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Short answer
A coordinate needs its reference context. We record the CRS, datum realization, axis order, units, relevant coordinate epoch and height type before combining datasets. Assigning a CRS describes existing numbers; a transformation calculates coordinates in another reference. A familiar basemap or an MSL label is not enough to validate that process.
- Changing a layer's CRS label and transforming its coordinates are different operations.
- A coordinate epoch describes when coordinates apply; it is not necessarily a transformation's parameter-reference epoch.
- A height conversion needs compatible references and an identified model, not just a subtraction.
Who this is for: Surveyors, GIS analysts and engineering teams combining or accepting coordinate datasets.
What belongs in the reference record?
The same pair of numbers can mean different locations when units, axes or reference definitions change. We ask for the actual CRS definition and its area of use, not only a shorthand name entered in a drawing. Where the reference is dynamic, the coordinate epoch can matter as well.
Keep the provenance of control and any transformations with the exported data. This makes it possible to investigate an offset without guessing which processing step introduced it.
| Field | What to record | Common failure |
|---|---|---|
| Horizontal reference | CRS definition, datum realization, axes and units | Assuming every geographic coordinate is interchangeable |
| Time context | Coordinate epoch when relevant, observation date and model reference epoch | Treating all dates as the same metadata field |
| Vertical reference | Height type, vertical datum, units and model/version | Calling every elevation MSL |
| Transformation | Operation, parameters or grids, area of use and software/version | Choosing a different label until the map appears aligned |
Technical references: QGIS 3.44: working with projections; PROJ: geodetic transformation; PROJ: time-dependent transformations.
Assigning a CRS does not move the coordinates
If a correctly surveyed file arrives without reference metadata, identify its original reference from evidence. Assigning that CRS tells the GIS how to interpret its existing values. If you need a different output CRS, select and apply an appropriate transformation instead.
A project display CRS can make layers appear together through on-the-fly operations without rewriting the source data. That display is useful, but it does not prove that the selected operation is suitable or that an exported file carries the intended reference. Check the actual output values, metadata and control comparisons.
Technical references: QGIS 3.44: working with projections; PROJ: geodetic transformation.
Ellipsoidal and orthometric heights are not interchangeable
GNSS and levelling workflows may report different height types. With compatible references and the usual signed geoid-undulation convention, H = h - N: h is ellipsoidal height, H is orthometric height and N is geoid undulation. N may be positive or negative; subtract its signed value.
The equation alone does not make incompatible datums agree. Record the ellipsoid/reference frame, vertical datum, units, model version and applicable area. A global model does not automatically reproduce a project's local levelling datum. NOAA's linked height example explains the relationship in a historical US setting; its GEOID96 model is not a recommendation for an Indian project.
Technical references: NOAA NGS: GPS height and geoid relationship (historical US example).
When does the epoch become important?
An observation date, a coordinate epoch and a transformation's parameter-reference epoch describe different things. Where time-dependent reference changes are relevant, omitting the coordinate epoch can leave a transformation under-specified.
We would preserve the recorded epoch and document the operation used rather than assuming software will infer it. Available time-dependent operations and supporting grids depend on the implementation and version. For a static reference, record that choice; do not add an arbitrary epoch simply to fill a column.
Technical references: PROJ: time-dependent transformations; PROJ: geodetic transformation.
Accept the data with checks, not a visual overlay
A visible offset is a symptom. It may involve a reference, measurement, georeferencing or import problem. We keep these possibilities separate instead of altering coordinates until an image overlay looks convincing.
- Obtain source and target reference definitions, axes, units and applicable area.
- Identify height type and the vertical model, if any, with its version.
- Record relevant epochs, control origins and the full transformation operation.
- Transform a small test set and compare independently established points where available.
- Report residuals and agreed tolerances separately for horizontal position and height.
- Export a sample and reopen it in the receiving system before accepting the whole delivery.
Technical references: QGIS 3.44: working with projections; PROJ: geodetic transformation.
Common questions
Will assigning WGS 84 fix a misplaced layer?
Not by itself. First identify the reference in which the original values were recorded. An incorrect assignment changes their interpretation; a suitable transformation is a separate operation.
Can I convert every GNSS height to MSL with a global geoid?
No. Identify the height type, required vertical datum, compatible reference frame and suitable model. A global model does not automatically reproduce a local levelling reference.
Is the observation date the coordinate epoch?
They may coincide in a particular workflow, but they are different concepts. Preserve the actual metadata and distinguish the coordinate epoch from any model or transformation parameter-reference epoch.
Official references
Sources checked 1 November 2026. The linked publishers remain the authority for their specifications and services.
- QGIS 3.44: working with projections
Layer and project CRS behaviour, coordinate interpretation and transformation choices.
- PROJ: geodetic transformation
Coordinate-operation workflows, transformations and supporting reference information.
- PROJ: time-dependent transformations
Time-dependent operations, coordinate epochs and software-dependent capabilities.
- NOAA NGS: GPS height and geoid relationship (historical US example)
Height relationship and reference consistency; not a geoid-model recommendation for India.