Coordinating Working Party on Fishery Statistics (CWP)

Geographic coordinates

In the context of fisheries data, geographic coordinates will consist of pairs of (x,y) numerical values handled in the spatial reference system considered (refer spatial reference system). If needed, these pairs may be extended to include a third dimension with a z coordinate to handle the depth.

In order to handle and exchange properly longitude/latitude pairs of geographic coordinates, the CWP recommends the following three methods:

  • Method 1: Treat latitude and longitude as numerical values enumerated in decimal degrees (DD)
  • Method 2: Treat latitude and longitude separately (e.g. in a table: two separate columns), in that way any ambiguity is avoided related to the SRS axis ordering (latitude/longitude or longitude/latitude order) and the identification of what coordinate is longitude and latitude. Refer note below on axis ordering latitude/longitude vs. longitude/latitude
  • Method 3: As alternative to method 2, treat latitude and longitude together using the OGC standard Well-Known-Text (WKT) format (refer below for details).

Note on axis ordering latitude/longitude vs. longitude/latitude

In the initial definition of the EPSG 4326 reference system (as defined by EPSG working group), coordinates should be specified as (latitude/longitude) pairs, instead of (longitude/latitude). Practically, backed by legal OGC standard specifications (including the WKT format definition specified by the OGC Simple Feature Access), the use of the (longitude/latitude) axis ordering was generalized. From 2005, with the ISO 19128:2005 specification EPSG:4326 refers to WGS84 geographic latitude first followed by the longitude, although most geographic information systems currently rely on OGC standards which supposes a longitude/latitude axis ordering.

Geographic coordinates WKT format

In Method 3 above, the recommended format for handling coordinates is the OGC Well-Known-Text format (WKT). Whatever the geometric shape, WKT provides a simple string representation of the geometry. In that way, a single column is enough to handle the dataset geo-reference. When using the WKT format with a geographic coordinate reference system such as EPSG:4326, it is mandatory to follow the axis order specified by the OGC standards, which is: longitude first followed by latitude.

It is also possible to handle both geometry WKT and SRS (identified by its SRID) in a unique format, named EWKT (for Extended WKT), by prepending the SRID to the WKT geometry.

Example: SRID=4326;POINT(-44.3 60.1)
In this example: -44.3 is the longitude, 60.1 is the latitude.

The EWKT format is not an OGC international standard but a format introduced with the PostGIS software. However, this format is increasingly used for geo-referencing and is recommendable by CWP as substitute for WKT, and a good practice to make sure the SRS is well-defined.

Use of geographic classification/coding systems

Alternatively,  a dataset may be geo-referenced using a reference geographic classification and coding system (e.g. FAO Major area codes, grid cell codes).

Use of Degrees-Minutes-Seconds (DMS)

In the case of handling geographic coordinates in official reports and publications, the geographic coordinates can be enumerated using the Degrees-Minutes-Seconds (DMS) notation. This notation is generally used in the legal context where the DMS appears to be more precise than the DD notation, and convenient for describing textually the geographic boundaries. However, in order to facilitate the digital exploitation of the geographic coordinates, reports handling geographic coordinates in DMS notation should be complemented by digital files providing geographic coordinates in decimal degrees (DD), and following the recommended geographic data formats above.

Resources for geographic coordinates

Open Geospatial Consortium (OGC). OGC Standards. [Cited 1 November 2020].

Well-known text (WKT) representation of geometry. [Cited 1 November 2020].