For the majority of England and Wales, the errors caused by the tropospheric effects and height variations between OS Net sites and your Network RTK rover position are generally well modelled by Network RTK providers (SmartNet). However, where these height differences increase (e.g. Snowdonia, Lake District and Scottish highlands) as illustrated below, it is recommended that the window averaging procedures as for surveying at the limits of the network and improving solution robustness be adopted to reduce height error. Note that it is possible to be significantly below the nearby OS Net base stations.
Ocean tide loading (OTL) is the time-varying displacement of the Earth’s surface due to the weight of the ocean tides. It can reach ±60 mm in height and ±20 mm in plan in the South-West Peninsula and Western Isles, although it is typically less than half of this magnitude. Instantaneous differences in OTL between a rover and base station can cause errors in the measured coordinates. Such errors will usually be greatest when the rover to base station separation distance is large, as OTL effects tend to cancel out over short baselines (few tens of km).
Mean height difference from nearest four OS Net base stations (April 2012):
The use of Network RTK reduces OTL error to the current system noise level throughout the majority of mainland Britain. In areas where OTL may be problematic, its effect can be almost completely removed by taking the mean of two sets of coordinates collected with 6 – 6½ hour separation.
Source: “Guidance Notes for GNSS Network RTK Surveying in Great Britain”, Issue 2, April 2012, Page 6-7, The Survey Association (TSA).