Kamal Abdellatif Abdalla
head of survey of section, al ain planning and
survey, Al Ain Municipality,
United Arab Emirates
The quality of the geodetic network of the Eastern Region of Abu Dhabi is assessed, recommending that the network should be re-measured regularly in order to keep track of possible movements of the sites and to maintain and control the overall quality of network and spatial reference system of the region
The United Arab Emirates Primary Geodetic Network (PGN) was used for the establishment of the Eastern Region of Abu Dhabi Emirate Spatial Reference System, known as Al Ain Geodetic Network (AGN). The network was observed using GPS techniques and processed by the Bernese GPS Software. The aim of this paper is to assess the quality of the results obtained for the AGN, the assessment includes:
- The overall quality and results of AGN network adjustment
- The design of the networks and selection of sites and monumentation
- Transformation between old geodetic system (Nahrawan datum) and the new AGN network based on WGS84 and ITRF2000 reference frame
The assessment of the quality of the geodetic networks consisted of an intensive checking of the GPS results, the quality of the individual campaigns, the consistency of GPS observations, and the ties between the networks and ITRF. These can be outlined in the following:
- The assessment of the overall execution of AGN network was limited to a review of the various steps of AGN processes that are relevant to the final results achieved. These include overall design of the network, selection of sites based on reasonable criteria and good monumentation, planning and col lection of the GPS observations and carrying out of new observations.
- Checking the results of AGN adjustment using the Bernese GPS Software: The main tasks carried out for the checking and results assessment are based on the following steps:
- Checking the antenna height reductions, the antenna phase centres values and the auxiliary information used in the processing, such as Earth rotation parameters and precise orbits
- Review of the general analysis strategy and the processing options used for the various steps of the Bernese GPS Software algorithms
- Evaluation of the procedures applied and results obtained for the link of the AGN to the ITRF2000
- Analysis of the connection of the AGN sites to the ITRF2000
- Critical evaluation of the adjustment of the AGN including additional stations and performing an independent assessment of the quality
- The overall characteristics of the network execution were only reviewed in so far as the main emphasis of the analysis was put on the impact of various aspects of the design on the AGN coordinates quality. The set up of the networks was done according to internationally accepted guidelines. This means, that a hierarchical design was used, starting from the sites given in the global reference frame ITRF2000 and subsequently proceeding to national PGN and regional networks such as AGN as well as Al Ain Terrestrial Geodetic sub-network (ATGN). This densification scheme (from large sparse networks to dense regional networks) is also applied within the International GPS Service (IGS) to contribute to the ITRF2000. Much care was taken so that each of the sub-networks is optimally tied to the higher order network by a set of sites common to both networks. These connecting sites had to be and were observed simultaneously during longer sessions than other points of less importance. The selected sites, to the extent possible, were geometrically well distributed over the region under consideration (Fig. 1).
- Selection of Sites and Monumenta-tion: Two standard types of criteria were applied to select the site locations for the AGN: these are criteria concerning the GPS observations and criteria concerning the stability of the site and the distribution within the region of interest. Satellite visibility, and possible radiowave disturbances were considered important for successful GPS observations. Accessibility and security of the site, good monumentation and geological considerations refer to the second class of criteria applied. The selection of sites is always a compromise between different criteria that may contradict each other. The geological stability was clearly the most difficult point to be fulfilled, because in larger areas of the region no bedrock formations were found. It is therefore possible that, despite the extremely careful and solid concrete monumentation, some of the points in the sand desert might undergo small-scale movements. It is therefore recommended, that the sites are re-measured in regular intervals in order to maintain and control the high precision of site coordinates initially obtained with both the PGN and AGN.
- Planning and Collection of the GPS Observations: A few general requirements have to be fulfilled when gathering GPS observations for high-precision networks, such as:
- Overlapping sites between sessions: In order to obtain consistent site coordinates for the entire network, the in network were tied together by overlapping sites. In the case of AGN network, the consistency between individual session results was realized by a few overlapping sites between subsequent sessions. This strategy was chosen to keep the baselines short to solve ambiguity resolution. The tie to the PGN was established through simultaneous observations at nine PGN sites.
- Occupy simultaneously as many sites as possible: This was done to the extent possible. The number of receivers available from the different participating organizations imposed some limitations.
- Observations at each site during independent sessions: All the PGN sites and additional ATGN sites (for transformation purposes) were measured on three independent sessions. Most AGN and additional ATGN sites were observed during 2-3 sessions. If there were failures during a session, the corresponding sites were reoccupied in an additional session.
- The Quality Check of Adjustment Results: Part of the assessment deals with the aspects of the GPS data processing and the results obtained. Antenna Heights, Antenna Phase Centers, Auxiliary Information, as one of the primary potential error sources. Thus the antenna heights, antenna phase center corrections and other auxiliary data (Earth rotation parameters, IGS precise orbits, etc.) used during the processing was checked.
Fig 1: Al Ain Geodetic Network (Scale 1:200000)
Fig 2: Quality of the 24h- and 12h-sessions of the combined APGN free network solution
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