Coordinate conversion vs transformation: CRS, datum, frame and epoch
A coordinate conversion changes representation within a datum, such as geographic coordinates to a map projection. A transformation relates different datums or reference frames and may require parameters, grids and epochs. An EPSG code identifies a CRS; it does not by itself specify every operation or accuracy needed for a survey.
Name the representation and the reference
| Term | Meaning | What to record |
|---|---|---|
| Geographic coordinates | Angular latitude and longitude, optionally ellipsoidal height | Axis order, angle units and whether height is included |
| Projected coordinates | Planar easting and northing in a projection | Projection, zone, units, false origin and base geodetic CRS |
| Geocentric / ECEF coordinates | Cartesian X, Y and Z about the Earth’s centre | Frame realisation, units and coordinate epoch |
| Coordinate epoch | The time at which coordinates of a moving point apply | A date or decimal year and the model used to change epochs |
Sources: PROJ: coordinate reference systems · PROJ: coordinate operations
EPSG:4326, EPSG:4979 and EPSG:4978 are different representations
EPSG:4326 describes a two-dimensional WGS 84 geographic CRS, EPSG:4979 includes ellipsoidal height, and EPSG:4978 is geocentric. A two-dimensional latitude/longitude pair contains no measured height. WGS 84 also has specific realisations: a generic WGS 84 label is insufficient when a task requires precise frame and epoch handling.
- Follow the interface’s declared axis order. Authority definitions and a web API’s longitude/latitude convention may differ.
- Do not put easting/northing into a latitude/longitude field because both accept decimal numbers.
- Preserve input units and decimal precision, but do not interpret additional decimal places as additional accuracy.
Sources: PROJ: coordinate reference systems
Height conversion needs a vertical reference
Ellipsoidal height is measured relative to an ellipsoid. Orthometric height is referred to a gravity-related vertical reference. In the common relation H = h − N, h is ellipsoidal height and N is geoid undulation; using it requires a geoid model and compatible reference conventions. Applying a horizontal projection does not perform this vertical operation.
- Record whether the deliverable uses ellipsoidal, orthometric or another defined height system.
- Check whether a required geoid grid is installed and valid for the location.
- Do not silently replace a missing measured height with zero for a three-dimensional datum transformation.
Sources: PROJ: coordinate operations
A seven-parameter transformation needs a convention
A three-dimensional Helmert transformation uses translations, rotations and scale. The position-vector and coordinate-frame conventions differ in the signs of the rotation terms. Parameters also have units, a direction and an area of intended use. A time-dependent operation may additionally use rates and a reference epoch; reversing a transformation is not the same as relabelling its source and target.
Sources: PROJ: Helmert transformation
Verify the result using independent control
- Record source and target CRS, frame realisations, coordinate epochs, axis order, units and height references.
- Save the chosen operation, parameter convention, grids and their versions.
- Check the operation’s area of use and stated accuracy against the project requirements.
- Transform a known control point that was not used to fit local parameters, then inspect residuals in the deliverable’s units.
- Use a round-trip calculation to detect implementation mistakes, but do not treat round-trip agreement as independent accuracy evidence.
Sources: PROJ: coordinate operations
Common questions
What is the difference between coordinate conversion and coordinate transformation?
A coordinate conversion changes the mathematical representation of coordinates within the exact same geodetic datum (e.g. from geodetic latitude/longitude to projected UTM easting/northing). A coordinate transformation relates coordinates between two different geodetic datums or reference frames (e.g. ITRF2014 to WGS84 or CGCS2000), typically requiring 3-parameter or 7-parameter Helmert translations, rotations, scale factors, and observation epochs.
Why is epoch important when transforming GNSS coordinates?
Tectonic plates move horizontally across the Earth at 2 to 10 cm per year. In dynamic reference frames like ITRF2020 or ITRF2014, station coordinates change continuously with time. Transforming a point to a static national datum (like CGCS2000 or NAD83) requires knowing the exact measurement epoch to apply plate motion models correctly.
References and methodology
Technical definitions follow the sources below. Worked examples are illustrative; product-specific thresholds are identified as PosFlow settings. Send a correction with the article URL and the relevant specification.