The apparent British sea slope is caused by systematic errors in the levelling-based vertical datum

Abstract

The spirit-levelling-based British vertical datum (Ordnance Datum Newlyn) implies a southnorth apparent slope in mean sea level of up to 53 mm deg -1 latitude, due to the datum falling on heading northwards. Although this apparent slope has been investigated since the 1960s, explanations of its origin have remained inconclusive. It has also been suggested that, rather than a slope, the British vertical datum includes a step of about 240 mm affecting all sites north of about 53 • N. In either case, the British vertical datum may be of limited use for any study requiring accurate heights or changes in heights, such as testing geoid models, groundwater and hydrocarbon extraction, the calibration and validation of satellite-based digital terrain models, and the unification of vertical datums internationally. Within the last decade, however, based on an apparent reduction in the slope to only −12 mm deg -1 latitude with respect to recent geoid models, it has been claimed that the British vertical datum does provide a physically meaningful surface for use in scientific applications. In this paper, we reinvestigate the presence of apparent south-north sea slopes around Britain and reported slopes in the vertical datum, using the EGM2008 global gravitational model, together with mean sea level and GPS data from British tide gauges, GPS ellipsoidal heights of 178 fundamental benchmarks across mainland Britain, and vertical deflection observations at 192 stations. We demonstrate a south-north slope in the British vertical datum of −(20-25) mm deg -1 latitude with respect to both mean sea level (corrected for the ocean's mean dynamic topography and the inverse barometer response to atmospheric pressure loading) and the EGM2008 quasigeoid model, while EGM2008 is shown to exhibit a negligible slope of (2 ± 4) mm deg -1 with respect to mean sea level. It is clear, therefore, that the slope can only arise from systematic errors in the levelling, although we are unable to isolate their exact origin. Using an offset detection method based on a penalized likelihood maximization using the Schwarz Information Criterion, we do not detect a step in the vertical datum affecting all sites north of 53 • N, but do identify regional distortions that we attribute to the inhomogeneity in both the levelling data used and the least squares adjustment procedures used to realize the datum. We conclude that the British vertical datum remains unsuitable for scientific purposes.

A Century of Sea Level Measurements at Newlyn, Southwest England

“…Values of the geoid, obtained from advanced space techniques, can be subtracted from the MSL values to give the Mean Dynamic Topography, which can in turn be compared to ocean estimates of spatial changes in sea level (e.g., Woodworth et al 2012). Such studies for the UK have confirmed that there is very little real MSL difference from north to south (e.g., see early studies of this topic by Ashkenazi et al 1994), and that the apparent latitudinal variation in MSL, relative to ODN, had been due to the ODN levelling errors (Penna et al 2013). Similar studies in Australia and North America have also resolved their anomalous historical sea level slopes (Featherstone and Filmer 2012;Higginson et al 2015).…”

“…However, oceanographers knew that there was no such current (or at least not large enough). An alternative explanation for the anomalous slope instead suggested systematic errors in the levelling (Hipkin et al 2004;Penna et al 2013).…”

Regional gravity field recovery using the GOCE gravity gradient tensor and heterogeneous gravimetry and altimetry data

“…Regarding shipborne data, the reference models derived from DTU13, i.e., mean sea surface (MSS) and mean dynamic topography (MDT), are used to extract the ellipsoidal and orthometric/normal height information. With respect to terrestrial data in the UK, the effects of the systematic errors are in its leveling system, where both the south‐north slope and regional distortions exist, and no valid transformation parameters can be used for vertical datum unification [ Penna et al ., ]. However, as EGM2008 has no slope when compared with the local corrected mean sea level values [ Penna et al ., ], we use it as the reference model for data reduction.…”

“…With respect to terrestrial data in the UK, the effects of the systematic errors are in its leveling system, where both the south‐north slope and regional distortions exist, and no valid transformation parameters can be used for vertical datum unification [ Penna et al ., ]. However, as EGM2008 has no slope when compared with the local corrected mean sea level values [ Penna et al ., ], we use it as the reference model for data reduction. This procedure is reasonable, as the accuracy of EGM2008 is at the decimeter level in Europe [ Pavlis et al ., ], and the associated errors for data reduction do not exceed 0.308 mGal, which are within the noise level of the data.…”

Assessment of EGM2008 over Britain using vertical deflections, and problems with historical data

“…[33]), with [3] advising that the ODN should not be used for scientific purposes. [28] have isolated this to systematic errors in the levelling that generate a south-north slope of around - (20)(21)(22)(23)(24)(25) mm/degree of latitude as well as regional distortions in ODN. As such, GPS-levelling should not really be used to test geoid models in Britain, though this has been done by, e.g., [35] (Fig 1).…”