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GPS Observing Methods

The concept of satellite position ﬁxing commenced with the launch of the ﬁrst Sputnik satellite by the USSR in October 1957. This was rapidly followed by the development of the Navy Navigation Satellite System (NNSS) by the US Navy. This system, commonly referred to as the Transit system, was created to provide a worldwide navigation capability for the US Polaris submarine ﬂeet. The Transit system was made available for civilian use in 1967 but ceased operation in 1996. However, as the determination of position required very long observation periods and relative positions determined over short distances were of low accuracy, its application was limited to geodetic and low dynamic navigation uses.

In 1973, the US Department of Defense (DoD) commenced the development of NAVSTAR (Navigation System with Time and Ranging) Global Positioning System (GPS), and the ﬁrst satellites were launched

in 1978.

The system is funded and controlled by the DoD but is partially available for civilian and foreign users. The accuracies that may be obtained from the system depend on the degree of access available to the user, the sophistication of his/her receiver hardware and data processing software, and degree of mobility during signal reception.

Global Positioning System logo

In very broad terms, the geodetic user in a static location may obtain ‘absolute’ accuracy (with respect to the mass centre of the Earth within the satellite datum) to better than ±1 metre and position relative to another known point, to a few centimetres over a range of tens of kilometres, with data post-processing. At the other end of the scale, a technically unsophisticated, low dynamic (ship or land vehicle) user, with limited access to the system, might achieve real time ‘absolute’ accuracy of 10–20 metres.

The GPS navigation system relies on satellites that continuously broadcast their own position in space and in this the satellites may be thought of as no more than control stations in space. Theoretically, a user who has a clock, perfectly synchronized to the GPS time system, is able to observe the time delay of a GPS signal from its own time of transmission at the satellite, to its time of detection at the user’s equipment. The time delay, multiplied by the mean speed of light, along the path of the transmission from the satellite to the user equipment, will give the range from the satellite at its known position, to the user. If three such ranges are observed simultaneously, there is sufﬁcient information to compute the user’s position in three-dimensional space, rather in the manner of a three-dimensional trilateration. The false assumption in all this is that the user’s receiver clock is perfectly synchronized with the satellite clocks.

1. GPS Observing Methods

The use of GPS for positioning to varying degrees of accuracy, in situations ranging from dynamic (navigation) to static (control networks), has resulted in a wide variety of different ﬁeld procedures using one or other of the basic observables. Generally pseudo-range measurements are used for navigation, whilst the higher precision necessary in engineering surveys requires carrier frequency phase measurements.

The basic point positioning method used in navigation gives the X, Y, Z position to an accuracy of better than 20 m by observation to four satellites. However, the introduction of Selective Availability (SA), see below, degraded this accuracy to 100 m or more and so led to the development of the more accurate differential technique. In this technique the vector between two receivers (baseline) is obtained, i.e. the difference in coordinates (ΔX, ΔY, ΔZ). If one of the receivers is set up over a ﬁxed station whose coordinates are known, then comparison with the observed coordinates enables the differences to be transmitted as corrections to the second receiver (rover). In this way, all the various GPS errors are lumped together in a single correction. At its simplest the corrections transmitted could be in a simple coordinate format, i.e. δX, δY, δZ, which are easy to apply. Alternatively, the difference in coordinate position of the ﬁxed station may be used to derive corrections to the ranges to the various satellites used. The rover then applies those corrections to its own observations before computing its position.

The fundamental assumption in Differential GPS (DGPS) is that the errors within the area of survey would be identical. This assumption is acceptable for most engineering surveying where the areas involved are small compared with the distance to the satellites.

Where the area of survey becomes extensive this argument may not hold and a slightly different approach is used called Wide Area Differential GPS.

It can now be seen that, using DGPS, the position of a roving receiver can be found relative to a ﬁxed master or base station without signiﬁcant errors from satellite and receiver clocks, ionospheric and tropospheric refraction and even ephemeris error. This idea has been expanded to the concept of having permanent base stations established throughout a wide area or even a whole country.

As GPS is essentially a military product, the US Department of Defense has retained the facility to reduce the accuracy of the system by interfering with the satellite clocks and the ephemeris of the satellite. This is known as Selective Availability (SA) of the Standard Positioning Service (SPS). This form of degradation has been switched off since May 2000 and it is unlikely, though possible, that it will be reintroduced as there are other ways that access to the system can be denied to a hostile power. The P can also be altered to a Y code, to prevent imitation of the PPS by hostile forces, and made unavailable to civilian users. This is known as Anti-Spooﬁng (AS). However, the carrier wave is not affected and differential methods should correct for most SA effects.

Using the carrier phase observable in the differential mode produces accuracies of 1 ppm of the baseline length. Post-processing is needed to resolve for the integer ambiguity if the highest quality results are to be achieved. Whilst this, depending on the software, can result in even greater accuracies than 1 ppm (up to 0.01 ppm), it precludes real-time positioning. However, the development of Kinematic GPS and ‘on-the-ﬂy’ ambiguity resolution makes real-time positioning possible and greatly reduces the observing times.

The following methods are based on the use of carrier phase measurement for relative positioning using two receivers.

1.1 Static positioning

This method is used to give high precision over long baselines such as are used in geodetic control surveys. At its simplest, one receiver is set up over a station of known X, Y, Z coordinates, preferably in the WGS84 reference system, whilst a second receiver occupies the station whose coordinates are required.

Observation times may vary from 45 min to several hours. This long observational time is necessary to allow a change in the relative receiver/satellite geometry in order to calculate the initial integer ambiguity terms.

More usually baselines are observed when the precise coordinates of neither station are known. The approximate coordinates of one station can be found by averaging the pseudo-range solution at that station.

Artist's impression of GPS Block IIR satellite in Earth orbit

Provided that those station coordinates are known to within 10 m it will not signiﬁcantly affect the computed difference in coordinates between the two stations. The coordinates of a collection of baselines, provided they are interconnected, can then be estimated by a least squares free network adjustment. Provided that at least one, and preferably more, stations are known in WGS84 or the local datum then the coordinates of all the stations can be found in WGS84 or the local datum.

Accuracies in the order of 5 mm ±1 ppm of the baseline are achievable as the majority of errors in GPS, such as clock, orbital and atmospheric errors, are eliminated or substantially reduced by the differential process. The use of permanent active GPS networks established by a government agency or private company results in a further increase in accuracy for static positioning.

Apart from establishing high precision control networks, it is used in control densiﬁcation, measuring plate movement in crustal dynamics and oil rig monitoring.

1.2 Rapid static

Rapid static surveying is ideal for many engineering surveys and is halfway between static and kinematic procedures. The ‘master’receiver is set up on a reference point and continuously tracks all visible satellites throughout the duration of the survey. The ‘roving’ receiver visits each of the remaining points to be surveyed, but stays for just a few minutes, typically 2–10 min.

Using difference algorithms, the integer ambiguity terms are quickly resolved and position, relative to the reference point, obtained to sub-centimetre accuracy. Each point is treated independently and as it is not necessary to maintain lock on the satellites, the roving receiver may be switched off whilst travelling between stations. Apart from a saving in power, the necessity to maintain lock, which is very onerous in urban surveys, is removed.

This method is accurate and economic where there are many points to be surveyed. It is ideally suited for short baselines where systematic errors such as atmospheric, orbital, etc., may be regarded as equal at all points and so differenced out. It can be used on large lines (>10 km) but may require longer observing periods due to the erratic behaviour of the ionosphere. If the observations are carried out at night when the ionosphere is more stable observing times may be reduced.

1.3 Reoccupation

This technique is regarded as a third form of static surveying or as a pseudo-kinematic procedure. It is based on repeating the survey after a time gap of one or two hours in order to make use of the change in receiver/satellite geometry to resolve the integer ambiguities.

The master receiver is once again positioned over a known point, whilst the roving receiver visits the unknown points for a few minutes only. After one or two hours, the roving receiver returns to the ﬁrst unknown point and repeats the survey. There is no need to track the satellites whilst moving from point to point. This technique therefore makes use of the ﬁrst few epochs of data and the last few epochs that reﬂect the relative change in receiver/satellite geometry and so permit the ambiguities and coordinate differences to be resolved.

Using dual frequency data gives values comparable with the rapid static technique. Due to the method of

changing the receiver/satellite geometry, it can be used with cheaper single-frequency receivers (although extended measuring times are recommended) and a poorer satellite constellation.

1.4 Kinematic positioning

The major problem with static GPS is the time required for an appreciable change in the satellite/receiver geometry so that the initial integer ambiguities can be resolved. However, if the integer ambiguities could be resolved (and constrained in a least squares solution) prior to the survey, then a single epoch of data would be sufﬁcient to obtain relative positioning to sub-centimetre accuracy. This concept is the basis of kinematic surveying. It can be seen from this that, if the integer ambiguities are resolved initially and quickly, it will be necessary to keep lock on these satellites whilst moving the antenna.

1.4.1 Resolving the integer ambiguities

The process of resolving the integer ambiguities is called initialization and may be done by setting up both receivers at each end of a baseline whose coordinates are accurately known. In subsequent data processing, the coordinates are held ﬁxed and the integers determined using only a single epoch of data.

These values are now held ﬁxed throughout the duration of the survey and coordinates estimated every epoch, provided there are no cycle slips.

The initial baseline may comprise points of known coordinates ﬁxed from previous surveys, by static GPS just prior to the survey, or by transformation of points in a local coordinate system to WGS84. An alternative approach is called the ‘antenna swap’ method. An antenna is placed at each end of a short base (5–10 m) and observations taken over a short period of time. The antennae are interchanged, lock maintained, and observations continued. This results in a big change in the relative receiver/satellite geometry and, consequently, rapid determination of the integers. The antennae are returned to their original position prior to the surveys.

It should be realized that the whole survey will be invalidated if a cycle slip occurs. Thus, reconnaissance of the area is still of vital importance, otherwise reinitialization will be necessary. A further help in this matter is to observe to many more satellites than the minimum four required.

1.4.2 Traditional kinematic surveying

Assuming the ambiguities have been resolved, a master receiver is positioned over a reference point of known coordinates and the roving receiver commences its movement along the route required. As the movement is continuous, the observations take place at pre-set time intervals, often less than 1 s. Lock must be maintained to at least four satellites, or re-established when lost. In this technique it is the trajectory of the rover that is surveyed and points are surveyed by time rather than position, hence linear detail such as roads, rivers, railways, etc., can be rapidly surveyed. Antennae can be ﬁtted to fast moving vehicles, or even bicycles, which can be driven along a road or path to obtain a three-dimensional proﬁle.

1.4.3 Stop and go surveying

As the name implies, this kinematic technique is practically identical to the previous one, only in this case the rover stops at the point of detail or position required (Figure 9.17). The accent is therefore on individual points rather than a trajectory route, so data is collected only at those points. Lock must be maintained, though the data observed when moving is not necessarily recorded. This method is ideal for engineering and topographic surveys.

1.4.4 Real-time kinematic (RTK)

The previous methods that have been described all require post-processing of the results. However, RTK provides the relative position to be determined instantaneously as the roving receiver occupies a position.

The essential difference is in the use of mobile data communication to transmit information from the reference point to the rover. Indeed, it is this procedure that imposes limitation due to the range over which the communication system can operate.

The system requires two receivers with only one positioned over a known point. A static period of initialization will be required before work can commence. If lock to the minimum number of satellites is lost then a further period of initialization will be required. Therefore the surveyor should try to avoid working close to major obstructions to line of sight to the satellites. The base station transmits code and carrier phase data to the rover. On-board data processing resolves the ambiguities and solves for a change in coordinate differences between roving and reference receivers. This technique can use single or dual frequency receivers. Loss of lock can be regained by remaining static for a short time over a point of known position.

The great advantage of this method for the engineering surveyor is that GPS can be used for setting-out on site. The setting-out coordinates can be entered into the roving receiver, and a graphical output indicates the direction and distance through which the pole-antenna must be moved. The positions of the point to be set-out and the antenna are shown. When the two coincide, the centre of the antenna is over the setting-out position.

1.4.5 Real-time kinematic on the ﬂy

Throughout all the procedures described above, it can be seen that initialization or reinitialization can only be done with the receiver static. This may be impossible in high accuracy hydrographic surveys or road proﬁling in a moving vehicle. Ambiguity Resolution On the Fly (AROF) enables ambiguity resolution whilst the receiver is moving. The techniques require L1 and L2 observations from at least ﬁve satellites with a good geometry between the observer and the satellites. There are also restrictions on the minimum periods of data collection and the presence of cycle slips. Both these limitations restrict this method of surveying to GPS friendly environments. Depending on the level of ionospheric disturbances, the maximum range from the reference receiver to the rover for resolving ambiguities whilst the rover is in motion is about 10 km, with an achievable accuracy of 10–20 mm.

For both RTK and AROF the quality of data link between the reference and roving receiver is important. Usually this is by radio but it may also be by mobile phone. When using a radio the following issues should be considered:

In many countries the maximum power of the radio is legally restricted and/or a radio licence may be required. This in turn restricts the practical range between the receivers.
The radio will work best where there is a direct line of sight between the receivers. This may not always be possible to achieve so for best performance the reference receiver should always be sited with the radio antenna as high as possible.
Cable lengths should be kept as short as possible to reduce signal losses.

Principle of Least Squares Applied to Surveying

12:18 PM Civil Engineering , Surveying

1. Introduction

‘Least squares’ is a powerful statistical technique that may be used for ‘adjusting’ or estimating the coordinates in survey control networks. The term adjustment is one in popular usage but it does not have any proper statistical meaning. A better term is ‘least squares estimation’ since nothing, especially observations, are actually adjusted. Rather, coordinates are estimated from the evidence provided by the observations.

The great advantage of least squares over all the methods of estimation, such as traverse adjustments, is that least squares is mathematically and statistically justifiable and, as such, is a fully rigorous method.

It can be applied to any over determined network, but has the further advantage that it can be used on one-, two- and three-dimensional networks.A by-product of the least squares solution is a set of statistical statements about the quality of the solution. These statistical statements may take the form of standard errors of the computed coordinates, error ellipses or ellipsoids describing the uncertainty of a position in two or three dimensions, standard errors of observations derived from the computed coordinates and other meaningful statistics described later.

The major practical drawback with least squares is that unless the network has only a small number of unknown points, or has very few redundant observations, the amount of arithmetic manipulation makes the method impractical without the aid of a computer and appropriate software.

The examples and exercises in this material use very small networks in order to minimize the computational effort for the reader, while demonstrating the principles. Real survey networks are usually very much larger.

A‘residual’ may be thought of as the difference between a computed and an observed value. For example, if in the observation and estimation of a network, a particular angle was observed to be 30°0' 0'' and after adjustment of the network the same angle computed from the adjusted coordinates was 30° 0' 20'', then the residual associated with that observation would be 20''. In other words:

computed value − observed value = residual

Any estimation of an over determined network is going to involve some change to the observations to make them fit the adjusted coordinates of the control points. The best estimation technique is the one where the observations are in best agreement with the coordinates computed from them. In least squares, at its simplest, the best agreement is achieved by minimizing the sum of the squares of the weighted residuals of all the observations.

2. Least squares applied to engineering

In practical survey networks, it is usual to observe more than the strict minimum number of observations required to solve for the coordinates of the unknown points. The extra observations are ‘redundant’ and can be used to provide an ‘independent check’ but all the observations can be incorporated into the solution of the network if the solution is by least squares.

All observations have errors so any practical set of observations will not perfectly fit any chosen set of coordinates for the unknown points.

Some observations will be of a better quality than others. For example, an angle observed with a 1'' theodolite should be more precise than one observed with a 20'' instrument. The weight applied to an observation, and hence to its residual, is a function of the previously assessed quality of the observation.

In the above example the angle observed with a 1" theodolite would have a much greater weight than one observed with a 20" theodolite. How weights are calculated and used will be described later.

If all the observations are to be used, then they will have to be ‘adjusted’ so that they fit with the computed network. The principle of least squares applied to surveying is that the sum of the squares of the weighted residuals must be a minimum.

2.1 A simple illustration

A locus line is the line that a point may lie on and may be defined by a single observation. Figure1(a), (b) and (c) show the locus lines associated with an angle observed at a known point to an unknown point, a distance measured between a known point and an unknown point and an angle observed at an unknown point between two known points respectively. In each case the locus line is the dotted line. In each case all that can be concluded from the individual observation is that the unknown point lies somewhere on the dotted line, but not where it lies.

Principle of Least Squares applied to Surveying

Figure 1 Locus lines

Figure 2 Intersection of locus lines

In the following, the coordinates of new point P are to be determined from horizontal angles observed at known points A, B, C and D as in Figure 2(a). Each observation may be thought of as defining a locus line. For example, if only the horizontal angle at A had been observed then all that could be said about P would be that it lies somewhere on the locus line from A towards P and there could be no solution for the coordinates of P. With the horizontal angles at A and B there are two locus lines, from A towards P and from B towards P. The two lines cross at a unique point and if the observations had been perfect then the unique point would be exactly at P. But since observations are never perfect when the horizontal angles observed at C and D are added to the solution the four locus lines do not all cross at the same point and the mismatch gives a measure of the overall quality of the observations. Figure 2(b) shows the detail at point P where the four lines intersect at six different points. The cross is at the unique point where the sum of the squares of the residuals is a minimum.

2.2 The mathematical tools

By far the easiest way to handle the enormous amounts of data associated with least squares estimation is to use matrix algebra. In least squares it is necessary to create a system of equations with one equation for each observation and each ‘observation equation’ contains terms for each of the coordinates of each of the unknown points connected by the observation. So, for example, in a two-dimensional network of 10 points where there are a total of 50 observations there would be a set of 50 simultaneous equations in 20 unknowns. Although this represents only a small network, the mathematical problem it presents would be too difficult to solve by simple algebraic or arithmetic methods.

Setting Out Construction Works with Satellite Positioning

8:50 PM Civil Engineering , Surveying

Satellite positioning

A 1960s surveying text book consulted in 1980 would reveal little change in twenty years. That is not true today, with modern technology, systems and software are being continually updated. Nowhere is this more obvious than with satellite positioning.

It has exploded onto the construction market changing some operations beyond all recognition. As technology improves, accuracy increases and costs come down it becomes more economical to employ it on smaller and smaller jobs.

Setting Out Construction Works with Satellite Positioning

Real-time kinematic (Image courtesy: The Pennsylvania State University)

What is it?

Satellite positioning is the determination of the position of a point using a satellite receiver. Satellite positioning is generally known as GPS or global positioning system after the American military system, which was first available for public use.

Unlike most surveying and setting out tasks, the skill required of the operator is minimal. The skill with GPS is with the management of the system’s input and output data. The satellite receiver does all the work in gathering the data and outputting or storing it as required. With setting out it can provide the operator with predetermined setting out coordinates.

How accurate is it?

Accuracy depends on the methods employed and the equipment used. For construction setting out centimetre level accuracy is achievable. This makes it suitable for many setting out tasks. Unlike traditional survey methods, each point is independent of the points around it, and therefore each point is of a similar accuracy. Degradation of accuracy (due to creep) with distance from the main station is no longer a problem.

If used in unsuitable conditions, accuracy may be compromised. An error in one point is not passed on to adjacent points.

What are the advantages?

When used for setting out, a single engineer with a setting out pole equipped with a satellite receiver can set out points almost as fast as he can mark them. With a road centre-line for example, the operator can walk the route and mark centre-line points at whatever frequency is required. The setting out information can be taken straight from the design on disk without the need to input a mass of figures. Work is unaffected by weather or daylight or a lack of it. Visibility between points is not required, so local obstructions (shrubbery, mechanical plant, low buildings, walls etc.) do not hinder the process. Productivity increases are considerable. As well as giving plan coordinates (Eastings and Northings), it will automatically provide heights as a matter of course. Satellite systems can also be integrated into computer-controlled plant, in which, for example, a grader has the road design in its memory. The grader blade is automatically adjusted to give the correct earthwork profile. This eliminates the need for a complete setting-out team along with their instruments, forest of timberwork, chainmen and their transport.

What are the disadvantages?

Cost is always an issue, but this has to be balanced against productivity. GPS is not suited to all locations. Due to the fact the position of the receiver is derived from observing a number of satellites, a clear view of the sky is necessary. This may make GPS unsuitable for city centre sites shielded by adjacent tall buildings. A received signal may give inaccurate results if deflected off the side of a building.

GPS is not suitable for tunnelling work. However GPS can be used very efficiently to establish a control either side of an obstruction under which tunnelling is required. GPS does not work well in tree-covered areas, again due to the need for a clear line of sight to the sky.

The height element of the output is of a lower order of accuracy than the plan coordinates. Additionally, heights given are not above mean sea level (as with traditional levelling), but above the mathematical model of the Earth, WGS84 (World Geodetic System 1984). Unfortunately, for Europe this does not run parallel to mean sea level. However the GPS output can be configured to give correct information.

GPS is not sufficiently accurate enough to obtain the 1mm precision that can be achieved with a theodolite.

Checklist

Is the site suitable for using GPS?
Are there obstructions to the horizon?
Can design information be obtained on disk?
Can the GPS system accept design information in the given format?
Is the output configured in such a way as to give heighting information that is correct for the site? If necessary check known benchmarks using GPS.
Will GPS give the required accuracy for the task?

Overview: Open and Closed Traverses in Surveying

2:47 PM Civil Engineering , Surveying

1. Background

A traverse is a form of control survey used in a wide variety of engineering and property surveys. Essentially, traverses are a series of established stations tied together by angle and distance. Angles are measured by theodolites or total stations; the distances can be measured by electronic distance measurement (EDM) instruments, sometimes by steel tapes. Traverses can be open, as in route surveys, or closed, as in closed geometric figures (Figures 1 and 2). Traverse surveying has been one of the most basic survey types being used for ages by surveyors. Closed traverse survey using total station has particularly become popular in recent times. It is however pivotal to understand the underlying concepts of open traverse calculations and closed traverse calculations.

Figure 1: Open traverse

Traverse computations are used to do the following: balance field angles, compute latitudes and departures, compute traverse error, distribute the errors by balancing the latitudes and departures, adjust original distances and directions, compute coordinates of the traverse stations, and compute the area enclosed by a closed traverse. In modern practice, these computations are routinely performed on computers and/or on some total stations or their electronic field books/data collectors. In this article, we will perform traverse computations manually (using calculators) to demonstrate and reinforce the mathematical concepts underlying each stage of these computations.

Figure 2: Closed traverse or loop traverse

In engineering work, traverses are used as control surveys

(1) to locate topographic detail for the preparation of topographic plans and engineering design plan and profiles,

(2) to lay out (locate) engineering works, and

(3) for the processing and ordering of earthwork and other engineering quantities. Traverses can also help provide horizontal control for aerial surveys in the preparation of photogrammetric mapping.

1.1 Open Traverse

An open traverse (Figure 1) is particularly useful as a control for preliminary and construction surveys for highways, roads, pipelines, electricity transmission lines, and the like. These surveys may be from a few hundred feet (meters) to many miles (kilometers) in length. The distances are normally measured by using EDM (sometimes steel tapes). Each time the survey line changes direction, a deflection angle is measured with a theodolite or total station. Deflection angles are measured from the prolongation of the back line to the forward line (Figure 1); the angles are measured either to the right or to the left (L or R), and the direction (L or R) is shown in the field notes, along with the numerical values.

Figure 3: Field notes for open traverse

Angles are measured at least twice to eliminate mistakes and to improve accuracy. The distances are shown in the form of stations (chainages), which are cumulative measurements referenced to the initial point of the survey, 0 + 00. See Figure 3 for typical field notes for a route survey. Open traverses may extend for long distances without the opportunity for checking the accuracy of the ongoing work. Thus, all survey measurements are repeated carefully at the time of the work, and every opportunity for checking for position and direction is utilized (adjacent property surveys and intersecting road and railroad rights-of-way are checked when practical.) Global positioning system (GPS) surveying techniques are also used to determine and verify traverse station positioning.

Many states and provinces have provided densely placed control monuments as an extension to their coordinate grid systems. It is now possible to tie in the initial and terminal survey stations of a route survey to coordinate control monuments. Because the Y and X (and Z) coordinates of these monuments have been precisely determined, the route survey changes from an open traverse to a closed traverse and is then subject to geometric verification and analysis. Of course, it is now also possible, using appropriate satellite-positioning techniques, to directly determine the easting, northing, and elevation of all survey stations.

1.2 Closed Traverse

A closed traverse is one that either begins and ends at the same point or begins and ends at points whose positions have been previously determined (as described above). In both cases, the angles can be closed geometrically, and the position closure can be determined mathematically. A closed traverse that begins and ends at the same point is called a loop traverse (Figure 2). In this case, the distances are measured from one station to the next and verified, using a steel tape or EDM instrument. The interior angle is measured at each station, and each angle is measured at least twice. Figure 4 illustrates typical field notes for a loop traverse survey. In this type of survey, distances are booked simply as dimensions, not as stations or chainages.

Figure 3: Field notes for open traverse

What is Surveying?

1:46 PM Civil Engineering , Surveying

Surveying is the art and science of taking field measurements on or near the surface of the Earth. Survey field measurements include horizontal and slope distances, vertical distances, and horizontal and vertical angles. In addition to measuring distances and angles, surveyors can measure position as given by the northing, easting, and elevation of a survey station by using satellite-positioning and remote-sensing techniques. In addition to taking measurements in the field, the surveyor can derive related distances and directions through geometric and trigonometric analysis.

Once a survey station has been located by angle and distance, or by positioning techniques, the surveyor then attaches to that survey station (in handwritten or electronic field notes) a suitable identifier or attribute that describes the nature of the survey station. Attribute data for a survey station can be expanded from a simple descriptive label to include a wide variety of related information that can be tagged specifically to that survey station.

Since the 1980s, the term geomatics has come into popular usage to describe the computerization and digitization of data collection, data processing, data analysis, and data output. Geomatics includes traditional surveying as its cornerstone, but it also reflects the now-broadened scope of measurement science and information technology. Figure 1 shows a computerized surveying data model. This illustration gives you a sense of the diversity of the integrated scientific activities now covered by the term geomatics.

Figure 1: Geomatics data model, showing the collection, processing, analysis, design, and plotting of geodata.

The vast majority of engineering and construction projects are so limited in geographic size that the surface of the Earth is considered to be a plane for all X (easterly) and Y (northerly) dimensions. Z dimensions (height) are referred to a datum, usually mean sea level. Surveys that ignore the curvature of the Earth for horizontal dimensions are called plane surveys. Surveys that cover a large geographic area—for example, state or provincial boundary surveys—must have corrections made to the field measurements so that these measurements reflect the curved (ellipsoidal) shape of the Earth. These surveys are called geodetic surveys. The Z dimensions (orthometric heights) in geodetic surveys are also referenced to a datum—usually mean sea level.

In the past, geodetic surveys were very precise surveys of great magnitude, for example, national boundaries and control networks. Modern surveys (data gathering, control, and layout) utilizing satellite-positioning systems are geodetic surveys based on the ellipsoidal shape of the Earth and referenced to the geodetic reference system (GRS80) ellipsoid. Such survey measurements must be translated mathematically from ellipsoidal coordinates and ellipsoidal heights to plane grid coordinates and to orthometric heights (referenced to mean sea level) before being used in leveling and other local surveying projects.

Engineering or construction surveys that span long distances (e.g., highways, railroads) are treated as plane surveys, with corrections for the Earth’s curvature being applied at regular intervals (e.g., at 1-mile intervals or at township boundaries). Engineering surveying is defined as those activities involved in the planning and execution of surveys for the location, design, construction, maintenance, and operation of civil and other engineered projects.* Such activities include the following:

1. Preparation of surveying and related mapping specifications.

2. Execution of photogrammetric and field surveys for the collection of required data including topographic and hydrographic data.

3. Calculation, reduction, and plotting (manual and computer-aided) of survey data for use in engineering design.

4. Design and provision of horizontal and vertical control survey networks.

5. Provision of line and grade and other layout work for construction and mining activities.

6. Execution and certification of quality control measurements during construction.

7. Monitoring of ground and structural stability, including alignment observations, settlement levels, and related reports and certifications.

8. Measurement of material and other quantities for inventory, economic assessment and cost accounting purposes.

9. Execution of as-built surveys and preparation of related maps, plans, and profiles upon completion of the project.

10. Analysis of errors and tolerances associated with the measurement, field layout, and mapping or other plots of survey measurements required in support of engineered projects.

Engineering surveying does not include surveys for the retracement of existing land ownership boundaries or the creation of new boundaries. These activities are reserved for licensed property surveyors—also known as land surveyors or cadastral surveyors.

*Definition adapted from the definition of engineering surveying as given by the American Society of Civil Engineers (ASCE) in their Journal of Surveying Engineering in 1987.

Difficulties faced during leveling in surveying

3:29 PM Civil Engineering , Surveying

Different difficulties are faced during leveling which makes the surveying process difficult to carry. Types of difficulties and their methods to overcome are discussed.

Leveling is a part of surveying, in which relative heights of different objects on or below the earth surface are measured. The undulations of ground can be measured using leveling. But it is not such an easy job.

Different ground conditions, lot of natural formations, structural parts etc. are interfered while leveling a ground. They provide difficulties to the surveyor and staff men. In such cases, there are some methods to resolve the difficulties and to continue the whole work.

Different Situations Those Makes Leveling Difficult
Some situations making leveling difficult are:

When the staff is close to the instrument
Leveling across a large lake
Leveling across a river
Leveling a cross a solid wall
Bench mark lies above line of collimation
Leveling along steep slope
Leveling across depression or rising ground

When the Staff is Close to the Instrument

Sometimes it is necessary to place the staff very near to the instrument for leveling. In that case, the instrument loses its focus and becomes blur. It is difficult to note down the exact staff reading.

This problem arises whenever the staff is as close as below the focusing limit. This can be solved by using white paper. Holding staff vertical and a white paper is slowly moved up and down in front of it.

The paper edge is observed through telescope and stopped at a point where the line of collimation meets edge of white paper. The staff reading at that point is noted.

Difficulties faced during leveling in Surveying

Leveling Across a Large Lake
If a large pond or lake is there across the leveling line, then it is difficult to perform the leveling across that pond. In that case, some assumptions are to be made to continue the work.

Generally, in ponds or lakes water is in still position so, we can consider it as a level surface. Two pegs are fixed at the two banks of pond as the top surface of peg is visible or coincide with water surface.

Now, the staff is placed at first peg and using level, RL is measured by taking foresight (FS). Then we assumed that the RL of second peg point is as same as first peg point. Finally, the level is shifted to the other bank side and back sight (BS) reading is taken from second peg point.

Leveling Across a River
In a river, water always flows so, we cannot consider it as leveled surface like in pond or lake. The water levels are different at two banks of a river. For this problem to resolve, reciprocal method of leveling is used.

Two pegs are fixed at two banks of river and RL of 1st peg positioned is taken in usual manner. RL of 2nd peg position is measured by obtaining the true difference level between two pegs using reciprocal leveling.

Leveling Across a Solid Wall

When a solid wall is interfered in leveling, it is difficult to measure RL of the points in that wall position. In such a case, two pegs are driven on both sides of the wall as the staff just touching the wall. Now find out the height of the instrument is measured by focusing any bench mark or Champion point.

Then, read the staff reading on one side of the wall note down it. Then using staff measure the height of the wall. Now we have all the data from which RL of every point can be calculated. Similarly, do this process on the other side of the wall.

Benchmark lies Above Line of Collimation
When the bench mark lies above the line of collimation such as on the bottom of bridge girder, lower surface of culvert etc., in such cases, fore sight reading of point A is taken first using the level positioned at O.

Now the staff is held at benchmark (under the girder) in inverted position from the girder. Now the back-sight reading is noted from the inverted staff and it is described with negative sign with showing “staff held inverted” in remarks column.

Leveling Along a Steep Slope
When it is required to perform leveling along steep slope areas like hilly regions it is difficult to obtain same values in back sight and fore sight readings. This problem can be resolved by positioning level set up in zigzag paths.

The instrument position in that zigzag path may be decided by keeping back sight and fore sight reading equal. Finally, RL of every point is calculated. This can be done for both positive and negative steep slopes.

Leveling Across Depression or Rising Ground

When a high-rise ground or depressed ground is reached while leveling, then the instrument should not be kept at peak positions like peak rise or lowest of ground. In case of rising ground, the instrument is fixed in a position such that the line of collimation should pass through the apex point of that high-rise ground.

Similarly, in case of depressed ground, the instrument is positioned at one side of depressed portion and staff is positioned at other side of depression.

Azimuths and Bearings in Surveying and their Comparison

12:00 AM Civil Engineering , Surveying

Azimuths

Figure :1 Azimuths

Azimuths are horizontal angles observed clockwise from any reference meridian. In plane surveying, azimuths are generally observed from north, but astronomers and the military have used south as the reference direction. Examples of azimuths observed from north are shown in Figure 1. As illustrated, they can range from 0° to 360° in value. Thus the azimuth of OA is 70°; of OB, 145°; of OC, 235°; and of OD, 330°.

Azimuths may be geodetic, astronomic,magnetic, grid, record, or assumed, depending on the reference meridian used.To avoid any confusion, it is necessary to state in the field notes, at the beginning of work, what reference meridian applies for azimuths, and whether they are observed from north or south. A line’s forward direction can be given by its forward azimuth, and its reverse direction by its back azimuth. In plane surveying, forward azimuths are converted to back azimuths, and vice versa, by adding or subtracting 180°.

For example,

If the azimuth of OA is 70°, the azimuth of AO is 70°+180° = 250°

If the azimuth of OC is 235°, the azimuth of CO is 235° - 180° = 55°

Azimuths can be read directly on the graduated circle of a total station instrument after the instrument has been oriented properly. This can be done by sighting along a line of known azimuth with that value indexed on the circle, and then turning to the desired course. Azimuths are used advantageously in boundary, topographic, control, and other kinds of surveys, as well as in computations.

Bearings

Figure :2 Bearings

Bearings are another system for designating directions of lines. The bearing of a line is defined as the acute horizontal angle between a reference meridian and the line.The angle is observed from either the north or south toward the east or west, to give a reading smaller than 90°.The letter N or S preceding the angle, and E or W following it shows the proper quadrant.Thus, a properly expressed bearing includes quadrant letters and an angular value.An example is N80°E. In Figure 2, all bearings in quadrant NOE are measured clockwise from the meridian. Thus the bearing of line OA is N70°E. All bearings in quadrant SOE are counterclockwise from the meridian, so OB is S35°E. Similarly, the bearing of OC is S55°W and that of OD, N30°W.When lines are in the cardinal directions, the bearings should be listed as “Due North,” “Due East,” “Due South,” or “Due West.”

Geodetic bearings are observed from the geodetic meridian, astronomic bearings from the local astronomic meridian, magnetic bearings from the local magnetic meridian, grid bearings from the appropriate grid meridian, and assumed bearings from an arbitrarily adopted meridian. The magnetic meridian can be obtained in the field by observing the needle of a compass, and used along with observed angles to get computed magnetic bearings.

Figure :3 Forward & Back Bearings

In Figure 3 assume that a compass is set up successively at points A, B, C, and D and bearings read on lines AB, BA, BC, CB, CD, and DC. As previously noted, bearings AB, BC, and CD are forward bearings; those of BA, CB, and DC, back bearings. Back bearings should have the same numerical values as forward bearings but opposite letters.Thus if bearing AB is N44°E, bearing BA is S44°W.

Comparison of Azimuths & Bearings

Because bearings and azimuths are encountered in so many surveying operations, the comparative summary of their properties given in Table 1 should be helpful. Bearings are readily computed from azimuths by noting the quadrant in which the azimuth falls, then converting as shown in the table.

Table :1 Comparison of Azimuths and Bearings

Book: Fundamentals of Surveying by S. K. Roy

3:11 PM Books , Civil Engineering , Miscellaneous Books , Surveying , Surveying Books

Modern surveying involves use of sophisticated scientific instruments. mathematical methods and computational techniques. In writing this book on surveying, I have tried therefore to explain comprehensively the principles of surveying instruments and derivation of mathematical formulae. Separate chapters have been written on 'Underground Surveys' and 'Computer Programs in Surveying' to incorporate the recent developments in this field.

Book: Fundamentals of Surveying by S. K. Roy

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Book: Engineering Surveying 5th Edition by W. Schofield

11:50 AM Books , Civil Engineering , Miscellaneous Books , Surveying , Surveying Books

Surveying may be defined as the science of determining the position, in three dimensions, of natural and man-made features on or beneath the surface of the Earth. These features may then be represented in analog form as a contoured map, plan or chart, or in digital form as a threedimensional mathematical model stored in the computer. This latter format is referred to as a digital ground model (DGM).

In engineering surveying, either or both of the above formats may be utilized in the planning, design and construction of works, both on the surface and underground. At a later stage, surveying techniques are used in the dimensional control or setting out of the designed constructional elements and also in the monitoring of deformation movements.

In the first instance, surveying requires management and decision making in deciding the appropriate methods and instrumentation required to satisfactorily complete the task to the specified accuracy and within the time limits available. This initial process can only be properly executed after very careful and detailed reconnaissance of the area to be surveyed. When the above logistics are complete, the field work – involving the capture and storage of field data – is carried out using instruments and techniques appropriate to the task in hand. The next step in the operation is that of data processing. The majority, if not all, of the computation will be carried out by computer, ranging in size from pocket calculator to mainframe. The methods dopted will depend upon the size and precision of the survey and the manner of its recording; whether in a field book or a data logger. Data representation in analog or digital form may now be carried out by conventional cartographic plotting or through a totally automated system using a computer-driven flat-bed plotter. In engineering, the plan or DGM is used for the planning and design of a construction project. This project may comprise a railroad, highway, dam, bridge, or even a new town complex. No matter what the work is, or how complicated, it must be set out on the ground in its correct place and to its correct dimensions, within the tolerances specified. To this end, surveying procedures and instrumentation are used, of varying precision and complexity, depending on the project in hand.

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Types of Surveys

5:54 PM Civil Engineering , Surveying

Surveying is the science and art of making the measurements necessary to determine the relative positions of points above, on, or beneath the surface of the earth or to establish such points. Surveying continues to undergo important changes.

Plane surveying neglects curvature of the earth and is suitable for small areas.

Geodetic surveying takes into account curvature of the earth. It is applicable for large areas, long lines, and precisely locating basic points suitable for controlling other surveys.

Land, boundary, and cadastral surveys usually are closed surveys that establish property lines and
corners. The term cadastral is now generally reserved for surveys of the public lands. There are two major categories: retracement surveys and subdivision surveys.

Topographic surveys provide the location of natural and artificial features and elevations used in map making.

Route surveys normally start at a control point and progress to another control point in the most direct manner permitted by field conditions. They are used for surveys for railroads, highways, pipelines, etc.

Construction surveys are made while construction is in progress to control elevations, horizontal positions and dimensions, and configuration. Such surveys also are made to obtain essential data for computing construction pay quantities.

As-built surveys are post construction surveys that show the exact final location and layout of civil
engineering works, to provide positional verification and records that include design changes.

Hydrographic surveys determine the shoreline and depths of lakes, streams, oceans, reservoirs, and other bodies of water.

Sea surveying covers surveys for port and offshore industries and the marine environment, including measurement and marine investigations by ship-borne personnel.

Solar surveying includes surveying and mapping of property boundaries, solar access easements, positions of obstructions and collectors, determination of minimum vertical sun angles, and other requirements of zoning boards and title insurance companies.

Satellite surveying provides positioning data and imagery, which is received by equipment, stored, and automatically verified in selected data coordinates with each satellite pass. Doppler and global positioning are used as standard practice in remote regions and on subdivided lands.

Global positioning system (GPS) utilizes a constellation of 24 high-altitude navigational satellites positioned in six orbital planes and spaced so that an operator of specialized equipment can receive signals from between five to eight satellites at all times.

Inertial surveying systems acquire coordinate data obtained by use of a helicopter or ground vehicle. Inertial equipment now coming into use has a dramatic impact on the installation of geodetic and cadastral control.

Photogrammetric surveys utilize terrestrial and aerial photographs or other sensors that provide data and can be a part of all the types of surveys listed in the preceding.

Basic Concepts of Maps

5:16 PM Civil Engineering , Surveying

SURVEY

1. Survey can be defined briefly as the science of measurement with the object of increasing our knowledge about the shape of earth and the features upon its surface. Mostly the end product in survey operation is a map.

MAP

2. A map is a representation of ground and different features and other information about ground on a paper, cloth, plastic sheet. Some of the uses of maps are as follows:-

a. Tourism

b. Road distance information

c. Navigation

d. Operational planning

e. Communication

f. Geological information

g. Climate information

h. Administration

i. Irrigation system

j. Engineering projects

k. Cadastral records

l. Environmental applications etc

TYPES OF MAPS

3. Maps are designed and made for different uses. Types depend on the nature of use. Some types are given as follows;

a. Topographic Maps

i. Small Scale Maps

ii. Large Scale Maps

b. Photo Maps

c. Relief Maps

TOPOGRAPHICAL MAPS

4. The topographical map is the one which shows all the information about natural and artificial objects, usually including such relief of the country as can within reason to be shown at scale employed. Scales vary between 1/5000 to 1/1000000 depending on the purpose for which the map is made.

a. Small Scale Maps: These maps show large area with small details.

b. Large Scale Maps: These maps show small area with greater details.

c. Some of the uses of Large Scale Maps are as follows:

i. Engineering Applications.

ii. Cadastral Purposes

iii. Any other special uses where greater details are required.

PHOTO MAPS

5. These are made by mosaics of aerial photographs and by satellite images.

RELIEF MAPS

6. These are 3-D maps made using different mediums like plastic etc.

SIGN CONVENTIONS

7. Different symbols have been designed to show different ground objects on maps. These symbols are listed on the bottom of every map in the form of a key or legend for guidance of users of maps.

INDEX TO MAP

8. All maps have been numbered on a regular system. At the bottom of each map the index shows the numbering of adjacent map sheets so that the users can conveniently demand the adjacent map sheets in case he has to work in that area.

MAP GRID SYSTEM

9. It comprises of a series of squares formed by vertical and horizontal lines called ‘Eastings’ and ‘Northings’ which are serially numbered e.g. 31,32,33 etc.

Grid System is used to identify the position of any ground point or object on map by a four figure or six figure references. On small scale map (1:million scale etc.) the series of squares are formed by longitudes and latitudes called graticules.

GEOGRAPHIC COORDINATE SYSTEM

10. The location of a point can also be indicated anywhere on the surface of Globe by its Longitude and Latitude. Longitudes are vertical lines joining the two poles of the Globe. These are numbered 0˚- 180˚ East and 0˚- 180˚ West of the prime meridian passing through Greenwich.

11. Latitude are horizontal lines running parallel to Equator and these are numbered 0˚- 90˚ North and 0˚- 90˚ South of Equator. On small scale maps we do not have grid lines and position of a location is indicated in terms of Longitude and Latitude.

TYPES OF NORTH

12. Different types of north are as follows:-

a. True North: The direction of North Pole from observer.

b. Grid North: The direction in which the grid lines point towards the top of map.

c. Magnetic North: It is the direction in which the compass needle points towards the magnetic pole.

d. MAGNETIC DECLINATION/VARIATION: The true north line and magnetic north line at a place seldom coincide with each other due to which there is a difference between them. This difference is called magnetic declination.

BEARING

13. Angle measured clockwise from a certain fixed North-South Line to an object.

SCALE

14. The proportion which the distance between any two points on map bears to the horizontal distance between the same two points on the ground.

METHODS OF EXPRESSING SCALE

a. IN WORDS

Words explain the distance on map that represents a certain distance on ground.

e.g 1 Inch=1 Mile etc.

b. BY REPRESENTATIVE FRACTION

The distance on map is represented by a fraction of corresponding distance on ground.

e.g 1:50,000 , 1/10,000 etc.

c. BY SCALE LINE

By drawing a scale line showing the digits or parts for measuring distance on the map

d. DIAGONAL SCALE

By drawing this type of scale line it is possible to measure distances accurately to the fraction of a secondary division of a scale line to a required standard

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