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.
