In order to be properly geo-referenced, statistical datasets require to be associated with the reference system used for geographic coordinates. This system is known as Spatial Reference System (SRS), sometimes also referred as Coordinate Reference System (CRS).

Each SRS is defined by a unique numerical Spatial Reference Identifier (SRID) but it is very common to find it named EPSG code or EPSG authority code in reference to the EPSG working group (European Petroleum Survey Group) that first established the registry of spatial reference systems worldwide (Spatial Reference).

The EPSG code is a character string compound by the EPSG prefix and the SRID. For example, the most common SRS used worldwide is the one used by the Global Positioning System (GPS). This spatial reference system is known as World Geodetic System 1984 (abbreviated WGS84). Its EPSG authority code is EPSG:4326 (SRID is 4326). Other registries than EPSG exist and are used for different purposes, such as the ESRI registry. Example of SRS in ESRI registry: ESRI:54012 (Eckert IV projection used for area calculation).

Another SRS notation, endorsed by the Open Geospatial Consortium (OGC), consists in using a Unique Resource Name (URN). This notation is highly recommended when possible. For EPSG:4326, this notation is: urn:x-ogc:def:crs:epsg::4326.

There is a large set of Spatial Reference Systems that were created for different purposes including positioning, spatial calculations or visualization.

SRS for data production, exchange and dissemination of datasets
Whatever the domain, and especially in the context of fisheries, geo-referenced datasets must be accompanied by the corresponding spatial reference system, specified by its EPSG code. The use of WGS84 (EPSG:4326) is recommended for the production/exchange/dissemination of datasets of regional or global extent.

SRS for areal calculation
When dealing with areal calculation, it is necessary to switch to a spatial reference system based preferably on metric units, and designed to conserve area proportions around the world, namely an equal areal projection. The SRS recommended for areal calculation is Eckert IV (ESRI:54012), or any other equal area projection system. Note that WGS84 is by nature a non-uniform, non-equal areal system, and must not be used for areal calculation.

SRS for visualization
The use of WGS84 (EPSG:4326) should be considered by default for any visualization of geo-referenced datasets. The use of Mercator projection system is not recommended. When distances and area proportions are important criteria for the visualization considered, an equal areal SRS should be also considered for visualization (see above).

European Petroleum Survey Group (EPSG).EPSG Geodetic Parameter Dataset . [Cited 1 November 2020]. https://epsg.org/

European Petroleum Survey Group (EPSG). World Geodetic System 1984 (WGS84/EPSG:4326)  [Cited 1 November 2020]. https://epsg.org/crs_4326/WGS-84.html

Open Geospatial Consortium (OGC). [Cited 1 November 2020]. https://www.ogc.org/

Spatial Reference System. [Cited 1 November 2020]. https://en.wikipedia.org/wiki/Spatial_reference_system

Spatial Reference. ESRI Projection 54012: World Eckert IV (equal area projection system). [Cited 1 November 2020]. https://spatialreference.org/ref/esri/world-eckert-iv/