J Iliffe

OSGM02: A NEW MODEL FOR CONVERTING GPS-DERIVED HEIGHTS TO LOCAL HEIGHT DATUMS IN GREAT BRITAIN AND IRELAND

Abstract The background to the recent computation of a new vertical datum model for the British Isles (OSGM02) is described After giving a brief description of the computational techniques and the data sets used for the derivation of the gravimetric geoid, the paper focuses on the fitting of this surface to the GPS and levelling networks in the various regions of the British Isles in such a way that it can be used in conjunction with GPS to form a replacement for the existing system of bench marks. The error sources induced in this procedure are discussed, and the theoretical basis given for the fitting procedure. Results for each major region (Great Britain, Ireland, and Northern Ireland) as well as the various independent island datums are described The problems to be expected when working between datums are discussed.

Interpolation of Tidal Levels in the Coastal Zone for the Creation of a Hydrographic Datum

Abstract As part of the U.K. Hydrographic Office (UKHO)-sponsored Vertical Offshore Reference Frames (VORF) project, a high-resolution model of lowest astronomical tide (LAT) with respect to mean sea level has been developed for U.K.–Irish waters. In offshore areas the model relies on data from satellite altimetry, while in coastal areas data from a 3.5-km-resolution hydrodynamic tide-surge model and tide gauges have been included. To provide for a smooth surface and predict tidal levels in unobserved areas, the data have been merged and interpolated using the thin plate spline method, which has been appropriately tuned by an empirical prediction test whereby observed values at tide gauges were removed from the solution space and surrounding data used to predict its behavior. To allow for the complex coastal morphology, a sea distance function has been implemented within the data weighting, which is shown to significantly enhance the solution. The tuning process allows for independent validation giving a ...

Potential performance levels of a combined Galileo/GPS navigation system

This paper presents the results of a research project that investigated the potential benefits of a combined Galileo/GPS navigation system. The research addressed in detail the two key required navigation performance (RNP) parameters of accuracy and integrity. The project was supported by Alcatel Space and was a contribution to the Galileo definition studies (supported by the European Community under the GALA project). The results show significant improvements in both accuracy and integrity (achievable through RAIM) when a combined constellation is used rather than Galileo alone.

A New Methodology for Incorporating Tide Gauge Data in Sea Surface Topography Models

As part of the Vertical Offshore Reference Frames (VORF) project sponsored by the U. K. Hydrographic Office, a new model for Sea Surface Topography (SST) around the British Isles has been developed. For offshore areas (greater than 30 km from the coast), this model is largely derived from satellite altimetry. However, its accuracy and level of detail have been enhanced in coastal areas by the inclusion of not only the 60 PSMSL tide gauges with long-term records around the coasts of the United Kingdom and Ireland but also some 385 gauges established at different epochs and for different observation spans by the U. K. Admiralty. All tide gauge data were brought into a common reference frame by a combination of datum models and direct GPS observations, but a more significant challenge was to bring all short-term sea level observations to an unbiased value at a common epoch. This was achieved through developing a spatial-temporal correlation model for the variations in mean sea level around the British Isles, which in turn meant that gauges with long-term observation spans could be used as control points to improve the accuracy of Admiralty gauges. It is demonstrated that the latter can contribute point observations of mean sea level (MSL) with a precision of 0.078 m. A combination of least squares collocation and interpolation was developed to merge the coastal point and offshore gridded data sets, with particular algorithms having to be developed for different configurations of coastal topology. The resulting model of sea surface topography is shown to present a smooth transition from inshore coastal areas to offshore zones. Further benefits of the techniques developed include an enhanced methodology for detecting datum discontinuities at permanent tide gauges.

Morphological and geographical evidence for the origin of Phobos' grooves from HRSC Mars Express images.

Abstract The surface of Phobos, the 27×22×18 km inner moon of Mars, is dominated by several families of parallel grooves. At least seven different groups of hypotheses have been advanced to explain their origin, but studies have always been limited by the fact that, until recently, much of Phobos was imaged at a resolution too low to show grooves. Now, however, the High Resolution Stereo Camera (HRSC) on board the European Mars Express mission has made 134 imaging fly-bys past Phobos. The pictures of the previously poorly imaged regions and much of the rest of the satellite have been returned with resolutions down to a few metres, facilitating the construction of a more complete map of the grooves. Each of the seven hypotheses was tested against the new data on groove morphology, positions and orientations, and it was found that six of the previous hypotheses could be discarded. The only hypothesis to pass all tests was that they are chains of secondary impact craters from primary impacts on Mars. An implication of these results is that previous estimates of an unusually thick Phobos regolith of 100–200 m depth are no longer necessary, and our conclusions place no constraints on the interior of Phobos, so recent evidence that Phobos is a ‘rubble pile’ is consistent with our work. The preferred hypothesis also sheds light on the origin of crater chains on Eros, and on impact processes in the early stages of crater excavation.

Accuracy of vertical datum surfaces in coastal and offshore zones

Abstract The Vertical Offshore Reference Frames (VORF) project is described, with a summary of the methodology and an explanation of the data sets used. The latter include satellite altimetry, tidal and geoid models, long and short term tide gauge data, and specially undertaken GNSS observations. This paper goes on to describe the theoretical basis for deriving spatially variant error estimates that respond to the varying quality of the input data. The paper then describes the testing programme undertaken by the United Kingdom Hydrographic Office, which has included 245 checks on datum connections at mostly coastal points, 63 comparisons between VORF-corrected tidal levels observed with GNSS and tide gauge data, and six specially commissioned offshore tide gauge deployments. It is shown that across the vast majority of the domain of applicability the VORF surfaces meet their target accuracies of 0·10 m inshore and 0·15 m offshore (both 1σ values) and the formal uncertainties are a fair reflection of the errors actually encountered. The main discrepancy between the modelled surfaces and the test data is found in the sharply varying tidal regime south of Portland on the south coast of England; however, preliminary results from incorporating the next generation of global ocean tide models show a marked improvement in this area.

Global Ocean Tide Models: Assessment and Use within a Surface Model of Lowest Astronomical Tide

The UK Hydrographic Office (UKHO)-sponsored Vertical Offshore Reference Frames (VORF) project aims to develop tidal level transformation models that are referenced to the GRS80 ellipsoid and thus compatible with GNSS positioning; in particular, heighting. Benefits include increasing the efficiency of hydrographic surveying, providing a stable consistent reference frame and enabling integration with land data in the coastal zone. Seven contemporary global ocean tide models are used to derive Lowest Astronomical Tide (LAT) surfaces which are each assessed by comparison with LAT values from the 7,389-strong UKHO tide gauge database, with the results correlated with distance from land. The proportion of truly offshore and pelagic gauges is relatively limited; however, the transition zone whereby the global ocean tide models commence to deteriorate in accuracy is evident at approximately 30km from the coast. The DTU10 model was selected as the strongest candidate overall. Subsequently, a thin plate spline method is used with the tide gauge dataset to enhance the DTU10 LAT surface in the coastal zone, creating a high resolution global LAT surface with respect to mean sea level. It is seen by cross-validation that the method may be used to predict LAT in near-shore locations with a standard error of 0.23 m.

The development and analysis of quasi-linear map projections

ABSTRACT Map projections are an essential component of coordinate systems used in applications such as surveying, topographic mapping, and engineering, and care needs to be taken to select ones that minimize distortion for each case. This article explores the selection process for near-linear features on the surface of the Earth and derives limits for the extent of a project that can be projected within specified distortion tolerances. It is then demonstrated that a multifaceted set of projections of the Earth may be used to extend this concept to the mapping of features such as highways and railways that are quasi-linear but do not exactly follow a standard geometrical line (a great circle or a small circle) on the surface of the Earth. A continuous, conformal coordinate system may be derived in such situations, extending to indefinite length and applicable over a swath of several kilometers width, but it cannot be extended to cover situations with extensive variations in height. Instead, the Snake Projection is analyzed, and it is shown that this can be used to develop continuous (non-zonal) projected coordinate systems for major engineering projects extending for hundreds of kilometers and having extensive height ranges. Examples are shown of the application to railway projects.

The Snake Projection: A Customised Grid for Rail Projects

Abstract Surveying on large engineering projects such as railway routes presents particular problems in terms of the coordinate system to be used, since they generally extend beyond the limit of an acceptable site grid. This paper presents a method for tailoring a projection in such a way that the scale factor is always close to unity within a few kilometres of a very long but gently sinuous construction project. It also corrects for height scale factor, and the result is that engineers using automated techniques can effectively ignore scale factor and height above sea level corrections when working on the project. This is now used by Network Rail contractors on the East Coast and West Coast main lines, where the scale factor on the track has been kept within ± 20 ppm of unity along a route of several hundred kilometres. The software has been developed to be applied to any railway project, but also has potential to be used on projects of similar geometry, such as major pipeline routes.