GEOID MODELLING IN EGYPT


By

 

Gomaa M. Dawod

 

Survey Research Institute, National Water Research Center, Cairo, Egypt

 

Currently on a temporally leave to: Umm Al-Quraa University, Holly Makkah, Saudi Arabia, Tel. +966501402093

 

Email: dawod_gomaa@yahoo.com

 

Preface:

 

 

This page is a voluntarily step to present a concise summary about geoid modelling trials and efforts in Egypt. It does not intend to serve as neither an official page nor a complete list about this issue. All interested geodesists and scientists in Egypt are encouraged to participate in such an effort, through sending fruitful comments and adding other materials on this topic. I appreciate any attempt to participate in the expansion of this page, and makes it freely available to all concerned individuals. I hope this step can lead to forming a scientific committee and a cooperative work about precise geoid modelling in Egypt.

 

 

Introduction:

 

The geoid has been an old geodetic interest within the geodetic community in Egypt very long time ago. A part of the definition of the Egyptian Geodetic Datum (EGD) in the early 1900s was the definition of a geoid undulation value. When EGD was defined in 1907, the Helmert 1906 International ellipsoid was chosen as a reference ellipsoid. Among other defining parameters, the geoid undulation (N) was set to zero at the Initial point of the geodetic networks (station F1 located at Al-Mouqatam mountain, East of Cairo). The national geoid of Egypt has been defined as the Mean Sea Level (MSL) computed at Alexandria tide gauge based on low and high water daily readings through the period 1898-1906 (Cole, 1944). Since 1907, the issue of geoid determination has been a central concern of the geodetic and surveying communities in Egypt. However, it becomes an essential demand as soon as the Global Positioning System (GPS) technique was extensively utilized in the surveying and mapping applications in Egypt, starting in 1985.

 

The geoid modelling and determinations in Egypt are grouped and summarized, in the following sections, on a date basis. Again, the collected materials are not a complete list on that topic, but they aim to provide a fine summary to the best of my knowledges.

 

 

 

Geoid modelling in Egypt in 1980s:

 

 

A milestone national geodetic research study has been carried out by Shaker (1982) aiming to perform, for the first time, a three-dimensional adjustment of the Egyptian geodetic networks in order to increase their reliability and precision.

 

The geoid solution of Alnaggar (1986) is considered the pioneering nation-wide (22N <latitude<32N, 25E<longitude <37E) geoid, developed using the Least-Squares Collocation (LSC) technique and heterogeneous geodetic data including terrestrial gravity, astronomic deflections of the vertical, and Doppler/levelling undulations. That geoid was relative to the WGS72 global geodetic datum.

 

 

Geoid modelling in Egypt in 1990s:

 

 

El-Tokhey (1993) has developed a geoid relative to the GRS80 datum using gravity, astronomic, Doppler, and GPS data. Additionally, Saad (1993) has investigated the accuracy improvements and the redefinition of the Egyptian vertical control networks.

 

Nassar et al. (1993) has developed the Ain Shams University (ASU93) national geoid solution. It was based on applying the least-squares collocation processing strategy to heterogeneous datasets including astro-geodetic deflections of the vertical,, gravity data from different sources, geometric satellite derived information. This geoid was relative to the WGA84 global geodetic datum.

 

El-Sagheer (1995) has applied the FFT techniques using a global geopotential model combined with terrestrial gravity data and heights from a local DTM (called DTM-95), which was used to predict free-air gravity anomalies in void areas.

 

El-Shazly (1995) has investigated the redefinition of the vertical datum of Egypt based on Analyzing Sea Surface Topography (SST) and levelling by GPS.

 

Shaker et al. (1997) has developed a  combined geoid where two types of geoid undulations (gravimetric and GPS-derived) are integrated in a unique solution.

 

Two very important achievements in the field of geodesy in Egypt have been completed in the 1990s: They both advance the geoid determination on a national scale. These milestones are:

 

 

(a) In 1995, two national GPS geodetic control networks have been established, by the Egyptian Survey Authority (ESA), to furnish a nationwide GPS skeleton for surveying and mapping applications (ESA 1995). The first network is the High Accuracy Reference Network (HARN) that covers the entire Egyptian territories and consists of 30 stations with approximate separation of 200 Km. The relative precision level of HARN is 1:10,000,000. The second network is the National Agricultural Cadastral Network (NACN) that is mainly covers the Nile valley and the Delta. NACN consists of 112 stations, with a station separation of 50 Km approximately, whose relative precision is 1:1,000,000.

 

 

(b) In 1997, an accurate gravity framework for Egypt has been established through the Egyptian National Gravity Standardization Network 1997 (ENGSN97). With a national homogeneous distribution and the utilization of precise instrumentation, the ENGSN97 serves as the accurate national gravity datum for Egypt. It consists of 5 absolute gravity stations (observed in a joint effort between the Egypt's Survey Research Institute (SRI) and the U.S. Defense Mapping Agency (DMA, known currently as NIMA) and 145 relative gravity stations connected to those absolute gravity points. The precise coordinates of almost all of the  ENGSN97' points have been observed by static GPS, along with precise levelling techniques (SRI, 1998, and Dawod and Alnaggar, 2000).

 

The ENGSN97' 150 gravity points (plus other 100 stations) have been utilized by Dawod (1998) to develop two national geoid models on a 5'x5' grids: (1) a gravimetric geoid, named SRI-GEOID98A , based on Fast Fourier Transformation (FFT) remove-compute-restore process using the OSU91A Global Geopotential Model (GGM) and a local Digital Terrain Model (DTM). (2) SRI-GEOID98B model was obtained through fitting the obtained gravimetric geoid to GPS/levelling data over 95 available stations.

 

Moreover, the MSL and SST have been investigated by several researchers. For example, Nassar et al. (1998) has modeled of the local sea surface topography in Egypt based on the zero frequency response analysis. Additionally, Nassar et al. (1999) has studied the accuracy improvements of the Egyptian vertical datum for national development projects. Also, Saad (1998) has performed a study on the productivity analysis for obtaining orthometric heights without leveling in Egypt.

 

 

Recent Geoid modelling in Egypt (2000-Now):

 

 

Nassar et al. (2000) has developed the Ain Shams University (ASU2000) national geoid solution. It was based on applying the least-squares collocation processing strategy to heterogeneous datasets including astro-geodetic deflections of the vertical,, gravity data from different sources, and the HARN national GPS network. This geoid was relative to the WGA84 global geodetic datum. The internal precision of this geoid solution is estimated to be better than 0.6 m on the average, while its external accuracy is better than 1 m on the average.

 

Moreover, it worth mentioning that there are other studies that have developed geoid models for particular areas of interest in Egypt: For example, Tscherning et al. (2001) has developed a geoid model for Lake Nasser area, south of Egypt. In this geoid, totally 197 gravity disturbances spaced approximately 3 km apart were used as well as 35 GPS/levelling points in the vicinity and on the Aswan Dam were combined in a LSC processing strategy. This geodetic network is a part of the geodetic activities of the National Research Institute for Astronomy And Geophysics (NRIAG). NRIAG operates a permanent GPS network of 9 stations, and 4 other networks of regularly repeated (gravity and GPS) observations (Mahmoud 2004).

 

Abd-Elmotaal (2002) has developed an Egyptian gravimetric geoid model, based on available free-air gravity anomalies and a local DTM, fitted to local GPS/levelling data.

 

A generation of geoid models for Egypt on a five-minute grid has been developed  by Saad and Dawod (2002): SRI2001A is a  gravimetric geoid computed using local gravity, a local Digital Terrain Model (DTM) and the EGM96 GGM, using a remove-compute-restore FFT processing methodology. SRI2001B is a geoid based on surface-fitting of the SRI2001A model to GPS/levelling.

 

Another geoid solution, called EGGG2003, has been developed by Abd-Elmotaal (2003) using the Window technique to create an adapted version of the EGM96 global model to get a better fit to the Egyptian gravity field in a Fast Fourier Transform (FFT) process.

 

Hassona (2003) has devolved a national 2'x2' geoid model, named EGM96EGIT, using the integral technique to tailor the EGM96 GGM to degree 650 utilizing local heterogeneous dataset.

 

In the context of developing local and regional DTMs, El-Sagheer (2004) has developed a local DTM model (called DTM-2003) through digitizing 1:250 000 and 1:100 000 hard-copy topographic maps. Hence, 144 000 data points have been used in the generation of the DTM-2003, so that its density is one data point per each 7 square kilometers. Also, Abd-Elmotaal (2004) has developed a digital height model for Africa, named AHF04.

 

Dawod (2005) has developed a geoid model for the Nile River as a part of the undergoing national project for updating the hydrographic and topographic maps of the Nile. Within this project, carried out by the Nile Research Institute (NRI), about 600 GPS/levelling stations will be established and precisely observed along both banks of the river (NRI, 2003). Such networks, when completed, will play fundamental role in the geoid determination in Egypt. This geoid model has been utilized to increase the reliability and integrity of the national Egyptian geodetic databases through estimating precise orthometric heights of the national GPS stations (Dawod and Ismail 2005).

 

 

A significant accomplishment in the field of geodesy in Egypt have been launched in the 2000s: SRI (in a joint cooperation with the Hydrographic Surveying Department, The Egyptian Naval Forces) has initiated a national project for establishing several tide gauge (TG) stations along the Egyptian coasts. In 2001, the first TG was installed at Alexandria through a state-of-the-art Sea Level Monitoring System (SLMS). It consists of a modern tide gauge instrument, a meteorological weather station, and a GPS receiver (SRI, 2002). In 2004, a second similar system has been installed at Marsa Matrouh. Planes are currently undergoing to establish new TGs at Hurgada, Al-Arish, Suez, and Port Said.

 

 

Mohamed (2005) has investigated the re-definition and realization of the Egyptian vertical geodetic datum. Data from several TG stations cover a time span, that constitutes a large period of the twentieth century, have been collected and analyzed. Recent heterogonous data from the new Alexandria SLMS have been also considered towards the assessment of the MSL changes in Egypt.

 

Abd-Elmotaal (2006) has developed and applied a high-degree tailored reference model by merging the available gravity anomalies with EGM96. Such a process gave better residual gravity anomalies than the original EGM96, and the variance was reduced by one third (ibid).

 

With the extensive utilization of GPS in surveying and mapping applications, the geodetic datum transformation still constitute a critical issue in Egypt. El-Shazly (2006) has investigated the utilization of the neural networks for coordinate transformation. Additionally, Saad And El-Sayed (2007) and (Shaker et al. 2007) have investigated new models to improve the Egyptian geodetic networks particularly in the transformation process between the WGS84 and the EGD.

 

Dawod (2008) has utilized local geodetic datasets (terrestrial gravity stations, GPS/levelling stations) to analyze the performance of seven of the recent GGM (released between 2003 and 2006) and four DTMs (3 global and one local) in the context of developing a precise geoid model for Egypt. Additionally, Dawod and Mohamed (2008) have studied the role of the geoid/quasi-geoid seperations when fitting global geoid model-based height anomalies to GPS/Levelling local datasets in Egypt.

 

Dawod et al (2009) assest the performance of the EGM2008 global geoid model and its performance over the north part of the river Nile and delta. It has been concluded that the EGM2008 is superior to earlier GGMs over the northern area of Egypt by a factor of 1.6.  An improvement of about 26% has been obtained after applying a regression model, and the standard deviation has been decreased to 0.17 m. As a result, the EGM2008 produces a crucial influence on integrated GPS surveys where the orthometric heights can be obtained without any additional costs.

 

 

Recent regional and global geoid modelling:

 

 

It worth mentioning that a current regional project (The African Geoid Project: AGP) is undergoing to develop a uniform precise geoid for Africa (Merry 2003) as a part of the regional AFREF project  to unify the vertical geodetic datums in Africa (Wonnacott 2006).

 

Moreover and in the context of global geoid modelling, it is valuable to state that a new GGM (the Earth Geopotential Model 2008) has been realeased by the US National Geospatial Intelligence Agency (NGA). EGM2008 is complete to degree and order 2160, with 5' x 5' resolution and 22 cm RMS global accuracy.

 

For more information about EGM2008:

 

http://earth-info.nima.mil/GandG/wgs84/gravitymod/egm2008

http://earth-info.nima.mil/GandG/wgs84/gravitymod/index.html

http://earth-info.nima.mil/GandG/wgs84/gravitymod/egm2008/NPavlis&al_EGU2008.ppt

 

 

 

Acronyms:

 

 AGP           African Geoid Project

AFREF       African Reference Framework

ASU           Ain Shams University

DTM           Digital Terrain Model

EGD           Egyptian Geodetic Datum

ENGSN97   Egyptian National Gravity Standardization Network 1997

ESA            Egyptian Survey Authority  

Esa1@idsc.net.eg

EGM96       Earth Geopotential Model 1996

EGM07/08   Earth Geopotential Model 2007/2008

FFT             Fast Fourier Transform

GPS            Global Positioning System

GRS80        Geodetic Reference System 1980

GGM           Global Geopotential Model

HARN         High Accuracy Reference (GPS) Network

LSC             Least-Squares Collocation

MSL            Mean Sea Level

NACN         National Agricultural Cadastral (GPS) Network

NRI             Nile Research Institute, NWRC

NRIAG    National Research Institute of Astronomy and Geophysics http://www.nriag.sci.eg 

NWRC        National Water Research Center

http://www.nwrc-eg.org/

SLMS         Sea Level Monitoring System

SRI             Survey Research Institute, NWRC

SST            Sea Surface Topography

WGS72      World Geodetic System 1972

WGS84      World Geodetic System 1984

 

 

List of interested geodesists in Egypt:

         

(Ordered Alphabetically !)

 

Abdallah A. Saad (Benha University) abahsa_31@yahoo.com

Adel El-Shazly (Cairo University) adel_shazly@hotmail.com

Ahmed A. Shaker (Benha University) ahmshaker@link.net

Ali A. Alsagheer (Benha University) alielsagheer@yahoo.com

Ali A. Tealeb (The National Research Institute of Astronomy and Geophysics) tealeb@nriag.sci.eg

Anwar Radwan (The National Research Institute of Astronomy and Geophysics) radwan99@yahoo.com

Dalal S. Anaggar (Chairperson of the Regional Center for Training and Water Studies, Vice Chairperson of the National Water Research Center, and Vice President of FIG). dalnagar@trainingcenter-eg.com , Web site: http://www.fig.net/council/vicepresident_alnaggar.htm

Gomaa M. Dawod (Survey Research Institute) dawod_gomaa@yahoo.com

Hoda F. Mohamed  (Survey Research Institute) hoda_faisal@yahoo.com

Hussien Abd-Elmotaal (Minia University) abdelmotaal@lycos.com

Mohamed El-Tokhy (Ain Shams University) 1 EL-Sarayat Street
Abbassia, Cairo, Egypt.

Moustafa A. Baraka (Cairo University) moustafa_baraka@hotmail.com

Mostafa Rabah (National Research Institute for Geodynamics and Astronomy) mostafa_rabah@yahoo.com

Raeed M. Hassona (Menoufia University) nileeast@hotmail.com

Salah M. Mahmoud (Chairperson, the National Research Institute of Astronomy and Geophysics)  salahm55@yahoo.com

 

 

 

Links to some International Geoid Organizations:

 

 

AGP: The African Geoid Project

http://www.geomatics.uct.ac.za/generic.php?m=/research/geodesy.php

AFREF: The African Reference Framework Project:

               http://geoinfo.uneca.org/afref/

Bureau Gravimétrique International (BGI)

http://bgi.cnes.fr:8110/ 

IAG (International Association of Geodesy)

http://www.iag-aig.org/

IAG Sub-Commission 2.2 on Spatial and temporal gravity field and geoid modelling,

http://www.geology.osu.edu/~jekeli.1/iag-commission2/sc2.htm

IAG Study Group SG2.2 on Forward gravity field modeling using global databases

 http://www.geology.osu.edu/~jekeli.1/iag-commission2/sg2.2.htm

IAG Sub-Commission 2.4 - Regional geoid determination

 http://www.geology.osu.edu/~jekeli.1/iag-commission2/sc4.htm

International Center for Global Earth Models (ICGEM)

 http://icgem.gfz-potsdam.de/ICGEM/Main.html

International Earth Rotation Service (IERS)

http://hpiers.obspm.fr/

International Federation of Survoyers (FIG)

http://www.fig.net/ 

International Gravity Field Service (IGFS)

http://www.igfs.net/

International Geoid Service (IGeS)

http://iges.polimi.it/

US National Geodetic Survey (NGS) Geoid group

                   http://www.ngs.noaa.gov/geoid/

 

 

 

References:

 

 

Abd-Elmotaal, H., (2002), Towards a Precise Geoid for Egypt, In Tziavos:  Gravity and Geoid 2002, The 3rd Meeting of the International Gravity and Geoid Commission, Thessaloniki, Greece, August 26-30, pp. 120-125.

Abd-Elmotaal, H., (2003), The Egyptian Geoid EGGG2003, Presented at the 23rd General Assembly of the International Union of Geodesy and Geophysics IUGG, Sapporo, Japan, June 30 - July 11.

Abd-Elmotaal, H., (2004), The AHF04 Digital Height Models for Africa, Presented at the IAG International Symposium on Gravity, Geoid, and Space Missions (GGSM2004), Porto, Portugal.

Abd-Elmotaal, H., (2006), High-Degree Geopotential Model Tailored to Egypt, Presented at the 1st International Symposium of The International Gravity Field Service (IGFS), Istanbul, Turkey, August 28 – September 1.

Alnaggar, D., (1986), Determination of the Geoid in Egypt Using Heterogeneous Geodetic Data, Ph.D. Dissertation, Cairo University, Egypt.

Cole, J.H., 1944, “Geodesy in Egypt” Survey Department Ministry of Finance, Egypt.

Dawod, G., (1998), A national gravity standardization network for Egypt, Ph.D. Dissertation, Shoubra Faculty of Engineering, Banha Branch, Zagazig University, Cairo, Egypt.

Dawod, G.,  and   Alnaggar, D., (2000), Quality control measures for the Egyptian National Gravity Standardization Network (ENGSN97), Proceedings of The Second International Conference on Civil Engineering, Helwan University, Cairo, Egypt, April 1-3, pp. 578-587.

Dawod, G., (2005), Developing a precise geoid model for hydrographic surveying of the River Nile, Al-Azhar University Engineering Journal (AUEJ), V. 8, No. 1, January, pp. 96 - 107.

Dawod, G., and Ismail, S., (2005), Enhancing the integrity of the national geodetic data base in Egypt, Proceedings of the FIG working week and GSDI-8 International Conference, Cairo, Egypt, April 16-21.

Dawod, G., (2008), Towards the redefinition of the Egyptian geoid: Performance analysis 

               of recent global geoid models and digital terrain  models, 

               Journal of Spatial Science,    V. 53, No. 1, pp. 31-42.

               http://www.mappingsciences.org.au/journal/june_08.html.

Dawod, G., and Mohamed, H., (2008), Fitting gravimetric local and global quasi-geoids to GPS/levelling data: The role of geoid/quasi-geoid variations in Egypt, CERM, V. 30, No. 1 (January), pp. 233-244

Dawod, G., Mohamed, H., and Ismail, S., 2009, Evaluation and Adaptation of the EGM2008 Geopotential Model along the Northern Nile Valley, Egypt: Case Study, ASCE Journal of Surveying Engineering, Abstract Available from:

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JSUEXX000001000001000001000001&idtype=cvips&gifs=yes

 

ESA (The Egyptian Survey Authority) (1995) The zero order GPS networks, Unpublished technical Report, ESA, Cairo, Egypt.

El Sagheer, A. (1995) Development of a digital terrain model for Egypt and its application for a gravimetric geoid determination, Ph.D. Thesis, Shoubra Faculty of Engineering, Zagazig University, Egypt.

El Sagheer, A. (2004) Towards updated concrete Digital Terrain Model for Egypt: DTM-2003, Civil Engineering Research Magazine (CERM), Al-Azhar University, V.26, No. 1, pp.158-179.

El-Shazly, A.H. (1995) “Towards the Redefinition of the vertical Datum of Egypt: an Analysis of Sea Surface Topography and Levelling by GPS” Ph.D. dissertation, Cairo University, Cairo Egypt.

El-Shazly, A., (2006), Neural networks compared to polynomials for coordinate transformation in Egypt, Civil Engineering Research Magazine, V. 28, No. 1, pp. 22-35.

El Tokhy, M., (1993), Towards the redefinition of the Egyptian geodetic control networks: Geoid and best fitting reference ellipsoid by combination of heterogeneous data, Ph.D. Thesis, Faculty of Engineering, Ain Shams University, Egypt.

Forsberg, R., (2007) The International Gravity Field Service and GGOS, Presented at the GGOS Retreat 2007 and Eighth Steering Committee Meeting, California, USA, February 19-21.

Hassona, R., (2003) Modelling of outer gravity field in Egypt using recent available data, Ph>D> Dissertation, Faculty of Engineering in Shebin El-Kom, Menoufia University.

Merry, C., (2003), The African Geoid Project and its relevance to the unification of African vertical reference frames, Proceedings of the 2nd FIG Regional Conference, Marrakech, Morocco, December 2-5.

Mohamed, H. (2005) Realization and redefinition of the Egyptian vertical datum based on recent heterogeneous observations Ph.D. Dissertation, Shoubra Faculty of Engineering, Banha University, Cairo, Egypt.

Mahmoud, S., (2004) Monitoring of crustal movements in Egypt using GPS techniques, Presented in the United Nations/United States of  America International Meeting on The Use and Applications of Global Navigation Satellite System, December 13 – 17, Vienna, Austria.

Nassar, M., Hanafy, M., and El-Tokhey, M., (1993), The Ain Shams University (ASU93) geoid solution for Egypt, Al-Azhar Engineering Third International Conference, Dec. 18-21.

Nassar, M., Baraka, M., and El-Shazly, A., (1998), Modeling local sea surface topography in Egypt based on zero frequency response analysis, Scientific Bulletin of Faculty of Engineering, Ain Shams University, V. 33, No. 1.

Nassar, M., Baraka, M., and El-Shazly, A., (1999), Accuracy improvements of the Egyptian vertical datum for national projects with emphasis on south valley project “Toshka”, Proceedings of the third conference on civil engineering, Military Technical College, Cairo, Egypt, March 8-10.

Nassar, M., El-Maghraby, M., El-Tokhey, M., and Issa, M., (2002), Development of a new geoid model for Egypt (ADSU2000 geoid) based on the ESA High Accuracy Reference Network (HARN), Scientific Engineering Bulletin, Faculty of Engineering, Ain Shams University, V. 35, No. 4.

NGA (the US National Geospatial Intelligence Agency),  (2007) http://earth-info.nga.mil/GandG/wgs84/gravitymod/new_egm/, Accessed Sept. 2007.

NRI (Nile Research Institute), (2003), The geodetic control networks along the fourth reach of the Nile, Unpublished technical report.

Saad, A., (1993), Towards the redefinition of the Egyptian vertical control network, Ph.D. Dissertation, Shoubra Faculty of Engineering, Zagazig University, Cairo, Egypt.

Saad, A. (1998), Productivity analysis for obtaining orthometric heights without leveling in Egypt, CERM, V. 20, No. 3, pp. 430-441.

Saad, A., and Dawod, G. (2002) A precise integrated GPS/gravity geoid model for Egypt, Civil Engineering Research Magazine (CERM), Al-Azhar University, V.24, No. 1, pp.391-405.

Saad, A., and El-Sayed, M., (2007) Simple model for improving the accuracy of the Egyptian geodetic triangulation network, The FIG Working Week 2007, Hong Kong SAR, China, May 13-17.

Shaker, A. (1982), Three dimensional adjustment and simulation of the Egyptian geodetic network, Ph.D. Thesis, Technical University in Graz, Austria.

Shaker, A., El Sagheer, A., and., Saad, A., (1997), Which geoid fits Egypt better?, Proceedings of the International Symposium on GIS/GPS, Istanbul, Turkey, Sept. 15-19.

Shaker, A., Saad, A., El-Sayed, M., and Ali, A., (2007) Remove-Restore technique for improving the datum transformation process, The FIG Working Week 2007, Hong Kong SAR, China, May 13-17.

SRI (Survey Research Institute), (1998), Establishment and calibration of the Egyptian National Gravity Standardization Network of 1997 (ENGSN97), SRI Technical Report 1.

SRI, (2002), The establishment of a modern sea level monitoring system towards the re-definition of the geodetic vertical datum in Egypt, SRI Technical Report 2.

Tscherning, C.C. Radwan, A, Tealeb, A., Mahmoud, S., Mohamed, A., Hassan, R., Issawy, E., and Saker, K., (2001), Local geoid determination combining gravity disturbances and GPS/levelling: A case study in the Lake Naser area, Aswan, Egypt, Journal of Geodesy, V. 75, pp 343-348.

Wonnacott, R., (2006) The AFREF project: Background, rationale and progress, Proceedings of the 5th FIG Regional Conference, Accra, Ghana, March 8-11.

 

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Updated: April 1, 2009

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