I’m getting my head around all the different terms and concepts related to GIS in the UK. Next, there’s a summary of what I understand (mostly based onhttps://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf). I'm (by far) not an expert on this topic, so if you feel I got something wrong, or something doesn't feel right just post a comment and I'll review it (change it, dispute it, etc..)
Worldwide, the World Geodesic System 1984 (WSG84) is a Terrestrial Reference System (TRS or datum) which origin is the Geocentre and which axis are aligned with the equator and main meridian. The three most used realization of this TRF are the United States military ‘broadcast’ realisation; the International Terrestrial Reference Frame (ITRF) precise scientific realisation; and the European Terrestrial Reference Frame (ETRF) Europe-fixed realisation which is related to ETRS89.
The European Terrestrial Reference System (ETRS89) is a datum that was coincident with WSG84 in 1989 and has been slowly sliding because of the tectonic plate movement since then.
In the UK, OS Net is a modern TRF that realizes the ETRS89 datum. OS Net is the base for the Ordnance Survey coordinates system.
The National Grid is datum based on the Airy ellipsoid and originally realized by the OSGB36 TRF. The observation of this TRF where originally based on triangulations.
The Ordnance Datum Newlyn (ODN) is an uni-axial datum based on Newlyn tide gauge. Originally was realized by a number of benchmark measured based on the mean sea level.
ODN TRF and OSGB36 have been replaced by a 3D TRF based on OS Net. This is achieved by a transformation software comprised by OSTN02 and OSGM02 (for ETRS-OSGB36, ETRS89-Orthometric heigh respectively).
Finally, there are the flat projections, map projections like the transverse Mercator. In the case of the UK, the National Grid is flat transformation based on a 100km grid and easting-northing coordinates. Its true origin is 49N 2W, a point near Guernsey in the Channel Islands. 2W is essentially in the middle of a bounding box covering the UK, so that the distortion of scale is equal as you go east or west of this central line. Once the projection has been calculated, 100km is added to the northing and 400km subtracted from the easting, the 'false origin', giving you a point to the north west near the Scilly Isles.so all the coordinates are positive.
Finally the SRID codes are: ETRS89: 4258 OSGB36: 27700 WSG84: 4326
Worldwide, the World Geodesic System 1984 (WSG84) is a Terrestrial Reference System (TRS or datum) which origin is the Geocentre and which axis are aligned with the equator and main meridian. The three most used realization of this TRF are the United States military ‘broadcast’ realisation; the International Terrestrial Reference Frame (ITRF) precise scientific realisation; and the European Terrestrial Reference Frame (ETRF) Europe-fixed realisation which is related to ETRS89.
The European Terrestrial Reference System (ETRS89) is a datum that was coincident with WSG84 in 1989 and has been slowly sliding because of the tectonic plate movement since then.
In the UK, OS Net is a modern TRF that realizes the ETRS89 datum. OS Net is the base for the Ordnance Survey coordinates system.
The National Grid is datum based on the Airy ellipsoid and originally realized by the OSGB36 TRF. The observation of this TRF where originally based on triangulations.
The Ordnance Datum Newlyn (ODN) is an uni-axial datum based on Newlyn tide gauge. Originally was realized by a number of benchmark measured based on the mean sea level.
ODN TRF and OSGB36 have been replaced by a 3D TRF based on OS Net. This is achieved by a transformation software comprised by OSTN02 and OSGM02 (for ETRS-OSGB36, ETRS89-Orthometric heigh respectively).
Finally, there are the flat projections, map projections like the transverse Mercator. In the case of the UK, the National Grid is flat transformation based on a 100km grid and easting-northing coordinates. Its true origin is 49N 2W, a point near Guernsey in the Channel Islands. 2W is essentially in the middle of a bounding box covering the UK, so that the distortion of scale is equal as you go east or west of this central line. Once the projection has been calculated, 100km is added to the northing and 400km subtracted from the easting, the 'false origin', giving you a point to the north west near the Scilly Isles.so all the coordinates are positive.
Finally the SRID codes are: ETRS89: 4258 OSGB36: 27700 WSG84: 4326