Home Articles Global Positioning System (GPS)

Global Positioning System (GPS)

Venant Balwegirira Mugemuzi
Land Surveyor & GIS, Surveys & Mapping Division
Telephone: 255 22 2121894, Fax: 255 22 2138962
E-mail:[email protected]

keywords: Survey marks, Control surveys, Funds & GPS

Abstract
Developing Countries have experienced a low pace of mapping their land, lack of up-to-dated maps for planning and execution of new projects and programmes. The question of timely data collection is still an issue taking in to consideration traditional technology of surveying, which is still dominant.

Lack of precise controls, to tie the subsequent survey works like cadastral surveys, hydrographic Surveys, engineering activities, ground photographic controls, topographic mapping and other uses, is a setback to many programmes and projects in Tanzania and some other developing countries. The geodetic framework of the past has been decimated following beliefs that the pillar mark positions of underground goldfields.

Following the authorisation to use GPS for activities other than Military purposes, more survey projects can be executed within a short time and the aforementioned programmes and projects can be executed accurately and timely by using the new technology of Global Positioning System (GPS). World Geodetic System (WGS84), as a multipurpose system, has done marvellously especially when the World is entering the new millennium by determining the position of any point on the earth instantaneously. Ships and aircraft track their location, velocity and time through GPS.

1.0 Introduction
Tanzania is located on the East Coast of Africa and south of the Equator. Its extends from meridians 30° to 39° East of Greenwich and 1° to 11° south of Equator.

The country is covered with the area 939,704 sq.kms. Mount Kilimanjaro with a permanent ice cap is 6,340mitres above sea level and the highest mountain in Africa, and in the deep trough-like depression within the Great Rift Valley of East Africa is Lake Tanganyika, lies the world’s second deepest lake.

2.0 Land Surveying in Tanzania
The United Republic of Tanzania came into being on 26 April 1964 following the merger of Zanzibar Islands and Tanganyika Mainland. However, matters relating to Surveys and Mapping are not a Union affair. Hence there are two separate departments dealing with surveying and mapping one in the Islands and the other in the Mainland. Throughout my paper, the name Tanzania is used to refer to the mainland unless otherwise stated.

Tanzania is divided into 20 administrative regions. Every region has a regional land surveyor being a member of regional advisory committee to support a Regional Administrative Secretary (RAS) on land Surveying matters. This survey officer is responsible to legal and technical matters related to surveying land for that particular region. Also every region is divided into Districts, at the moment we have 104 district covering the whole country.

Surveys and Mapping being an organisation in the Ministry of Lands, Human Settlement Development usually prepare good plans for execution of projects. Such projects are related to cadastral surveys, ground photographic control surveys, hydrographic surveys. Other projects like engineering surveys are mostly taken by private firms.

3.0 Control surveys in Tanzania
The traditional technology of data collection is still dominating 90% of the whole country and the controls are passive where monuments are used to extend new surveys. Control points are categorised by orders, from zero order, which is highest accuracy to 5th order. These categories are varying in their naming, monumentation, spacing, and specified accuracies. Method commonly applied before GPS technology was hierarchical triangular network (horizontal co-ordinates, attained cm-dm precision). The observations start by astronomical observations and base measurements, then employed other methods depending on which available survey equipment. E.g.Triangulation was employed when theodolites were available, trilateration was employed when Electromagnetic Distance Measurement (EDM) were available, traversing as well employed when theodolites and EDM are both available or total stations nowadays. The network established since colonial era, very few points are still existing. Sometimes it is hardly to locate any control point in the area of 50 square kilometres to the area outside the township instead of the normal required of one control point within two square kilometres or lesser than to cover the whole country. Some of the monuments were decimated and some are suffering from radio-waves construction made very nearby on which mostly preferred on the highest terrain.

3.1 Survey Marks
The monuments, which are known as survey marks, are trigonometric stations, pillars, benchmarks, boundary beacons, Iron pins in concrete (IPC), wooden pegs, whether above or below the surface of the ground on which it is fixed. Either can be placed or set-up by or under the direction of a land surveyor for purpose of any survey (Silayo, 1985). In all classes of Surveying in Tanzania survey marks giving the exact position plannimetric and height for main survey station should left on the ground. These vary from temporary ones, such as wooden pegs used in setting out works, to permanent ones, such as triangulation pillars and fundamental benchmarks.

The usefulness of a survey depends on the precision with which these marks are located, their permanence, the accuracy of their fixation and positional restoration should then be removed or damaged.

3.2 Problem of decimating Survey Marks
There exist a wrong belief among some tribes in Tanzania that underneath the survey marks such as pillars there is wealth of minerals such as gold, diamond, copper extra. Such people in the case of mining their assumption causes to remove, destroy displacement or alter the position of, or defacing, mutilating, obliterating the survey marks. Also development activities such as road construction and agricultural activities destroy the survey marks. A research carried out in various towns and cities in Tanzania about two decades ago gave the results that between 5% and 20% of ground monuments are destroyed every year in urban areas (Blashut T.J., 1979).

Intervisibility between the ground marked control points is oftenly lost permanently by new construction along the line of sight or blocked temporarily by parked cars causing delay in new surveys. City traffic and construction excavations causes ground movements that lead to the displacement of survey marks.

  4.0 Impact of limited funds on executing projects
Planned projects remain uncompleted for long time, and cause some conflict when being late on implementation. For example any cadastral survey in Tanzania by the law require to use town planning proposed drawings to guide any demarcation which is being done. But town-planning drawings are normally prepared using outdated base maps with neither field verification nor any details are being picked in the site. This brings about conflict with cadastral plans due to lack of parcel/plots conformity after field survey. Instead of starting with, Planning, Surveying, Building and Occupation (PSBO) we are falling in Building, Occupation, Planning and Surveying (BOPS) popularly known as squatter upgrading.

Densification of controls as a base for any subsequent survey requires enough money for monumentation and field execution especially in the traditional survey environment either aerial survey projects which are required for ground photo controls in the case of base maps preparation or field completion need enough money which are not available.

4.1 High demand of surveyed land
Demand for surveyed parcels of land in urban areas is very high for example in Tanzania, is increasing daily due to people migrating to urban centres from rural areas seeking employment and better social amenities and natural increase of population. Lack of funds for surveying parcels of land in time and the awareness of people to own the surveyed and register land, have been noticed as one major problem.

5.0 Global Positioning System (GPS)
The Global positioning System was developed to replace the U.S. Navy Navigational Satellite System (NNSS), also called TRANSIT or DOPPLAR System. This System was composed of six satellites orbiting at altitudes of about 1100 km with near circular polar orbits. The TRANSIT System was developed by the U.S. Military, primarily to determine the positions by co-ordinates of vessels and aircraft. Civilian use of this Satellite system was eventually authorised, and the system became used worldwide both for Navigation and Surveying. The GPS was developed to replace the TRANSIT system because of two major shortcomings in the earlier system.

1.The main problem with TRANSIT was the large time gaps in coverage. Since normally a satellite would pass overhead every 90 minutes, users had to interpolate their position between ”fixes” or passes.

2. The second problem with TRANSIT system was its relatively low navigation accuracy.

In contrast, GPS answers the questions ”what time, what position, and on what velocity is it?” Such questions are answered quickly, accurately, and inexpensively anywhere on the globe at any time. Remondi (1991).

5.1 Space Segment (Satellite Constellation)
The Space Segment of the system consists of 24 GPS satellites. In-order to provide a continuous global positioning capability, a scheme to orbit a sufficient number of Satellites, to ensure that at least four were always electronically visible, was developed for GPS. Figure 1 shows the system consists of constellation of 24 satellites (21 satellites constellation with 3 active on-orbit spaces) in six evenly spaced orbital planes placed in circular 12 – hours orbits inclined at 550 to equatorial plane. In any event, this constellation provides a minimum of four satellites in good geometric position at any of the 24 hours of a day anywhere on the earth’s surface. Depending on selected elevation angle, there is often more than the minimum number of satellites available to enable the receiver to compute the position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. It is during these periods of 4 satellites and above that Surveyors perform any method of GPS Surveys Figure 2. In fact, assuming a 100 elevation angle, there are brief periods where up to 10 GPS Satellites are visible on the earth’s surface.

5.2 Global Positioning System is a solution technology
Advancement of space technology lead to develop Global Positioning System (GPS), although it was intended to be used to different purposes in army activities. The alternative of surveying is another reported a timely breakthrough to rescue the aforementioned problems. It is a multipurpose system, which has done marvellously especially when the World is entering the new millennium by determining the position of any point on the earth instantaneously. Ships and aircraft track their location, velocity and time through GPS. GPS is the best for networking, provided the space between antenna of the receiver and sky is clear and at least four satellites are available. That measurement provides World Geodetic Co-ordinates (WGS84) in three dimensional co-ordinate systems that is fixed to the earth and has its origin at the centre of the earth. During measurement requires occupying at least three local known points for our case in Tanzania of UTM co-ordinate system to be used to determine parameters for transforming from WGS84 to the mapping plane.

5.3 World Geodetic System of 1984 (WGS 84).
The struggle to tie different regional datums and the advent of Satellite-based positioning systems assert the need for a global geodetic reference system.

World Geodetic System 84(WGS84) in the determination of true shape of the earth through mapping as the regional datums and the advent of Satellite-based positioning Systems, asserts the need for a global geodetic reference system.

The reference ellipsoid of WGS84 is essentially that of the International Union of Geodesy and Geophysics (IUGG) Geodetic Reference System 1980 (GRS80) with some minor changes. The International geodetic community at the IUGG’s 17th quadrennial meeting, held in Canberra, Australia in 1979, as best representing the size and shape of the earth adopted this ellipsoid.

World Geodetic System has a long history starting in the early 1960s, when the United States Department of Defense introduced the WGS60. It was created from a global Database of conventional geodetic measurements, satellite observations, data from High-Precision Short-Range Navigation (HIRAN), Airborne trilateration, or range measuring systems developed during and after World War II. In the years following the introduction of WGS60, the accuracy and number of satellite observations greatly increased and led to the development of WGS66 and subsequently WGS72.

WGS72 was initially adopted as the Conventional Terrestrial System (CTS) for describing the orbits of the Global Positioning System (GPS) Satellites in their navigation messages (signals). But as with WGS60 and WGS66, the accuracy of WGS72 eventually was found wanting. WGS72 was superseded by WGS84 and has been used for GPS navigation messages since January 1987.

GPS Nomina Constellation
24 Sattellites in 6 Orbital Planes
4 satellites in each Plane
20,200 km altitudes,55 Degree Inclination

Figure 1. Space Segment (Satellite Constellation by Dana)

[X, Y, Z & T]
Figure 2: Measurements of code phase arrival times from at least four satellites are used to estimate four unknown: position in three dimensions (X, Y, Z) and GPS time (T).
By V.B.Mugemuzi

5.4 GPS Technology in Tanzania
The first time Tanzania introduced GPS receivers, was in 1992 under the Urban Sector Engineering Project (USEP) part of which covered the topographical mapping of nine Cities in Tanzania. The said project was executed jointly by three Companies Viak AS, Blom AS and Norconsult AS all of them from Norway with the collaboration to Surveys and Mapping Division (SMD) of the Ministry of Lands and Human Settlements Development. Since then many projects have been done to increase controls in WGS84 using GPS receivers available at SMD. The long-term aim is to cover the whole country. The recent ICAO project, to establish WGS84 Controls at the airports in Tanzania, aims to provide a very precise geometrical reference for Tanzania majors Airports. This task will serve as a backbone for future surveys and the future GPS-based navigation infrastructure. The actual survey consisted of GPS measurements at 8 main airports in Tanzania; two of them are first order points located at Dar es Salaam and Mbeya airports. The remaining six are located at Kilimanjaro, Zanzibar, Mtwara, Dodoma, Mwanza and Kigoma airports. The distribution intends to provide precise co-ordinates in the international reference frame (ITRF/WGS84). The points provide important navaid positions as well as the position and orientation of the runways. NeSA B.V. from the Netherlands did the fieldwork in co-operation with the Tanzania Directorate of Civil Aviation and the full participation of Tanzanian Surveys and Mapping Division.

6.0 The Immediate Need of Tanzania
As mentioned above, Tanzania is already close to WGS84 through the ICAO project. The main thrust now is to use this basic framework to densify GPS Control networks. In order to improve on this framework, some primary levelling and gravimetric observations are urgently need to complete basic parameters for computing the reference spheroid. Local resources can not accommodate this work which rightly puts Tanzania on the Global System now termed the Global Village Reference Frame. The intended goals will be achieved through joint global efforts upon addressing such existing pressure, where moral and material support are required. The scientist congregation at Taipei, not only discussing modern technology among of members attending the conference, but further to open the eyes for investors to collaborate on projects like control densification and chances of education to accelerate surveys and mapping activities in Tanzania.

Thank you for paying attention.

References

  • Alfred Kleusberg, 1992. Precise Differential Positioning and Surveying, Department of Surveying Engineering, University of New Brunswick
  • B. Hofmann-Wellenhof, H. Lichtenegger and J. Collins, 1992. GPS theory and Practice
  • Jan Van Sickle, 1996. Global Positioning System (GPS) for Land Surveyors.
  • Venant B. Mugemuzi, 1991. The question of preserving Survey marks in Tanzania. A paper presented on the technical committee of former Ardhi Institute then University College of Lands and Architectural Studies – Dar es Salaam Tanzania.
  • Venant B. Mugemuzi, 1999. Accelerating Cadastral Survey Processes to Improve Up-to-date Digital Parcel Information in Tanzania. Professional Masters Degree report in GIS for Cadastral Applications. ITC, Enschede, The Netherlands.
  • Venant B. Mugemuzi, 2000. Interpretation of WGS84 and Advantages. A paper presented on the Tanzania Aeronautical Information Service Officer’s Association (TAISOA) being the 2nd Annual General Meeting (AGM) held on 14 April 2000 in Dar Es Salaam.