasigeoid - A reference surface associated with normal heights that is generally not an equipotential surface.
- Topographic surface - The physical land surface includes mountains and valleys; the geoid passes beneath or above terrain independently of that terrain's elevation.
- Geoid model - A computed representation or estimate of the geoid, rather than the physical reference surface itself.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of geoid this model covers, and on what evidence? provenance
What the second pass must settle
- Which defining authority and W0 convention should the completed publication adopt or explicitly compare?
- Which representative global and regional realizations should be researched to establish practical recognition criteria?
- What documented criteria reliably distinguish physical geoid approximations from hybrid products marketed as geoid models?
- For which applications must time dependence and permanent-tide differences be modeled explicitly?
- What independent regional validation evidence supports usable uncertainty thresholds, especially where terrestrial gravity coverage is sparse?