Limiting span/effective depth ratio = Basic span/effective depth ratio * m.f.
Actual span/effective depth ratio = Span / effective depth
If actual span/effective depth ratio < limiting span/effective depth ratio, deflection is o.k. Else (i.e. if actual span/effective depth ratio > limiting span/effective depth ratio), the section is redesigned either by providing more than required reinforcement so as to increase limiting value or increasing thickness of slab so as to reduce actual value.
Check for shear, as is the case in the section for beam.
(d) Reinforced Concrete Wall: Reynolds and Steedman (op. cit.) explains that the design procedures for reinforced concrete walls are similar to those for columns. In the case of reinforced concrete walls, the design axial force, according to BSI (op. cit.), may be calculated on the assumption that the beams and/or slabs transmitting loads into the wall are simply supported.
Furthermore, considerations must be given to the appropriate conditions stated in section 3.9.3 of the code.
(e) Pad Footing: Unlike the structural member already discussed earlier, the design procedure for pad footings involve the two limit states simultaneously. The ground bearing capacity is generally expressed at the working state (SLS); hence, the area of the foundation required to sustain the estimated load must be determined based on the working loads. Thereafter, the exerted pressure is then expressed in the SLS (Oyenuga, op. cit.).
Having decided the shape of the footing (specifically from the working drawings), the design procedures are as follows:
Calculate the plan area of the footing using the permissible bearing pressure of soil and the critical loading arrangement for the SLS.
With the appropriate values for the thickness (h) and the effective depth (d), check that the shear stress at the column face is less than 5N/mm2 or 0.8√fcu, whichever is the lesser value.
Check the thickness for punching shear, compared with that provided in the code.
Determine the ultimate net pressure at the ULS, through dividing the design ultimate loads by the area provided.
Express the ultimate net pressure in the form of a udl, and obtain a corresponding ultimate design moment using the simple cantilever equation:
M = wl2/2 (2.31)
where l = distance from the column’s centre to the edge of the slab.
Determine the reinforcement required to resist bending in the same way for beams
Make a more accurate check of the punching shear, having established νc precisely, according to section 3.7.7 of the code.
Check the shear stress at critical sections. It must be noted, as stated in clause 220.127.116.11, that the critical section in an isolated pad footing is taken as that at the face of the column or wall supported. Also, the punching shear zone is considered as the critical perimeter around the column (i.e. the perimeter at 1.5d from the column).
(f) Cantilever Retaining Wall: The design procedures for a cantilever retaining wall are generally in two stages, namely: Stability analysis – ULS, and Bearing Pressure analysis – SLS. It is well established (Oyenuga, op. cit.; Mosley et al, op. cit.; MacGinley and Choo, op. cit.) that the stability requirements are in terms of resistance to sliding and overturning: the effect of the balancing forces must outweigh that of the sliding forces, and the overturning moments generated by the sliding forces must be adequately resisted by the moments generated by the counter-balancing forces. Also, the bearing pressure conditions require that the ultimate pressures at the heel and toe of the base must not exceed the allowable bearing pressure of the soil.
The step-by-step procedure of the fundamental design principle above are as follows:
Determine the active pressure, Pa from
Pa = Ka * γ * H (2.32)
where Ka = coefficient of active pressure
For situation where angle of slope of retained material, β = 0, therefore
Ka = tan2 (45 – θ/2) ≡ (1 – Sin θ)/(1 + Sin θ) (2.33)
θ = angle of repose (internal friction) of soil
Transform all pressures to forces by multiplying with their respective area of pressure diagram.
Determine the sum of vertical forces and horizontal forces respectively.
Check for stability as follows:
μ(1.0 * N) ≥ 1.6Hf
where N = total vertical forces
Hf = total horizontal forces
μ = coefficient of friction
Resisting moment/Overturning moment ≥ 2.0
where Resisting moment = sum of moments of each of the vertical forces about a point A, and
Overturning moment = sum of moments of each of the horizontal forces about a point A.
The values – 1.6 and 2.0 – are conservative factors of safety.
Calculate the bearing pressures on the ground under the base and compare these with the permissible bearing pressures.
P = N/BD ± 6M/BD2 (2.34)
P1 = N/BD + 6M/BD2 (2.35)
P2 = N/BD – 6M/BD2 (2.36)
P1 and P2 are the upper and lower limit values, respectively, of the bearing pressure at any point along the base. P1 and P2, to satisfy bearing pressure requirements, must be less than the permissible bearing pressure of soil.
Calculate the moments on members of the entire structure and, consequently, determine the area of reinforcement required in each of them.
Check for deflection, shear, and crack conditions on the wall and heel.
(g) Drainage Box Culvert: The design procedure for this member is similar to that of retaining wall, except that the units are just subject to pressures due to earth material outside and the pressure exerted by the liquid inside.
Once the end moments have been determined from the appropriate method of structural analysis, the critical moment on each member is determined analytically. Then, the amount of reinforcement required to make the structure perform satisfactorily is provided. To achieve this requires experience and good engineering judgement. Because, though stability against floatation is important, the proportion of reinforcement and concrete must fulfil the requirements of leak-proof, deflection and freedom from cracking.
18.104.22.168 Detailing: This is the presentation of the results of the design calculations in diagrammatic form for the purpose of executing the project (Oyenuga, op. cit.). One may wonder, “After a demanding design calculations, why is detailing necessary?” Boughton (1971) answers, “because of the composition of the construction industry, there is danger in just conveying the wishes of the design engineer to other members of the construction team in any way”. Thus, to ensure that the possibility of error in conveying ideas, it is advisable that the contractor should know the precise requirements with regard to sizes and positioning of reinforcement, cover thickness, concrete strengths, e.t.c. The detail drawing gives all this information and will normally also provide dimensions and outlines of the structural unit.
Since there is a known variation in the experience of steel reinforcement fixers, it is essential that a detail drawing is easy to read and understand. It is of little use, Boughton (op. cit.) emphasises, to produce a drawing that looks impressive to an engineer but cannot be fully understood by the man actually placing and fixing the reinforcement. Structural designers, especially engineering students, must not forget the basic fact that the detail drawing is the only positive link between the design engineer and the contractor; and that the site visit and meetings are of secondary importance to good, clear detail drawings.
A detailed drawing and a thorough working knowledge of reinforced concrete detailing are of vital importance to any reinforced concrete design project. Indeed, the detailing knowledge and requirement can often affect the basic design method. Thus, detail drawing, as well as calculations, should be thoroughly checked (specifically, more). Since with calculations, a safety factor on the behaviour of the structure can compensate for an error, whereas a wrongly placed system of reinforcement or the omission of some bars due to badly produced drawing can lead to local failure.
Boughton (op. cit.) lists the following vital points that must be indicated on a good, clear detail drawings:
A reasonable scale should be adopted for each unit.
Grid lines, where used, should run in sequence on plans, numbers, e.t.c. (from top to bottom) and letters A, B, C, e.t.c. (from left to right) so that beams and columns can be easily referenced when shown in isolation on detail drawings, or called up on schedule sheets.
Plans, elevations and sections should be clearly defined.
Sections through plans should always, also, be taken in a uniform direction, usually left downward.
Dimension lines of structural units, where no general layout drawing is provided, should always be taken outside the member to avoid confusion; however, a general layout drawing is preferable in which case the reinforcement drawing will not show unit dimensions.
For clarity sake, section’s outline should be in thicker line form other than those of the plans and elevations.
Reinforcement should be in heavy line since it is the most important item on the drawing.
An indication of the reinforcement with one (or two, if alternate) typical bar only in full should be shown on plans and elevations. The bar should also be fully located on either plan or section.
Bars should be called up separately for each unit, and not repeated where a similar bar is used in another unit on the same drawing.
Bars should be referenced in their likely order of placing to make the steel reinforcement fixers’ job more straightforward.
Each drawing should start from bar mark 1.
Cover should be shown on the section where it varies from one unit to another on a drawing. Where it is constant, it can be called up in the notes.
Where only one type of reinforcement is used throughout a drawing, it is unnecessary to indicate its type on every set of bars since it can be called up in the notes’ column.
Certain standard abbreviations can be used in calling up reinforcement, e.g.
For type of reinforcement:
Mild Steel bars ----------- R
High Tension bars ------- Y
For placement of reinforcement:
Bottom face --------------- B
Top face ------------------- T
Near face ------------------ N. F.
Far face -------------------- F. F.
Each face ------------------ E. F.
Both ways ----------------- B. W.
For arrangement of reinforcement:
c/c --------------------------- Centre to centre
thro’out --------------------- Throughout
(N. B. All these should be mentioned in the notes’ column.)
The bar mark and size should be grouped in a single numeral where the diameter precedes the bar mark.
Spacing of reinforcement should be in 25mm increments from 50mm above.
Normal bar diameters used should be of the order: 6, 8, 10, 12, 16, 20, 25, 32 and 40mm.
Bars should be called up in the following manner: No. required/ Type of steel/ Diameter or size/ Bar mark/ spacing (if required)/ location/ any special consideration, e.g. 20 – R1205 – 150 c/c T means 20 no. of mild steel bars of 12mm diameter, bar mark 5, are required at 150mm centre to centre at the top of the slab.
Reynolds and Steedman (op. cit.) points out that it has long been realised that the calculated strength of a reinforced concrete member cannot be attained unless the reinforcement it contains is detailed effectively and efficiently. Unless reinforcement bars are detailed correctly, tests show that the actual strength of reinforced concrete member is considerably lower than calculations indicate. Apart from observing the points that make for a good detail drawing, considerations must be given to few interconnected salient factors that can make detailing to be effective and efficient. Basically, one must know the length and size of bars that would bring about efficient construction.
For instance, as few different sizes of bars as possible should be used, thus reducing the number of bars to be bent and placed. Also, the longest bar economically obtainable should be used, but regard should be paid to the facility with which a long bar can be transported and placed in position. Moreover, over certain lengths, it is more economical to lap two bars than to buy long bars, as there are standard length for various diameters of bar.
22.214.171.124 Scheduling of Steel Reinforcement: It is the method by which steel reinforcements are given dimensions and markings, so that information on quantity, shape and size can be provided for supply (Boughton, op. cit.). It is also referred to as bar bending schedule. The method of scheduling should be uniform throughout the bar bending schedules for any structure. Thus, all scheduling of reinforcement must conform to BS 4466: 19699.
Generally, a bar bending schedule sheet should contain the following information:
Member – the location in which the bar is used.
Bar mark – the number of the bar in its sequence on the detail drawing.
Type and size – the type of steel used and its diameter.
Number of members – the number of identical units, which can occur in each member.
Total number – the number of members × the number of bars in each member.
Length of each bar – the overall length in metres and millimetres allowing for bending tolerances e.t.c. This should always be rounded off to the nearest 5mm.
Shape -- this shows the bending of the bar with critical dimensions indicated.
Dimension columns – these relate to the dimension letters shown in shape’s picture.
It must be noted that the preparation of bar bending schedule, as part of the detail drawings, is the responsibility of the engineer (Boughton, op. cit.). Schedule sheets are used, not only by the steel supplier and steel fixers; but, also, by the quantity surveyor in order to prepare the bill of quantities (BOQ). Bar shapes should be within the range of preferred shapes shown in BS 4466(op. cit.), since bending of steel contributes considerably to the cost of reinforcement. Thus, angle cranks should be avoided, particularly on larger diameters bars, except where absolutely necessary. Bar lengths should remain within easily manageable sizes, where possible, must not exceed the maximum lengths produced by steel reinforcement manufacturers. Bar numbers should run in sequence and should not be repeated for separate bars. In general, the objective of the bar schedule is to make everybody’s job, in the construction team, a little easier.
Geographical Information System (GIS)
All forms of human activity include, and involve, a measure of geography. Whether you are a geologist seeking a well of the ‘black gold’ or a transport planner looking for the shortest and easiest route between two places, the problems you will face are the age old question of geography: where, when and how.
We, as human beings, possess a certain understanding of our immediate surroundings, i.e. our neighbourhoods and communities, through a natural sense of place. However, as we increase the scale of our vision to a local, national or international scope, our knowledge and ability to relate things decreases significantly. At such stage, it becomes a daunting task for us to make decisions based on the conflicting data obtained from our environmental features.
Nowadays, GIS technology allows us to gather and organise, analyse and manipulate, and interpret large volumes of data about geographical features in a way that greatly enhances making a well-informed decision.
2.3.2 Definition of GIS
Basically, GIS Development Centre (2000) explains that GIS is an acronym for specific terms:
Geographical – This term is used because GIS tend to deal primarily with ‘geographical’, ‘spatial’ or ‘graphical’ features. The features can be referenced or related to a specific location in space. The features may be physical, cultural or economic in nature. Features on a map, for instance, are pictorial representations of spatial objects in the real world.
Information – This represents the large volumes of data, which are usually handled within a GIS. Every geographical object has their particular set of data, which cannot be represented in full details on the map. Hence, all these data have to be associated with the corresponding spatial object so that the map can be complete. When these data are associated with respective graphical feature, they get transformed to information. This implies that all information is data, but all data are not information.
System – This term is used to represent the approach taken by GIS, whereby complex features are broken down into their component parts for ease of understanding and handling; but are considered to form an integrated whole.
Foote and Lynch (1995) state that, because of its vast areas of application, there is no single universal definition for GIS as a technology. Thus, various definitions have evolved from the various aspects of GIS. It will be worthwhile to look at some of these definitions so as to get a proper understanding of the technology.
A GIS is an information system that is designed to work with data referenced by spatial or geographical coordinates (Star and Estes, 1990). In other words, it is both a database system with specific capabilities for spatially referenced data as well as a set of operations for working with data. GIS Development Centre (op. cit.) defines GIS as a computer-based information system used to digitally represent and analyse the geographic features present on the Earth’s surface and the attributes/events (non-spatial attributes linked to the geography under study) that take place on it. GIS is a set of tools for collecting, storing, transforming and displaying geographically referenced spatial data with its corresponding attribute information to meet a specific requirement (Chouchan, 2002). From another viewpoint, Environmental System Research Institute (ESRI) (1990) defines GIS as an “organised collection/integration of computer hardware, software, geographic data and personnel designed to efficiently capture, store, update, manipulate, analyse and display all forms of geographically referenced information”.
From all these definitions, it will be noted that, “every object/feature present on the Earth’s surface can be geo-referenced” is the fundamental key of associating any database to GIS; and that the ultimate objective of GIS is the capturing, storing, checking, integrating, manipulating, analysing and displaying of these geographical data, which are spatially referenced to the Earth (Chouchan, op. cit.).
2.3.3 Fundamental of GIS
GIS is a special-purpose digital database in which a common spatial coordinate system is the primary means of reference. Thus, a comprehensive GIS, according to Burrough (1986), requires a means of:
(i) Data input from maps, aerial photogrammetry, satellites, surveys and other sources.
(ii) Data storage, retrieval and query.
(iii) Data transformation, analysis and modelling.
(iv) Data reporting such as maps, reports, plans, e.t.c.
Thus, GIS is an integrating technology, by linking a number of discrete technologies into a whole entity that is greater than the sum of its parts. This entails the components of GIS, devices needed for GIS, associated technologies for GIS, and so on.
126.96.36.199 Components of GIS
GIS Development Centre (op. cit.) explains that GIS consists of five key components listed below:
This is as illustrated in Figure 2.1 below. As clearly shown on Figure 2.1, it is seen that the GIS is a process that needs each of the components to be effective in carrying out the purpose for which it was built.
(a) Hardware – It consists of the computer devices and the computer system in which the GIS will run. The computer and its peripherals form the backbone of the GIS technology. The choice of hardware is influenced by the size of data and the project type. It may include some, or all, of these: monitor, mouse, keyboard, Central Processing Unit (CPU), scanners, digitiser, printer, plotter, e.t.c.
(b) Software – The GIS software provides the functions and tools needed to store, analyse and display geographic information. The common softwares in use are MapInfo, ArcView, AutoCAD Mapping, e.t.c. The software available can be said to be application-defined. If the user intends to carry out extensive analysis on GIS, the ArcView is the preferred option.
(c) Data – It is a collection of attributes (numeric, alphanumeric, figures, pictures) about entities (things, events, and activities). It contains an explicit geographic reference, such as latitude and longitude coordinates or an implicit reference, such as address, owner’s name, area, etc.
FIGURE 2.1 Components of GIS
(d) People – GIS users range from technical specialists, who design and maintain the system, to those who can use it to help them perform their every day’s work. The people who use can be broadly into two classes, but the more important is the class of CAD/GIS analysts, whose works are to vectorise the map objects.
(e) Method – And above all, a successful GIS operates according to a well designed plans and business rules, which are the models and operating principles unique to each organisation. There are various techniques used for map creation and further usage for any project. The map creation can either be automated raster to vector creator, or it can be manually vectorised using the scanned images. The source of these digital maps can either be map prepared by any survey agency or satellite imagery.
A GIS, as earlier defined, is a computer-based system that is used to digitally reproduce and analyse the features present on the earth’s surface and the events/activities that take place on it. In the light of the fact that almost 70% of these data has geographical reference as its denominator, it becomes pertinent to understand, or be familiar, with the technological means by which data can be represented geographically. These technologies include Global Positioning System (GPS), Remote Sensing, e.t.c; each is briefly explained below.
(a) GPS: The GPS consists of 24 Earth-orbiting satellites. These satellites, in function with a GPS receiver, allow the determination of the precise longitude, latitude and altitude of any feature anywhere on the surface of the earth (Brain and Harris, 2002).
The GPS satellites determine the coordinates by which each feature is geographically referenced from the basic concept of trilateration. Trilateration is a basic geometric principle that allows a point to be located if its distances from other already determined locations are known.
The strength of a GPS receiver lies in its ability to find the receiver’s distance from four (or more) GPS satellites. Once it determines its distance from the four satellites, the receiver can calculate its exact location and altitude on Earth. If the receiver can only find three satellites, then it can use an imaginary sphere to represent the Earth and can give you location information (latitude and longitude), but no altitude information. For a GPS receiver to know the location of any feature, it has to determine two things:
The location of at least three satellites from the feature.
The distance between the feature and each of the satellites.
To measure distances, GPS satellites send out radio signals that the GPS receiver can detect. The receiver measures the amount of time it takes for the signal to travel from the satellite to the receiver; and knowing that the signals, being electromagnetic radiations, travel at the speed of light (3.0 x 108m/s), the receiver can then basically calculate the distance between the satellites and the feature. Although, some complex mathematical models of a wide range of atmospheric conditions are involved (Brain and Harris, op. cit.). To find the satellites, the receiver simply stores an almanac that tells it where every satellite should be at any given time.
The most essential function of the GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with the information in its almanac so that it can automatically determine the receiver’s position on Earth (invariably that of the feature, since the receiver is placed on it). Thus, the basic information that GPS receiver provides is the latitude, longitude and altitude of its current position.
Hence, GPS has evolved to become a vital technology to acquire raw, positional data that can be inputted into GIS database. These data are the coordinates of the feature/point on the Earth’s surface in the form of latitude10 and longitude11 and altitude.
(b) Remote Sensing: This is the science and art of obtaining useful information (spatial, spectral, temporal) about an object, area or a point through the analysis and interpretation of image data acquired by a recording device that is not in physical, intimate contact with the object, area or point under surveillance (Chouchan, op. cit.). Simply put, Remote Sensing is any means, other than direct observation, that determines the attributes and location of a feature. This implies that, without direct contact, some means of transferring information through space must be utilised. In remote sensing, information transfer is accomplished by using electromagnetic radiation.
Remote Sensing is a complementary technology to aerial photogrammetry, whereby ‘remotely sensed’ information gathered by satellites in outer space is used for geographical analysis and cartographic production. Hence, remote sensing technology is also an important tool for the collection of geo-spatial data of an entity for use in GIS, which analyses and manages these data.
Remote Sensing produces large volumes of spatial data, which can be handled only by efficient geographic handling and processing system that will transform these data into useable information. GIS utilises these maps as its primary source of spatial data and Remote Sensing produces such spatial data in the form of maps. A typical of such maps from Remote Sensing is the map showing the coastal areas of Lagos State as shown in Figure 2.2. The map is taken by one of the various remote sensing satellites to observe and collate the major roads in Lagos and Victoria Islands. As seen from the map, the features range from barely visible to invisible, because of the altitude of the remote sensing satellites and the sizes of the features.
The proliferation of GIS is explained by its unique ability to assimilate data from widely divergent sources, to analyse trends over time, and to spatially evaluate impacts caused by development. This implies that GIS, to be effective, needs the experience and knowledge of its operators/analysts; since GIS is an extension of people’s analytical thinking. It has no in-built solutions for any spatial problems! Its work depends upon the outlined processes by the analysts. Thus far, GIS has been explained the challenge now is how it works – the process.
2.3.4 GIS Process
GIS involves complete understanding about patterns, space and processes needed to solve a problem. It is a tool acting as a means to achieve certain objectives quickly and effectively. Its applicability is realised when the user fully understands the overall spatial concept by which GIS operates and analyses his specific application in the light of that established process. This process is what is to be explained here.
Two important similar terms, but different really in GIS, are data and information. Data is a collection of attributes (numeric, alphanumeric, figures, pictures) about entities (things, events, activities). On the other hand, information is the
Figure 2.2 Map showing the coastal areas of Lagos State, Nigeria
organisation of data such that it is valuable for analysis, evaluation and decision-making. In other words, information is processed data. This implies that data, in its raw form, is not useful directly to the user. Hence, GIS involves the transformation of data to information. The process by which GIS does that entails problem definition, data acquisition, data structuring and analysis, interpretation of results, and decision-making information.
188.8.131.52 Problem Definition
GIS application is basically the customisation of existing GIS software to meet specific needs. The needs may be as simple as a set of preferences that are stored for each user, or they may be a very complex query that selects a group of layers, identify features of interest, etc. Hence, considering the vast areas of application, every GIS user must define the problem that (s)he wants to solve with the aid of GIS tool. A clear-cut definition of the problem(s) will assist in determining, at the start, whether or not the basic functions of GIS can solve it. If not, it can be programmed using the GIS macro-language for complex problems (Wahi, 2000).
Moreover, a clear-cut problem definition will help to fashion out the best ways to go about the remaining stages of the GIS process, so as to arrive at a well-informed decision and achieve the objectives. It will determine the sources of data, its method of collection, etc. It will help to identify what parameters play a significant role in the selection of spatial facts and what parameters do not.
184.108.40.206 Data Acquisition
As it can be observed from the foregoing discussion, without data there can be no GIS; because it is a technology that is data-driven. As a result, there is the need to acquire the required data from the most appropriate and reliable source in conformity with the problem definition. Data acquisition is the process of identifying data sources, collecting data, verifying collected data and inputting the verified data (Burrough, op. cit.).
The sources of data can be grouped into primary source (field work) and secondary source (other means of getting data). Usually, primary source of data is adopted. Primary source involves getting the required data from the exact location of interests, i.e. mainly by site surveys.
To carryout site surveys, the method of data collection must be identified. Data can be collected by manual (traditional) method of survey, remote sensing, use of GPS, or photogrammetry. For simplicity and cost-effectiveness, manual method is predominant in this part of the world. Surveys are conducted by technical personnel, who use compass, linear measurement devices, and maps to establish spatial location, extract spatial data, observe and record aspatial data (attributes of the spatial feature).
Upon collection of the required data (spatial and aspatial), these data need to be verified so as to improve its accuracy. Importantly, the primary requirement for the source data is that the locations for the variables are known. These locations can be annotated by x, y and z coordinate of longitude, latitude and altitude (elevation). The verification of the data must entail identifying the essential and correct data, and filtering out the irrelevant data. This stage is very important to the GIS process, because the reliability of GIS mainly depends upon the accuracy of the data collected, the way it is integrated and displayed for the purpose of extracting information for decision-making (Chouchan, op. cit.). Thus, the verification of the collected data must ensure the completeness, accuracy and consistency of these data.
The stage of data verification prepares the data to be acceptable to the GIS database. This acceptance is by inputting them into the database. Data input involves transforming the data from ‘physical’ form to ‘digital’ form. It entails keyboard entry of aspatial attributes and locational data into the system, scanning the field information into the system (i.e. converting the data from an existing map to a digital, raster representation), use of digitiser, etc. The choice of mode of data input depends on the type of data source, the database model of the GIS (scanning is easier for raster representation, while digitising is for vector representation), density of data, etc.
The success of the data acquisition stage results in a prepared data format that can be analysed modelled and restructured to achieve the desired objectives, leading to well-informed decisions.
220.127.116.11Data Structuring and Analysis
Data structuring, in GIS, involves storage, retrieval and manipulation of data, so that they can be analysed on certain basis. The GIS has a data of multiple information layers that can be manipulated, to evaluate relationships among the desired elements in a computer system. GIS uses layers, called ‘themes’, to overlay different types of information. Each theme represents a category of information. This is illustrated in Figure 2.3. From the diagram, it can be seen that GIS database handles large volume of data, process them and transform them into a wide variety of usable information: geographical, social, political, environmental and demographic. Moreover, each layer has been carefully overlaid on the others so that every location is precisely matched to its corresponding locations on all the other maps (Foote and Lynch, op. cit.).
FIGURE 2.3 Diagram showing typical layers of data in a GIS
GIS stores these layers, connected by a common geographical frame of reference, and allows the information displayed on the different layers to be compared and analysed in combination. Not all analyses will require using all of the map layers simultaneously; hence, this simple yet powerful mode of abstraction – GIS – allows the users to capture on the information that are of interest to them. For instance, with regards to figure 2.3, users may want to consider the relationship between the layers of land use and infrastructure. Furthermore, information from two or more layers might be combined and then transformed into a new layer for use in subsequent analyses.
In general, the analysis functions of GIS use the spatial and aspatial data in the database to answer questions about the real world, based on the objectives of the process (i.e. the problem definition). The analysis facilitates study of real-world processes developing and applying models. Such models illuminate the underlying trends in geographical data, and thus make new information available. Essentially, the objective of geographical data analysed is to transform data into useful information to satisfy the requirements or objectives of decision-makers at all levels. An important use of the analysis is the possibility of predicting events in another location or at another point in time. A major method of data analysis is the database query. Database query simply asks to see already stored information in the GIS database. The query may be by attribute (relational data) or by geometry (locational data). The database query is usually performed by a sophisticated function known as Standard Query Language (SQL) to search a GIS database.
The power of GIS, as it has been explained, lies in its ability to identify relationships between features based on their locations and their attributes. Upon analysis of these data, the results are displayed – waiting for them to be acted upon. For users to act on these results, they must be able to interpret them.
18.104.22.168 Interpretation of Results
GIS results are displayed in the form of digital maps, which are produced from the layers of data stored in the GIS database. These layers are stored using one of two distinctly different data models, known as raster and vector.
In raster model, according to GIS Development Centre (op. cit.), a feature is defined as set of cells on a grid. All of the cells on the grid are of the same shape and size, and each one is identified by a coordinate location and a value which acts as its identifier (features are represented by a cell or a group of cells that share the same identifier). In vector model, a feature is represented as a collection of begin and end points used to define a set of points, lines or polygons, which describes the shape and size of the feature. The vector model is particularly useful for representing highly discrete data types such as roads, building and the like.
These digital maps represent geographical features or other spatial phenomenon by graphically conveying information about locations and attributes. Locational information describes the position of particular geographical features on the Earth’s surface, as well as the spatial relationship between the features. Attribute information describes characteristics of the geographical features represented such as its name, or number, and quantitative information such as its area or length. Locational information is usually represented by:
Point Feature – for discrete feature represented as single location. It defines map object too small to show as a line, e.g. tree, telephone pole, etc.
Line Feature – is a set of connected, ordered coordinates representing the linear shape of a map object that may be too narrow to be displayed as an area, such as roads, fence, etc.
Area Feature – is a closed figure with length and width, whose boundary enclosed a homogeneous area, such as lake, state, etc.
In addition to feature locations and their attributes, other technical characteristics that define maps and their uses, and that aids in the correct interpretation of GIS results, are:
Map Scale – This indicates how much the given area has been reduced. The map scale, or extent of magnification, is expressed as a ratio. A typical type I sthe representative fraction of the form:
1: X implies that ‘1’ is a single unit of distance on the map, and ‘X’ is the distance on the ground. Examples are 1:1250, 1:250,000, etc.
Map Accuracy – This refers to the relationship between the geographical position on a map and its real-world position measured on the surface of the Earth. Many factors are responsible for this, including quality of source data, map scale, etc. The most important issue to remember about map accuracy is that the more accurate the map, the more it costs in time and money to develop.
Map Extent – The aerial extent of map is the area on the Earth’s surface represented on the map. It is the limit of the area covered, usually defined by rectangle just large enough to include all mapped features. The size of the study area depends on the map scale. The smaller the scale, the larger the area covered.
The stage of interpretation of result of analyses makes GIS more of mapping software that links information about where things are with information about what things are like, so as to give a better understanding of things.
22.214.171.124 Decision-Making Information
The old adage “better information leads to better decisions” is as true for GIS as it is for other information systems. A GIS, however, is not an automated decision-making system but a tool to query and analyse map data in support of the decision-making process. To get that information, you need the right set of tools, which GIS provide.
The decision on where, when and how to develop a land-use policy, locate a landfill, or a sewage treatment facility, build a water treatment plant, all involve a process that rely heavily not only on the understanding of critical environmental, socio-cultural, political factors, etc; but also the ability to integrate these factors into a common decision-making process for well-informed decision to be taken. Almost all of the questions and issues faced in real-world situations have a geographical component in them. Questions such as when, how, why, or what, all have an obvious or hidden geographical component. Therefore, a GIS with its ability to link and display different data sets on the basis of a common geography apparently becomes the perfect set of tools for supporting a decision-making process.
2.3.5 Application of GIS ( to Civil Engineering)
GIS’s are being used widely applied to Natural Environment, Built Environment, and Human Environment. GIS applications can be undertaken only when three-piece geographical information are collected and stored for every aspect under study: what is it, where is it and how is it related to other aspect (GIS Development Centre, op. cit.). GIS technology has enabled us to integrate social, economic, demographic and environmental database, and to understand the complexities and interrelationships between features of natural and human environments.
For Civil Engineering purposes, GIS is used in the following areas:
Planning and maintenance of transportation facilities including roadways and railways, bridges and tunnels, air and sea ports; as well as improving the efficiency of transportation means.
Land Use planning – this involves zoning policies, land acquisition, maintenance and regulation of ownership of land development regulations.
Facilities Management – this includes locating and improving the state of facilities, such as locating underground networks of pipes and water distribution.
Project Monitoring and Supervision.
GIS applications are extensive. GIS is now used in research and business for a wide range of expertise including environmental resource analysis, tax assessment, real estate analysis, archaeological analysis, natural resources management, street network etc.
VILLAGES’ WATER SUPPLY SCHEMES 3.1 Project Background
CNL is planning to provide potable water to a variety of villages in its area of operation. Thus, CNL contracted Reid Crowther, in 1999, to come up with a feasibility report on the project. Reid Crowther carried out the feasibility study with an affirmative result for the supply of water to the project area.
The project area is situated in the River Niger delta in Delta State. The project area is generally swampy and covered with mangroves, containing numerous river sub-channels. Villages are mostly constructed on small man-made or natural islands. Access into the villages is by boat or canoe.
CNL approved the report, and Reid Crowther was retained to carry out the preliminary design for the Villages’ Water Supply Scheme with the objective to provide water to the quality that meets WHO Recommendations. The preliminary design process, as required by CNL, commenced in July 2001. The villages are divided into schemes for easy design process and coordination. Scheme1 includes Tisun, KoloKolo, Deghele and Bateren; Scheme 2 includes Opia and Ikenyan; Scheme 3 includes Makaraba and Okoyitoru; while Scheme 4 includes Adagbraza and Asantuwagbene. Each design process is centred on each scheme taking the best arrangement of the water supply units which best minimise cost and allows for easy construction process, without jeopardising the objective of the project.
After submitting the preliminary design report, CNL undertook an evaluation, in collaboration with the villages of the implications of the various options Reid Crowther developed for the villages. This resulted in a set of local considerations needed to be included in a revised design report to be prepared by Reid Crowther.
A series of correspondence ensued between CNL and Reid Crowther on those issues between August 2001 and April 2002. This led to the production of Revision A of the design report on the Villages’ Water Supply Scheme, which the trainee was actively involved in, as illustrated in Figure 3.1. As it can be seen from the figure, the flow process involves abstraction from borehole, aeration, filtration, chlorination, storage and distribution.
3.2 Work Carried Out and Experience Gained
The work carried out under the production of the Revision A of the Villages’ Water Supply Scheme’s revised design report centres around the design of the reinforced concrete units associated with the water supply scheme. Specifically, they include:
Review of the water treatment flow process.
Load estimation for the structural design of the reinforced concrete members.
Roof truss analysis of filter building’s roof.
Design, detailing and preparation of bar bending schedule of beams, walls and wall footing for the aerator-supporting structure.
Design, detailing and preparation of bar bending schedule of slab, column and column footings for the filter building.
Design, detailing and preparation of bar bending schedule of retaining wall for the water treatment site.
Design, detailing and preparation of bar bending schedule of the tank supports for the treated water tank on the ground.
Design, detailing and preparation of bar bending schedule of the drainage channel for the filter building.
For the sake of brevity, complete design packages for the drainage channel in the filter building and the retaining wall of the water treatment site are provided under Appendix B and C.
The experience gained while involved in this project is enormous and includes:
A practical understanding of what detailing entails.
Bars are bought in weight specifications (in tonnes) and not in length.
Standard lengths of the various diameters of bars, namely:
6m for Ǿ8mm bars and less;
8m for Ǿ8mm – Ǿ12mm bars;
12m for Ǿ16mm and Ǿ20mm bars;
18m for Ǿ25mm bar; and
20m for Ǿ30mm bar and above.
A practical understanding of the water treatment process, especially having to understand the operations of the Davnor BioSand Filter better, as shown on Figure 3.2. As seen from the figure, the Davnor ‘BioSand’ Filter Unit is a modern innovative technological product with different operational mode from the conventional filters. Hence, there are various steps to be taken for various water conditions in order to achieve the minimum water quality standards. For instance, unlike the slow-sand and rapid-sand filters, which are used for specific raw water qualities, the Davnor ‘BioSand’ Filter Unit for all raw water qualities providing that they are used at the appropriate stage of the water treatment process.
Figure 3.2 The Davnor ‘BioSand’ Filter Treatment Unit
PROPERTY IDENTIFICATION EXERCISE FOR LAGOS STATE
Generally, big cities have big problems. From New York, Paris, London, Banjul, Beijing, Johannesburg, to Lagos, the list of mega cities is growing steadily. Among these, Lagos is ranked to be among the largest metropolitan cities in the world alongside such places as Los Angeles, Mexico city, Delhi and Peking. It is estimated, by international organisations, that Lagos, currently, is a city of about 15million people. Also, by projection, that Lagos would be home to about 25million people by the year 2015. At that time, Lagos would be the third largest city in the world behind Tokyo and Mumbai. These high figures pose great problems to government in making life meaningful to the residents (popularly called Lagosians).
In order to effectively plan for the provisions of infrastructure, such as improved health and welfare facilities, good network of roads, controlled use of land resources, improved environmental facilities, and so on, the Lagos State Government initiated the Property Identification Exercise (PIE) in February 2001. The project was contracted out to Reid Crowther through its subsidiary company (LRC Nigeria Limited).
To effectively tackle the complex and conflicting aspects of Lagos as an entity, Reid Crowther set up a GIS database for Lagos State’s geographic information on land development. This implies that the whole exercise revolves around data – its collection, storage, integration, retrieval and transformation – to arrive at well-informed decision-making policies on the geographical entity called Lagos State.
Considering the kind of society we are in, it has been agreed, right from the onset, to employ the simplest techniques to achieve the objectives of the project. Thus, against its wish of using the latest technologies such as GPS and RS, LRC has to employ manual method of data collection including taking of photographs to run the GIS process.
Work Carried Out and Experience Gained
The work carried out under the PIE project includes:
Spot location of map features on site.
Collection of geometrical data and other relevant information pertaining to land parcels and their usage.
Analysis and verification of field data for upgrading old maps to show existing features on site.
Input of field data into GIS database.
Updating and maintenance of GIS database.
The experience gained from this project revolves around a technology that is data-driven. It includes paying careful attention to minute details of data, understanding the dynamics of data, methods of collection and analysis of data, application of GIS to manage engineering projects, map reading and importance of site visits to get proper interpretation of data.