coordinates

Reference Frames

Continental drift causes the continents to move around and change shape slowly. In the longer term it builds them up and tears them down.

The UK is part of the Eurasian continent, which is moving north-east at about 2.5 centimetres per year. The line on the ground at Greenwich that marks the Prime Meridian is moving with the drift.

That didn't used to matter because most surveying equipment wasn't accurate enough to notice, but now that we have GNSS receivers accurate to two or three centimetres, it does matter. If you measure the position of anything fixed to the ground in the UK, then come back two years later and measure its position again, it will have moved by 5 centimetres.

For an organisation like the Ordnance Survey, responsible for making maps within one country, a convenient way to deal with this is to define positions on the ground relative to another position on the ground in the same country. The real positions will all move due to continental drift, but the maps will still be consistent within themselves. That leads to the idea of a reference frame. essentially a version of latitude and longitude with a Prime Meridian fixed in some agreed way, for example, fixed to objects on the ground.

WGS84

The World Geodetic system 1984 (WGS84) is fixed to the mean movement of the continents, so each continent moves against it constantly. In a world where the axis of spin, the magnetic field and everything on the land and on the sea bed is moving around, WGS84 is the nearest thing we have to a notion of where things really are.

My Trimble Catalyst GNSS receiver figures out its position in the WGS84 reference frame but I can set it to convert the results to ETRF89 or one of a range of other reference frames suitable for individual countries.

ITRF

The International Terrestrial Reference Frame is a close friend of WGS84 but calculated in a different way. The current version is ITRF2020. A quick Google search produces references like this , explaining that positions in ITRF2020 and WGS84 are almost the same.

As explained later, I use the Canadian Government's Precise Point Positioning service to figure out the position of my base station and it produces results within the ITRF framework. They recently switched from an older version to ITRF2020.

ETRF89

Within Europe it makes sense to use the European Terrestrial Reference Frame (ETRF89). This is fixed to GNSS ground stations in Europe and Asia and drifts with them. It's convenient for cartography in Europe because if you measure a position in this reference frame and come back a few years later to measure it again, it will not appear to have moved.

At the start of 1989, ETRF89 was set to coincide with ITRF, but continental drift constantly pulls the two apart. By the year 2000 they disagreed by 25 centimetres. To prevent ETRF89 getting too far removed from reality it has to be refactored every few years, producing different versions. The version ETRF2000 coincided with ITRF at the start of 2000. Since then there's been more drift and the current version ETRF2020 coincides with ITRF as it was at the start of 2020.

This means that when you record a position using ETRF89 you have to note which version of the framework you used. If you use ETRF2020 and come back thirty years later to find the same spot, you will either have to use ETRF2020 again or use whatever the latest version is, and correct the result. Getting that wrong could at least lead to you wasting money digging a hole in the wrong place. If you're trying to resolve a boundary dispute, a little detail like that could land you in serious trouble.

Other reference frames are defined that work well for cartography in particular parts of Europe.