Mada za sehemu hiiSimple Survey And Map MakingMada 5
- concept of land survey
- specialised types of survey
- compass survey
- plane table survey
- Levelling survey
Since ancient time, land surveying has been used to set important marks on the land. The established marks, also referred to as control points were used to establish position of features. For instance, in ancient Egypt surveyors called rope stretchers used the control points and simple geometry to re-establish marks of boundaries swept-off by annual floods of the Nile River. The name rope stretchers originated from a marked rope which was their principal tool of survey, and the today's chain survey originated from this marked rope surveying. It was associated with making linear measurement between the established points or stations. In his work, "The sea island Mathematical Manual" published in 263 AD, the ancient Chinese mathematician, Liuhui, described ways of measuring distant objects. The work of Liuhui founded the growth of survey which was later recognized by Romans as a profession.
Thus, land surveying or geomatics as a profession can be defined as the science, art, and technology of determining the relative positions of points or features above, on, or beneath the earth's surface. The determination of such relative position involves measurements of distances, height and angles; which results to graphical or numerical presentation of measured values.
Land surveying supplies data by which accurate space-based plans and maps of the earth's surface or part are made. Survey can aid proper land management and administration in acumen. The planning of proper layout of streets and water supplying network, division of arable land for farming and other services are significant contributions of land survey.
Land survey is part and parcel of human life and his development. Land surveying aims at establishing boundaries, creating navigation maps, and creating plan and maps for different land uses. To accomplish such tasks, humans have been innovating tools for taking linear, angular, and area measurements.
While the purpose of surveying has remained the same since ancient Egyptian survey (3000 BC), surveying instrument have evolved drastically with technological development. The linear measurement instrument have evolved from chains and Gunter's steel band through steel band and metal tapes to Electronic Distance Measurement (EDM) instruments. Subsequently, the Global Positioning Systems (GPS) devices have improved efficiency and greater accuracy of measurement than any other preceding instruments.
Evolution of distance measuring instruments
Just as it was for linear measurements, instruments for angular measurements have also evolved significantly from the Egyptian Groma (which has been perpetuated by the cross-staff and its successor, the optical square) through dioptra to the compass like-instruments called astrolabe. The dioptra and astrolabe were followed by sextants, which are more professional and accurate. Evolution of modern angular measurements began with compasses that were followed by Transit, Theodolites and Total stations. A total station is a multipurpose surveying instrument that combine the functions of Transit level or Theodolite and electronic distance measurement (EDM) into a single instrument.
Evolution of angular measuring instruments
Modern angular measurement instruments use principles of electronics to calculate angles and distances. Morphologically, there is only a slight difference between Theodolite and Total station. The main difference is on their function and applications. Total station is used to measure both angles and distances while Theodolite is used to measure angles only.
However, in this chapter you will learn basic surveying techniques using basic instruments to understand principles of surveying, which also apply when using modern instruments.
Most of land surveying involves three important processes:
- reconnaissance
- fieldwork
- office work
Reconnaissance
is the first process in land surveying in which the surveyor gets a general view of the area to be surveyed. In this step, a surveyor gathers information related to the area to be surveyed then for familiarization with its landscape and get an overview of what may be required before the commencement of a fieldwork. This stage helps the surveyor in planning for the execution of the survey project, particularly in identifying appropriate survey methods, instruments, required man power, preparation of budget and time schedule.
Fieldwork
is the actual execution of survey work which involves observations and measurements of distances and angles, recording of measurements in a field notebook, preparing field sketches and performing simple calculations.
The office work
is the last process which is carried out in the office and involves correction of survey data, reduction of levels, calculation of coordinates, preparation of plans, maps and other graphics, as well as calculation of areas and volumes of Earth quantities.
Although land surveying is usually classified on the basis of multitude of criteria,
- geodetic survey
- plane survey
Geodetic land survey
assumes the Earth as curved surface (ellipsoid) and that any computation must consider the ellipsoidal nature of the surface.
Plane surveys
assume the earth's surface as flat.
However, the terms plane surveys and plane table surveying should be treated different. The earlier is a type of surveys, the later represents a technique of plane surveying. Geodetic methods are employed in solving a relatively large land masses, usually over 250 square kilometers at national and continental scale, and widely spaced monuments or features. Plane surveys on the other hand, are used in mapping areas covering less than 250 square kilometers. Contrary to geodetic survey which treats all lines joining stations as arcs, plane surveys consider all lines joining two or more points as straight lines.
A demand of high accurate data in geodetic surveys requires instrument of high precision, accuracy and economy than those employed in plane surveys.
Prior to 1970, accurate observation of angles and distances to collect spatial data in geodetic surveys was difficult and pains taking. Angles were measured using precise ground-based Theodolites while distances were measured using special tapes made from metal with low coefficient of thermal expansion. Although these instruments are still used for angles and distances measurement to date, satellite positioning has almost replaced other instruments in geodetic survey. The global positioning system relies upon signals transmitted from satellites for its operation. The entire scope of satellite systems used in positioning is referred to as global navigation satellite systems (GNSS)
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