WGS 84

WGS 84 is a three-dimensional global reference system developed by the US authorities for determining coordinates with the GPS system in real time.

If you do an absolute GPS measurement, i.e. without support from for example permanent reference stations, you will get coordinates in WGS 84. The uncertainty of such positions is usually around 10 meters, but depends to a large extent on which GNSS receiver is used and what the surrounding environment looks like.

Reference systems and relative positioning

If you need to determine positions with lower uncertainty, you must use so-called relative measurement and combine your measurement data with observations from geodetic markers on the ground, for example through one of Swepos's services. The position is expressed in the same reference system as the coordinates of the ground-based points. In Sweden the reference system SWEREF 99 is used.

Alternatively, so-called Precise Point Positioning (PPP) can be used, which is also based on a network of permanent reference stations for calculating accurate track and clock products. 

WGS 84 is a dynamic reference system

Just like ITRF, WGS 84 is a dynamic system consisting of both coordinates and velocities and can thus be expressed in any epoch. Since Europe moves by a few centimetres per year, this means that WGS 84 (approximately current epoch) and SWEREF 99 (plate tectonic epoch 1989.0) are slowly drifting apart.

The latest realisations of WGS 84 correspond at the centimetre level to the ITRF solution to which they are connected. WGS 84 is updated regularly, like ITRF, but not at the same time.

WGS 84 is based on the WGS 84 ellipsoid which is basically identical to the ellipsoid GRS 1980. The difference is only 0.1 millimeters in half the minor axis.

Relation to SWEREF 99

In Sweden there is no regular realisation of WGS 84. Instead SWEREF 99 is used. WGS 84 and SWEREF 99 currently (2021) differ by 7‑8 dm and the difference increases by a few centimeters per year.For many GPS applications in Sweden, WGS 84 and SWEREF 99 can be considered the same, because in practice it is difficult to measure with lower uncertainty in WGS 84.

Transformation of WGS 84 coordinates

Since there is no regular realisation of WGS 84 in Sweden, there are also no transformation connections between WGS 84 and SWEREF 99.

For transformation between latitude/longitude in WGS 84 (or SWEREF 99) and plane coordinates in SWEREF 99 TM you can use Simple coordinate transformation.

If you have coordinates that are stated to be in WGS 84, but which have lower position uncertainty, it is usually in practice another globally adapted reference system, such as an ITRF solution. There are high quality transformation parameter sets between ITRF2014 and SWEREF 99. Note that the epoch of the coordinates must be specified because WGS 84/ITRFyyyy are dynamic systems.

About WGS 84 realisations

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