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The Field of Surveying and the Role of the Surveyor

In its broadest sense, the term surveying encompasses all activities that measure and record information about the physical world and the environment. The term is often used interchangeably with geomatics which is the science of determining the position of points on, above or below the surface of the earth.

Humans have been undertaking surveying activities throughout recorded history. The oldest records indicate that the science began in Egypt. In 1400 BCE, Sesostris divided the land into plots so tax could be collected. The Romans also made significant developments in the field with surveying a necessary activity in their extensive building works across the empire.

The next period of major advancement was the 18th and 19th centuries. European countries needed to accurately map their land and its boundaries, often for military purposes. The UK national mapping agency, the Ordnance Survey was established at this time and used triangulation from a single baseline in the south of England to map the entire country. In the United States, the Coast Survey was established in 1807 with the remit of surveying the coastline and creating nautical charts in order to improve maritime safety.

Surveying has progressed rapidly in recent years. Increased development and the need for precise land divisions, as well as the role of mapping for military requirements have led to many improvements in instrumentation and methods.

One of the most recent advances is that of satellite surveying or Global Navigation Satellite Systems (GNSS), more commonly known as GPS. Many of us are familiar with using sat-nav systems to help us find our way to a new place, but the GPS system also has a wide range of other uses. Originally developed in 1973 by the US military, the GPS network uses 24 satellites at an orbit of 20,200 km to provide positioning and navigation services for a range of applications such as air and sea navigation, leisure applications, emergency assistance, precision timing and providing co-ordinate information when surveying.

The advances in air, space and ground based surveying techniques are in part due to the great increase in computer processing and storage capacity that we have seen over recent years. We can now collect and store vast amounts of data on the measurement of the earth and use this to build new structures, monitor natural resources and help develop new planning and policy guidelines.

There are several types of surveying:

Land Survey: The primary role of the land surveyor is to find and mark certain locations on the land. For example, they could be interested in surveying the boundary of a certain property or finding the coordinates of a specific point on the earth.

Cadastral Land Surveys: These are related to land surveys and are concerned with establishing, locating, defining or describing the legal boundaries of land parcels, often for the purpose of taxation.

Topographic Surveys: The measurement of land elevation, often with the purpose of creating contour or topographic maps.



Geodetic Surveys: Geodetic surveys locate the position of objects on the earth in relation to each other, taking into account the size, shape and gravity of the earth. These three properties vary depending where on the earth's surface you are and changes need to be taken into account if you wish to survey large areas or long lines. Geodetic surveys also provide very precise coordinates that can be used as the control values for other types of surveying.

Engineering Surveying: Often referred to as construction surveying, engineering surveying involves the geometric design of engineering project, setting out the boundaries of features such as buildings, roads and pipelines.

Deformation Surveying: These surveys are intended to ascertain whether a building or object is moving. The positions of specific points on the area of interest are determined and then re-measured after a certain amount of time.

Hydrographic Surveying: This type of surveying is concerned with the physical features of rivers, lakes and oceans. The surveys equipment is on board a moving vessel with follows pre-determined tracks to ensure the entire area is covered. The data obtained are used to create navigational charts, determine depth and measure tide currents. Hydrographic surveying is also used for underwater construction projects such as the laying of oil pipelines.

The requirements for becoming a geomatics surveyor vary from country to country. In many places, you need to obtain a license and / or become a member of a professional association. In the U.S., licensing requirements vary between states and in Canada, surveyors are registered to their province.

At present, the UK suffers from a shortage of qualified land / geomatics surveyors and many organisations have struggled to recruit over recent years.

In the UK, a graduate surveyor's starting salary usually ranges between £16,000 and £20,000. This can rise to £27,000 - £34,000 ($42,000-$54,000) once chartered status is achieved. Chartered status is gained from either the Royal Institute of Chartered Surveyors or the Chartered Institute of Civil Engineering Surveyors. A Masters degree is useful but not essential. Postgraduate qualifications also allow the opportunity to specialise in a specific area of the industry such as geodetic surveying or geographical information science. Entry to the industry with a foundation degree or Higher National Diploma is possible at lower levels such as assistant surveyor or in a related technician role.

 

Geodetic Surveying

The art of Surveying the earth surface considering its shape and size is called Geodetic Surveying . Geodetic Surveying is suitable for finding out the area of any region on the earth surface, the length and directions of the border lines, contour lines and location of basic points.

It is assumed that the shape of earth is spheroid. The convention held by the International Geodetic and Geophysical Union in 1924 assumed 41, 852, 960 ft as the earth's diameter at the equator and at the poles the diameter is 41, 711, 940 ft. Computation of the equatorial diameter was based on the fact that due to gravitational attraction the earth was flattened exactly by 1/297. Thus, measurement of distances are taken along curved surfaces and not along straight lines. Therefore for Geodetic Surveying, earth's both the diameters are considered. The latitudes and longitudes are determined considering the spheroidical shape of the earth. The points which are used to find out the shape, size and coordinates of the earth surface is called Geodetic Datum in Geodetic Surveying . Hundreds of such points are marked for carrying out Geodetic Survey.

Geodetic Surveying: Finding exact location of an object

Triangulation: As the name indicates, a triangle is incorporated to find out the location of the point in respect of latitude and longitude. The measurements of the sides of the triangle and the angles in the triangle which is drawn with respect to the particular point is found out. With the help of these measurements, longitude and latitude of the triangulation point is calculated.

Bench mark: In Geodetic Surveying, benchmarks are also used for determining the height or elevation of a point. The surveyor gives a permanent mark in the area which shows the benchmark for ages.

GPS based control station: The GPS or Global Positioning System based control station capture the radio signal given by the satellite. This signal is then processed and analyzed to find out the latitudes and logitudes of the given point.

Main instrument forGeodetic Surveying:

Theodolite: It is the basic surveying unit used for Geodetic Surveying. Theodolite consists of a telescope which is placed on a swivel and it can be rotated both horizontally and vertically. Triangulation point are determined by the theodolite in Geodetic Surveying. Two circles-one vertical and another horizontal, are used to read out the readings. But in the modern theodolite the reading is done electronically. Geodetic Surveying can be done by geographers, engineers and surveyors specialised in related disciplines.

Use of Geodetic Surveying:

Engineering purposes: The engineers uses Geodetic Surveying for finding out the exact location of the concerned point or area. Latitudes and longitudes are needed for any engineering constructions.

Construction purposes: The builders used Geodetic Surveying for finding out the direction of the buildings or their exact location for vaastu shastra.

Land Surveying and assessment: The vertical elevation and the horzontal attributes , the latitude and longitudes of the area surveyed are found out through Geodetic Surveying .

Geodetic Surveying is thus considered as an important method of Surveying.

 

Part II

Geodetic Datums

Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Hundreds of different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements.

Modern geodetic datums range from flat-earth models used for plane surveying to complex models used for international applications which completely describe the size, shape, orientation, gravity field, and angular velocity of the earth. While cartography, surveying, navigation, and astronomy all make use of geodetic datums, the science of geodesy is the central discipline for the topic.

Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters. Different nations and agencies use different datums as the basis for coordinate systems used to identify positions in geographic information systems, precise positioning systems, and navigation systems. The diversity of datums in use today and the technological advancements that have made possible global positioning measurements with sub-meter accuracies requires careful datum selection and careful conversion between coordinates in different datums.

The Figure of the Earth

Geodetic datums and the coordinate reference systems based on them were developed to describe geographic positions for surveying, mapping, and navigation. Through a long history, the "figure of the earth" was refined from flat-earth models to spherical models of sufficient accuracy to allow global exploration, navigation and mapping. True geodetic datums were employed only after the late 1700s when measurements showed that the earth was ellipsoidal in shape.

Geometric Earth Models

Early ideas of the figure of the earth resulted in descriptions of the earth as an oyster (The Babylonians before 3000 B.C.), a rectangular box, a circular disk, a cylindrical column, a spherical ball, and a very round pear (Columbus in the last years of his life).

Flat earth models are still used for plane surveying, over distances short enough so that earth curvature is insignificant (less than 10 kms).

Spherical earth models represent the shape of the earth with a sphere of a specified radius. Spherical earth models are often used for short range navigation (VOR-DME) and for global distance approximations. Spherical models fail to model the actual shape of the earth. The slight flattening of the earth at the poles results in about a twenty kilometer difference at the poles between an average spherical radius and the measured polar radius of the earth.

Ellipsoidal earth models are required for accurate range and bearing calculations over long distances. Loran-C, and GPS navigation receivers use ellipsoidal earth models to compute position and waypoint information. Ellipsoidal models define an ellipsoid with an equatorial radius and a polar radius. The best of these models can represent the shape of the earth over the smoothed, averaged sea-surface to within about one-hundred meters.

Earth Surfaces

The earth has a highly irregular and constantly changing surface. Models of the surface of the earth are used in navigation, surveying, and mapping. Topographic and sea-level models attempt to model the physical variations of the surface, while gravity models and geoids are used to represent local variations in gravity that change the local definition of a level surface.

The topographical surface of the earth is the actual surface of the land and sea at some moment in time. Aircraft navigators have a special interest in maintaining a positive height vector above this surface.

Sea level is the average (methods and temporal spans vary) surface of the oceans. Tidal forces and gravity differences from location to location cause even this smoothed surface to vary over the globe by hundreds of meters.

Gravity models attempt to describe in detail the variations in the gravity field. The importance of this effort is related to the idea of leveling. Plane and geodetic surveying uses the idea of a plane perpendicular to the gravity surface of the earth, the direction perpendicular to a plumb bob pointing toward the center of mass of the earth. Local variations in gravity, caused by variations in the earth's core and surface materials, cause this gravity surface to be irregular.

Geoid models attempt to represent the surface of the entire earth over both land and ocean as though the surface resulted from gravity alone. Bomford described this surface as the surface that would exist if the sea was admitted under the land portion of the earth by small frictionless channels.

Global Coordinate Systems

Coordinate systems to specify locations on the surface of the earth have been used for centuries. In western geodesy the equator, the tropics of Cancer and Capricorn, and then lines of latitude and longitude were used to locate positions on the earth. Eastern cartographers like Phei Hsiu used other rectangular grid systems as early as 270 A. D.

Various units of length and angular distance have been used over history. The meter is related to both linear and angular distance, having been defined in the late 18th century as one ten-millionth of the distance from the pole to the equator.

Latitude, Longitude, and Height

The most commonly used coordinate system today is the latitude, longitude, and height system.

The Prime Meridian and the Equator are the reference planes used to define latitude and longitude.

The geodetic latitude (there are many other defined latitudes) of a point is the angle from the equatorial plane to the vertical direction of a line normal to the reference ellipsoid.

The geodetic longitude of a point is the angle between a reference plane and a plane passing through the point, both planes being perpendicular to the equatorial plane.

The geodetic height at a point is the distance from the reference ellipsoid to the point in a direction normal to the ellipsoid.

Earth Centered, Earth Fixed X, Y, and Z

Earth Centered, Earth Fixed Cartesian coordinates are also used to define three dimensional positions.Earth centered, earth-fixed, X, Y, and Z, Cartesian coordinates (XYZ) define three dimensional positions with respect to the center of mass of the reference ellipsoid. The Z-axis points toward the North Pole. The X-axis is defined by the intersection of the plane define by the prime meridian and the equatorial plane. The Y-axis completes a right handed orthogonal system by a plane 90 east of the X-axis and its intersection with the equator.

Geodetic Datums

Datum types include horizontal, vertical and complete datums. Hundreds of geodetic datums are in use around the world. The Global Positioning system is based on the World Geodetic System 1984 (WGS-84). Parameters for simple XYZ conversion between many datums and WGS-84 are published by the Defense mapping Agency. Coordinate values resulting from interpreting latitude, longitude, and height values based on one datum as though they were based in another datum can cause position errors in three dimensions of up to one kilometer. Datum conversions are accomplished by various methods. Complete datum conversion is based on seven parameter transformations that include three translation parameters, three rotation parameters and a scale parameter. Simple three parameter conversion between latitude, longitude, and height in different datums can be accomplished by conversion through Earth-Centered, Earth Fixed XYZ Cartesian coordinates in one reference datum and three origin offsets that approximate differences in rotation, translation and scale. The Standard Molodensky formulas can be used to convert latitude, longitude, and ellipsoid height in one datum to another datum if the Delta XYZ constants for that conversion are available and ECEF XYZ coordinates are not required.

 


Date: 2015-12-24; view: 115


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