The IGS Station and its Role in Land Delivery in Zambia
L. Combrinck1 and P. Nsombo2
1. HartRAO Space Geodesy Programme, P. O. Box 443, Krugersdorp 1740, South Africa, e-mail: firstname.lastname@example.org
2. Department of Surveying, School of Engineering, University of Zambia. P. O. Box 32379 Lusaka, Zambia. E-mail: email@example.com
The installation of this IGS station (ZAMB) now makes it possible to realize reliable coordinate solutions for all GPS surveys. This in turn will lead to reliable mapping and survey operations and consequently contribute to a quicker land delivery process.
2.0 Re-observation of the Zambian primary, secondary and tertiary control stations.
It is well known that many of the existing national networks in Eastern and Southern Africa are tied to the Arc of the 30th Meridian as is the case of Zambia. Thus in 1999, and in conjunction with the Zambia Survey Department (ZSD), and with support and cooperation of the Swedish International Development Agency (SIDA), and the Royal Institute of Technology, (KTH); a GPS campaign was carried out to re-survey control stations around the Arc. This GPS survey comprised ten (10 No.) Zambia Primary (ZP) control points and one (1 No.) Zambia Tertiary (ZT) control point. Initially, over 20 control points were selected but could not all be observed due to varied problems -mainly being inaccessibility and limitation in time. The configuration of the net is illustrated in figure 1 below.
Fig. 1. Part of the GPS control network in Northern and Eastern Zambia
The Equipment used during the campaign were Leica’s RTK 300 series the static 200 series. A total of six receivers were used. Centring plates shown in figure 2 were used in the survey (as shown below).
Fig. 2 Centring plate
2.1 Computation of the net
Three stations, ZT500, ZP114 and ZP129 continuously observed during the campaign. Thus three receivers roved. Observations took six working days. ZT500 was Computed from HRAO using the Bernese software, while all the other stations were computed using the SKI software. Baseline distances ranged between 46 – 790km. Preliminary results show loop misclosures ranging from 0.2 to 1.5ppm.
Transformations to Clarke 1880 using the Molodensky-Badekas model, were performed in 4 configurations as below:
We present the estimates of the transformation parameters obtained from the first set, i.e. using all the eleven points. The geocentric geoidal heights were estimated from Nsombo (1998). The local geoid is estimated using the abridged Molodesnky formula.
DX = 152.359 ± 0.425 m
DY = 60.814 ± 0.425 m
DZ = 297.384 ± 0.425 m
Rx = -11.915 ± 0.371²
Ry = 1.424 ± 0.788²
Rz = 8.451 ± 1.445²
M = -8.328 ± 1.773 ppm
2.2 RTK Pilot Project Chilenje South, Lusaka
In collaboration with KTH, a pilot project was undertaken to demostrate the capabilities of GPS Real Time Survey (Real Time GPS Kinematic mode, or RTK) for cadastral purposes. This is how a pilot project area was selected in Chilenje South, Lusaka to test the RTK GPS equipment brought from Sweden. Approximately a complex of 200 small house plots had to be set out. The results from the tests not only confirmed our expectation but also proved that with respect of economy and the precision required, the method is the quickest and more reliable than any other known today. Point position errors was estimated at @ ±0.029 m , which is within accuracies that could be achieved by the conventional methods (Sjöberg et. al. 2002).
Fig. 3. Pilot project in Chilenje
2.3 Re-survey of the Greater Lusaka control network
Between July and September, 2001, the whole control network over Greater Lusaka (encompassing areas beyond the Lusaka District boundary) was re-observed using Leica’s 200 series receivers. Two M.Sc. students who were undertaking minor field studies carried out the work.
Four sub-networks were established, with ZT500, ZS73, ZT28, ZT523, being reference stations. However, ZT500 was chosen to be the base of the networks, due to the, soon to be established, IGS station. The observation time for the references was set to two hours, with 10 second's observation rate, in order to get high accuracy and to be able to resolve the ambiguities. The distance between reference and rover points in the smaller networks were much shorter so the observation time was set to thirty minutes at three seconds observation rate. Every point was planned to be observed in two sessions. The figure 4 below shows the network. Average loop misclosures lie between 1.3 and 2.9ppm.
Transformation parameters for 13 common points yielded the following parameters for the region:
DX = 162.146 ± 0.041 m
DY = 82.466 ± 0.041 m
DZ = 280.571 ± 0.041 m
Rx = 7.963 ± 1.036²
Ry = 1.851 ± 1.297²
Rz = -8.889 ± 1.211²
M = 12.629 ± 4.421ppm
Again the local geoid was estimated by using abridged Molodensky formula.
Fig. 4. Re-survey of Greater Lusaka control points
Part of the two students’ work was to design and establish a new network in a part of the Lusaka having relatively rapid development. Hence 8 new points where established in South of Chilenje South.
Fig. 5. New reference marks in Lusaka
This network was designed bearing in mind the use of the points in conventional methods. Hence intervisibility was considered.
3.0 The IGS station
The IGS station ZAMB was processed in a regional network of 25 IGS stations which includes a subset of eight IGS global stations: HRAO (South Africa), SUTH (South Africa), CAS1 (Antarctica), ASC11 (Ascension Island), KERG (Kerguelen Islands), MALI (Kenya), GOUG (Gough Island) and MAW1(Antarctica).
In determining ZAMB's preliminary ITRF coordinates, sixteen ZAMB 24 hour rinex files were processed in this regional network. Processing was done using the GAMIT package. The primary solutions were performed with loose apriori uncertainties to provide an unconstrained initial solution. The GAMIT solutions files were further processed by the GLOBK Kalman filter. This additional stage uses the estimates and associated covariance matrices for station coordinates, orbital parameters, earth-rotation parameters, and source positions generated from analyses of the primary
The a priori well known positions of the eight IGS global sites were used to constrain the reference frame by minimising the position deviations of these stations. Only translation parameters were estimated. Orientation of the reference frame was maintained through the IGS orbit and IERS Earth
orientation values. The preliminary results are encouraging and can be used as interim ITRF coordinates. The ITRF values will improve as more data are collected. A firmware upgrade to the SNR8000 GPS receiver (planned for July 2002) will also improve data quality.
ITRF 2000 geocentric coordinates epoch (2002.44) for ZAMB:
X = 5415353.0847 (m) ± 71 mm
Y = 2917209.7255 (m) ± 46 mm
Z = -1685888.9341 (m) ± 18 mm
WGS84 coordinate values epoch (2002.44) for ZAMB:
Latitude = 15° 25' 31.94930" S
Longitude = 28 18' 39.63772" E
Height = 1324.909 m
Having the station coordinates so determined, we will then be able to connect all local networks to this station, bearing in mind ZAMB observation rates and other parameters. We now give an example of a recent GPS campaign undertaken in June 2002.
Fig. 6. Zambia Tertiary, ZT500 station at Mulungushi House, before installation IGS GPS antenna
Fig7. ZAMB AshTech VZ –12 GPS antenna
3.1 GPS Campaign in Eastern Zambia
As a follow up to the previous works in 1999 and 2000, KTH together with UNZA resumed the GPS surveys, but this time to carry out the work in Chipata, Eastern Zambia. Three GPS receivers (Leica’s 299/233/333 series) were used. ZAMB was a considered part of the network. Computations of this network is in progress.
TT111, TT110 and TT113 are points lying on the peripheral of the town, monumented on the surrounding hills. Results of the earlier observations were preliminary. Thus the Chipata campaign was aimed at determining better coordinate solution for the three points. These points are deemed crucial as they will obviously help the ZSD regional office densify the town reference marks used for property surveys to mention just one.
Fig. 9. GPS surveys in Eastern Zambia
The installation of the IGS station is certainly very welcome as it will be possible now to realize reliable networks. The transformation parameters will now be determined more accurately. The station will now enable us calculate the networks sighted above. Moreover, it will now be possible to carry out surveys with only one GPS receiver thereby making it possible for private surveyors and surveying companies to cut costs. This will also entail higher productivity and possibly a quicker land delivery. However, it is still not possible to acquire GPS receivers. Partnerships with potential users/providers of GNSS technologies will be the solution.
The following seem to be the immediate tasks to be undertaken following the installation of the station:
· Re-compute the geocentric geoid over Zambia and subsequently the local geoid
· Establishment of control points especially in rural areas
· Re-computation of the GPS networks
The above tasks will certainly call for funding and the partnerships earlier mentioned is the obvious way out.
2. Ansenjo E.; Nsombo P. ; Sjoberg L. E. and Stoimenov G. (2002). Sustainable Development in Zambia SAREC/SIDA KTH Report.