Projektionsepoche

Projektionsepoche, in geodesy and cartography, refers to the date or time at which the parameters of a map projection are defined as valid. The term combines Projektion (projection) and Epoche (epoch) and is commonly used in German-language literature to mark the temporal reference for projecting geographic coordinates onto a plane, especially when the underlying geodetic reference frame is time-dependent.

Purpose and use

The concept is important when the geodetic reference frame itself changes over time due to crustal motion, plate tectonics, or updates to reference realizations. In such cases, projected coordinates can drift if the epoch of the projection is not specified. By assigning a projection epoch, GIS users and geodetic analysts ensure that coordinates are interpreted consistently for a given date, preserving spatial accuracy.

Context and examples

In many systems, a projected coordinate reference system (CRS) is defined for a specific epoch, such as 2010.0, within a dynamic reference frame like ITRF or a country-specific frame linked to ETRS89. Static CRSs may omit an epoch, while time-dependent transforms rely on a defined projection epoch to reconcile changes in the underlying datum over time.

Technical notes

The epoch is metadata that accompanies projection parameters and transform models. It interacts with transformation algorithms (e.g., Helmert shifts) to maintain coherence between geographic coordinates and their planar projections across different times.

See also

Coordinate reference system, dynamic reference frame, epoch, ITRF, ETRS89, geodesy.