Geoid model derived from GPS/levelling data 

Context in source publication

Context 1

... the densification of the horizontal network (one of the main tasks of the department of Geodesy of the National Geographical Institute - NGI), about 4200 ground markers have been determined with GPS observations. Almost 80% of these points (3400) spirit levelling was performed. This means that, for (almost) the whole Belgian territory, points with both ellipsoidal and orthometric heights, are available with a mean density of 1 point every 10 km 2 . Recently the NGI made a first and promising attempt to derive a geoid model from these data ( Figure 2). – Mean baseline length: 5 km; – Mean GPS occupation time: 40 minutes; – Baselines computed for 90% with GPSurvey (Trimble) and for 10% with SkiPro (Leica); – Processed in regional projects with links to BEREF resulting in ETRS89 coordinates; – Spirit levelling accuracy: = 1.5 cm . The obtained model has been compared with the EGG97 model. – Residuals (see Figure3): < Min. -0.150 m < Max. 0.127 m ; – Adjusted fit parameters: < Standard deviation: 0.032 < Bias ( m ): -2.393 ± 0.001 < NS tilt ( m/ 1000 km ): 0.658 ± 0.013 EW tilt ( m/ 1000 km ): -0.584 ± 0.010. Belgium is a federal state with three regions: Brussels, Flanders and Wallonia. For each of these regions a network of permanent GPS stations is being set up, mainly for RTK purposes. Some characteristics of the networks: – (Flemish Positioning Service) – The network will cover the Flemish region (northern part of Belgium); – The Flemish Government (administration responsible for GIS) takes the initiative; – Should become operational during the fall of 2002; – 36 sites situated on rooftops, equipped with Leica RS500 receivers and choke ring antennas; – Data transfer through dedicated network + some leased lines; – Control software: GPSNet; – Free RTK service via mobile phone; – Data for post processing will be freely available on the internet; – For more information: info.flepos@vlm.be or – Walcors (Wallonia Continuous Operating Reference System) – The network will cover the Walloon region (southern part of Belgium); – The Walloon Government (administration responsible for cartography) takes the initiative; – Should become operational during the fall of 2002; – 23 sites situated on steel pillars (height 8 m), equipped with Leica RS500 receivers and choke ring antennas; – Data transfer through dedicated network; – Control software: GNSmart; – Free RTK service via mobile phone; – Data for post processing will be freely available on the internet; – For more information: gps@wallonie.be or http:// gps.wallonie.be – GPSBru (Brussels GPS Network) – The network will cover the Brussels region (in the centre of Belgium); – The National Geographical Institute (NGI) takes the initiative; – Should become operational during the fall of 2002; – 4 sites situated on rooftops, equipped with Leica RS530 receivers and choke ring antennas; – Data transfer through dedicated network; – Control software: not determined yet; – Free RTK service via mobile phone; – Data for post processing will be freely available on the internet; – For more information: pvo@ngi.be or . The position of the reference sites of all three networks will be determined and monitored by the NGI. All stations will be linked to the BEREF network (Belgian densification of EUREF). The ROB has continued in 2001-2002 to operate its network of 7 permanent GPS stations (see Figure 4). Presently, the network is at the end of a transition phase: the old ROGUE SNR-8000 receivers are replaced by new ASHTECH receivers. In addition, the data download and data transfer software to the ROB is adapted to work with the ASHTECH receivers. Next to the ASHTECH Z-XIIT receiver that was installed in Brussels in March 2001, we have replaced the receivers in Dourbes, Dentergem, Meeuwen and Bree by ASHTECH receivers. We expect to change the Rogue receiver in Waremme by an ASHTECH one and to change the old ROGUE SNR-8000 in Membach by a ROGUE AOA SNR-12 ACT in a very near future. Four of the ROB stations: Brussels (BRUS), Dentergem (DENT), Dourbes (DOUR) and Waremme (WARE) belong to the permanent EUREF network (EPN) since 1996. One of them belongs also to the IGS network (Brussels since 1993) and submits hourly data. Dentergem and Dourbes submit also hourly data to EUREF and, in a near future, Waremme will start to submit hourly data to EUREF too. In addition to this, the ROB continues to operate, since 1996, the “ROB”EUREF Local Data Centre, the “ROB” EUREF Local Analysis Centre and it is responsible for the coordina- tion of the activities related to the EUREF permanent network (B RUYNINX C. and R OOSBEEK F. 2002[1]). The ROB's EUREF local data centre receives each day the data from its 7 permanent GPS tracking stations. Both daily and hourly observation files are made available. Data with a sampling rate of 30 sec. are available for all our stations through the EUREF and IGS network. We have also 10 sec. data available for all stations except Membach and 1 sec. data available upon request for all stations except Membach and Waremme. The goal is to run all procedures fully automatically: – The ROB compresses the received data using the Hatanaka compression scheme, which was especially developed for the efficient compression of RINEX data; – Data holding files are generated. These ASCII files give a quick overview to the users which data files are available to the users. In addition to this, these data holding files allow checking quickly if a data file has been properly transferred to the Data Centre. Within its function as EUREF local analysis centre, the ROB analyses daily the GPS data of a network of about 28 EUREF tracking stations. The results of the daily ROB data analysis flow each week to the EUREF Combination Centre where they are combined with the solutions of the other EUREF Local Analysis Centres. It should be mentioned that the ROB has initiated a new “rapid” data analysis method using the rapid IGS products. It typically runs at 19h00 UTC the day after observation. In addition to the coordinate information, the daily GPS data analysis gives long-term information about the behaviour of the permanent tracking stations, which is essential for the monitoring of their stability. As the Central Bureau of the EUREF Permanent Network, the ROB coordinates the activities of the tracking stations, data centres and analysis centres. It distributes standards and guidelines. The web site containing all the information related to the permanent network can be found at the follow- ing address: More ...