In the Eastern Region of Abu Dhabi, because much of the existing vertical control has been lost to destruction or disturbance, and because the network was not dense enough to support use of GPS to derive elevations, Al Ain Town Planning initiated a project for height determination based on gravimetric determination of geoid. Prior to GPS, horizontal control was most easily established by measuring between hilltops. Similarly, vertical control was (and still is) most easily established by leveling along railroad or highway corridors. Even once GPS became a practical technology, vertical control monument locations were still problematic as control stations since some are near buildings and along the roads covered by trees and others are not easily accessible. Nevertheless, it can be seen in Al Ain that its a great advantage in property surveys to have control values distributed all through the developed areas, because the surveyor saves the time otherwise required to survey ("run") control in from nearby (sometimes several miles or more away) geodetic stations. There are several phases involved in establishing a geoid surface that can be used to correct GPS derived heights so that they are consistent with the existing orthometric height reference system. Initially, it is necessary to assess the existing control, and confirm its validity and consistency. Only once this has been done can a geoid be determined and then fitted to the existing vertical control points. 2. Existing vertical control. The current height network in the Eastern Region is based mainly upon a series of leveling runs carried out by BKS Surveys between 1983 and 1987. A total of about, 214 levelling circuits were carried out over a total distance of 2396 km. The accuracy was specified as 10 mm per square root of levelling run in kilometres. The aim was to provide one point for every square kilometre in areas to be covered by 1:500 and 1:1000 mapping, and one point for every 5 km2 in areas to be covered by 1:2000 mapping. Of the 2000 ground markers, 814 were spirit levelled. The rest were heighted by reciprocal vertical angles. Many of these points have subsequently been destroyed during development work, and that only as few as 25% may still exist in some areas. The datum used for this levelling was the Port Rashid datum of Dubai, established by levelling carried forward from Dubai to points BTP219 (Al Faqa) and BTP226 (Schweib). These two points mark the northern extent of the levelling work; the southern extent is at Al Qua. In addition to the levelling and trigonometric heighting established by BKS, two other sources of height information are available in Al Ain Town planning database. The first of these are the points identified by the initial prefix “K”, which relate to a boundary survey carried out along the border with Oman by KLM. The second set of points is that identified by the initial prefix “G”, which were geodetic points established by the Survey Section of Al Ain Town Planning in the 1990s. Concrete monuments were installed in all non-rocky and non-sandy locations. In rocky locations where excavation proved difficult a shortened central tube was used. In sandy locations liable to erosion, the three-metre pipe marker was installed with witness posts. A total of 1712 monuments of all types were installed to provide the vertical control network of the eastern region. Vertical control analysis showed that there are several discrepancies in the control points used in the region. In the first place, the heights of the KLM points were actually determined by trigonometric heighting which leads to a lower accuracy, as well the points were refereed to Ras Ghantut datum, while that used by BKS to determine the heights of the BKS points referred to Port Rashid mean sea level datum. The comparison between BKS and KLM indicates a difference between datums of approximately 4 m, which indicates that, it is possible, that there is a further confusion between datums. Secondly, there is also a problem with the way in which the “G” points were established. These points were positioned with GPS during the 1990s, using “B” points as control and deriving transformations using the SKI software. The effect of this is to derive heights that are neither orthometric nor ellipsoidal, but what can be termed “linearly corrected orthometric”: that is, they are ellipsoidal heights that have been converted (implicitly, rather than by design) to orthometric using a model of the geoid that is defined as a plane through the values at the control points. This approach is generally valid over short distances (several hundred metres, or perhaps some kilometres), where to a certain extent the geoid can be modelled as an inclined plane. The evidence here, however, is that it has been done over several tens of kilometres: over these distances, the necessary assumptions break down completely. An examination of the form of the EGM-96 geoid shows that errors of around 0.5 m could be introduced in this way, and possibly larger since EGM-96 is a smoothed model of the geoid. In addition, if any “K” points were used as control, then the errors would be even worse than this. It is therefore necessary to ignore both the “K” points and the “G” points in assessing the quality of the vertical control network, this leaves only BKS points to from the vertical control of the Eastern Region. 3. Relations between Orthometric heights and Geoid The GPS measured heights are measured from the ellipsoid, therefore, they need to be converted into an orthometric height system. The current methods of converting GPS elevations to orthometric elevations (Acharya and Popp, 1994) are:
The residuals shown in table.1 are of a magnitude that is consistent with an underlying more detailed structure of the geoid than is modelled by EGM-96. The analysis showed that the “B” points are in an orthometric height system and are sufficiently widely spread to form the basis of an on-going vertical control system. The “K” points are few in number and based on the Ras Ghantut local datum; the “G” points are not in an orthometric height system. In adopting the “B” points as the sole basis for the orthometric height datum, however, an assessment will have to be made of the extent to which “K” and “G” points have already been used to control mapping, and appropriate corrections will have to be applied.  Fig. 1 WGS84 Geoid of the Eastern region of Abu Dhabi Emirates 4. The Earth Geo-potential Model EGM96 The EGM96 model is the result of collaboration between the National Imagery and Mapping Agency, the NASA Goddard Space Flight Centre, and the Ohio State University. Major terrestrial gravity acquisitions by NIMA since 1990 include airborne gravity surveys over Greenland and parts of the Arctic and the Antarctic, surveyed by the Naval Research Lab (NRL), and cooperative gravity collection projects, several which were undertaken with the University of Leeds. These collection efforts have improved the data holdings over many of the world's land areas, including Africa, Canada, parts of South America and Africa, Southeast Asia, Eastern Europe, and the former Soviet Union. In addition, there have been major efforts to improve NIMA's existing 30' mean anomaly database through contributions of various countries in Asia. |