LESSON ON Procedures of Photogrammetric Mapping

Procedures of Photogrammetric Mapping

The procedures in photogrammetric mapping include project planning (recce), flight planning (or image acquisition planning), imaging, ground control (ground truth) establishment, image restitution (interior, relative, absolute and exterior orientations), QA/QC analysis, collection of vector data and final map production.

 1 FLIGHT PLANNING FOR AERIAL IMAGE COVERAGE

PHOTOGRAMMETRIC PROCEDURES

As presented in the block schematic above, photogrammetry, the science of measurement by aerial photographs, consists of five stages which are summarized below:

Site Identification and flights/image-acquisition planning. Aerial surveying is generally carried out over large areas which involve extensive photography or image coverage. Such surveys are, therefore, made by the government, organizations or large private companies. In general, prior to the acquisition of the images, the site must be identified, possibly using existing maps or mosaics. All the possible ways of covering the area with images are considered and the most appropriate method selected. For aerial photography, the imaging will usually be done in strips and so the process(of image acquisition) will involve the determination of the number of images per strip and the number of strips to cover the area. Of course, flight planning makes use of the photographic parameters such as focal length, image frame size, flying height and desired ground sampling distance (gsd) and the amount of overlaps needed both between successive images and between strips. Consideration for photo scale is also made. The photo scale is directly linked with the accuracy of the map which is the final product. After agreeing on the map accuracy with the client, the scale of the aerial photography is then determined. For topographical maps of small and medium scales, the height accuracy attainable and the image quality are the governing factors for the choice of the scale, whereas for large scale maps, planimetric accuracy is more important. The largest scale of maps that can be produced photogrammetrically in an economic way is 1: 500. It is therefore essential to ensure that both planimetric and height accuracy requirements, are met with the decided photo scale. The flying height of the camera depends on the scale of the photography. For simple plotting, the average scale of the photography is generally kept the same or slightly larger than the desired scale of the final compiled map. Hence, by knowing the average scale of the photography and the scale of the compiled map, the flying height of the aircraft above mean ground level, may be calculated. If planimetric accuracy is important, the flying height depends on the photo scale chosen to meet the planimetric accuracy. On the other hand, if height accuracy is of prime importance, the flying height may be first decided, depending on the required contour interval. The flying height is often related to the contour interval of the final map.

The following points may be noted :

(i) The area of ground covered by each photograph increases with the increase of the flying height of the aircraft and hence less number of photographs are required for any particular area of land.

(ii) The scale of the photography increases with the decrease in the flying height and consequently more details for greater height accuracy are obtained.

(iii)Due to increased flying height, the haze and dust reduce the quality of the photographs.

(iv)The cost of the flying at greater flying height is excessive as compared to that of low flying height.

The details of this process are given later. In the case of satellite imagery, the total number of frames to cover the area is calculated from the available frame size, and the amount of overlaps that exist on the images. In some cases, only one frame or two may be sufficient for the purpose when the frame is large.

LESSON ON PHOTOGRAMMETRIC PROCEDURES PART 3

NUMBER OF PHOTOGRAPHS NECESSARY TO COVER A GIVEN AREA

In the preliminary estimate, the number of photographs required is calculated by dividing the total area to be photographed by the net area covered by a single photograph.

Let       A = total area to be photographed.

            l = length of the photograph in the direction of flight

            w = width of the photograph normal to the direction of flight

            s = scale of photograph =  (i.e., 1 cm = s metres)

            L = net ground distance corresponding to l

            W = net ground distance corresponding to w

            a = net ground area covered by each photograph = L x W

P_t = percentage overlap between successive photographs in the direction of flight (expressed as a ratio)

P_w = side lap (expressed as a ratio).

Since each photograph has a longitudinal lap of  the actual ground length (L) covered by each photograph is given by

                                                                                                  (43)

Similarly, the actual ground with (W) covered by each photograph is given by                                                                                                     (44)

Hence the ground area (a) covered by each photograph

                            (45)

The number of the photographs (N) required is given by      

N = A/a                                                                        (46)

If, however, instead of the total area A, the rectangular dimensions (i.e., length and width) of the ground are given, the number of the photographs required are computed by calculating the number of strips and the number of photographs required in each strip and multiplying the two.

Let       L₁ = dimension of the area parallel to the direction of flight

            L₂ = dimension of the area normal to the direction of flight

            N₁ = number of photographs in each strip

            N₂ = number of strips required

            N = total number of photographs to cover the whole area.

Now net length covered by each photograph = L = (1 – P_l) sl

  Number of photographs in each strip is given by

                                                                 (47)

Similarly, net width covered by each photograph = W = ( 1 – P_w) sw

Hence the number of the strips required are given by

                                                              (48)

Thus, the number of photographs required is

                          (49)

Lesson Note On Principles of Town planning

Principles of Town Planning

Town planning cannot be studied in isolation. It involves the study of various subjects such as engineering, architecture, surveying, transportation planning etc. The intention of the town planning is to satisfy the needs of our future generations and prevent the haphazard growth of the town. Some of the guiding principles of town planning are as follows:

1. Zoning

The town should be divided into suitable zones such as commercial zone, industrial zone, residential zone, etc and suitable rules and regulations should be formed for the development of each zone.

2. Green belt

Green belt is non-development zone on the periphery of the town. It prevents the haphazard sprawl of the town restricting its size. In essence, a green belt is an invisible line designating a border around a certain area, preventing development of the area and allowing wildlife to return and be established. Greenways and green wedges have a linear character and may run across the town and not around the town.

3. Housing

Housing has to be carefully studied and designed to suit the local population. Care should be taken to see that there is no development of slums since it would be responsible for degrading the life of the citizens. There are various types of housing styles. When a landuse plan is made, zones for independent housing, midrise buildings, high rise buildings are allocated. Landuse maps are of two types. Type 1 helps us study the landuse on a broad range. All we can see are the residential, commercial and recreational zones.

4. Public buildings

Public buildings should be well grouped and distributed throughout the town. Unnecessary concentration of public buildings should be avoided. Factors such as parking facilities, road widths have to be taken into consideration while allocating the space for public buildings.

5. Recreation centres

Recreation centres have to be given importance while designing a town. They are necessary for the recreational activities of the general public. They include parks for walking and cycling, amusement parks etc.

6. Road systems

Road network hierarchy is very important. The efficiency of any town is measured by the layout of its roads. A nicely designed road system puts a great impression in the minds of people, especially the visitors to the town. The provision of a faulty road system in the initial stages of town formation proves to be too difficult and costly to repair or to re-arrange in future.

7. Transport facilities

The town should be provided with suitable transport facilities so that there is minimum loss of time from place of work to the place of residence. Efficiency in transport facilities includes both public and private networks. Public transportation network includes access to buses, trains, trams and trolleybuses. Efficiency in using the public transport will determine the success of that town in terms of design.

Town planning has gained a lot of importance today. New towns are being developed. It has become very important for the town planners to concentrate on old development as well as the new development. It is essential that old and new development are linked properly. Energy efficiency in planning should be the goal of any town planner, urban designer or an Architect.

LAND USE PLANNING IN NIGERIAN

LAND USE PLANNING IN NIGERIAN

          Introduction

Land use planning is historically about management of problemsemanating from growth and change. At the inception of physical; planning, its activities were directed towards improving the health, safety and visual quality of the physical environment. The British government introduced land use planning practice in Nigeria during their rule in 1860-1960, but the Lagos township improvement ordinance of 1863 actually marked the beginning of physical or land use planning in Nigeria, and was aimed art controlling developments and urban sanitation in Lagos.

The Lagos Township ordinance was extended to cover the entire countryafter being amended and renamed as the township ordinance No. 29 of 19 17, and was concerned with the classification control, and regulation ofconstruction of buildings, streets, and bridge within them. The wide scopegiven to it gave rise to it’s being seen as the first systematic attempt at land use planning in Nigeria. It’s main aim was the board principles of municipal responsibility, segregation between European rules and the African, Environmental Hygiene and Sanitation.

In 1928, the Lagos Township ordinance was enacted following the outbreakof bubonic plague in Lagos. As a result, the Lagos Executive Development Board was setup in 1929 as the authority responsible for planning and development Lagos. 1928 town and country planning ordinance (No. 4) based on the 1932 town and country planning act of the United Kingdom.

Shortly after independence in 1960, there was increase in population which resulted in the growth of towns and cities, and this necessitates some control mechanism to ensure orderly development of settlements inNigeria.

In 1978, land use act was enacted to curb land speculates, ease process of land acquisition by government. The 1946 town and country planning law remained in force until 1992 when the first indigenous urban and regional planning law was enacted (Decree 88 of 1922). The law provides a binding legal basis that is most current and matching the tempo of changes for physical and development activities throughout the country.

          ROLE AND FUNCTION OF LAND USE PLANNING

i.        The main task of town planning is to reconcile competing claims for the use of limited land so as to provide a consistent, balanced and orderly development of land use (Foley, 1960’s).

ii.      The central function of land use planning is to provide a good physical environment, a physical environment of such a good quality meaning and development essential for the promotion of a healthy and civilized life.

iii.   Town planning as part of a broader social program (Foley, 1960) Also function to provide low density residential areas, faster local community life and control sprawl or conurbation growth.

          NATURE OF LAND USE IN NIGERIA

In most urban areas, land use has not effectively put under control. Most developers adopt various means to get structures in place in any part of the city.

Most Nigeria towns are undergoing rapid urbanization process; master plans are by nature too rigid to change easily in period of urbanization. Thus no sooner are the prepared that they become out of context with reality because of rapid development associated with cities of the 20” century.

          ZONING SYSTEM

Zoning system is a process through which a community divides itself into two or more district or zones allowing only designated land uses in each district and imposing certain development activity. Adopting a zoning ordinance is an exercise of regulating powers and ordinance provision must be reasonable, based on a comprehensive plan and related to health, safety, morals and general welfare of the community. Zoning is one of themany measures of government to guise the use and development of privately owned real estate.

Rural zoning is not nearly as wide spread as urban zoning, it early use was to reduce public service costs and prevent personal hardship in land cleared areas.

In Nigeria, especially the Federal Capital City (FCC) zoning is considered a useful tool for combating problems such as:

i.                   Protecting public health and safety by preventing development that could lead to ground and surface water pollution, keeping flammable structures at distance apart.

ii.                 Preservation and developing the economic base. This is of particular significance in areas where economic existence is based on natural resources, such as agriculture, recreation or running.

iii.              Improving the quality of the physical environment by screening or eliminating unsightly uses of land use as junk poles and damps, prevent lake pollution etc

iv.              Protecting properly values from being undermined by the development of incompatible uses on adjacent or nearly properties.

          ZONING REGULATION

There are three ways to categorized regulation contained in zoning ordinance.

i.                   The first distinguished between regulating provisions that seek to control the location of different kinds of land uses and those intended to influence characteristics controlling provisions concerning physical aspects within the boundaries of the plot, or tract of land involved as such as the relative position of improvement and the amount of open space.

ii.                 Zoning regulations can also be categorized according to the geographic area of application.

iii.              The third classification separates zoning restriction according to the physical characterized that are affected such as:

a. Those intended to determine population density such as lot size and coverage and type of dwelling.

b. Those concerning land structure relationships minimum set back distance, frontage, and side yard and backyard requirements for residence are almost universal.

c. Those directly controlling land use list of permitted, conditionally permitted, and prohibited uses are set-aside in certain land areas.

Understanding Engineering Surveying, (Alignment and Route Location). Steps/Procedure

Understanding Engineering Surveying, (Alignment and Route Location). Steps/Procedure

In the course of road construction, before finalizing the Alignment of our highways, there are generally four (4) guidelines for the Alignment and the Route Location also known as the Engineering Surveying of the highway that must be carried out. These include

1.     Desk Study Or Map Study  
2.    Reconnaissance (Recce) Survey 
3.    Preliminary Survey
4.   Final Survey (Location Survey)

1.  Desk Study or Map Study:-

           STEPS:

                                 i.      Get the topographic map of area (if available), the likely intended route of the road   may be marked on it.

                               ii.      Study all available information in the office for contours, ponds, valleys, lakes, mountain pass, bridge etc.

                              iii.    Study town planning master plan drawing if available

                             iv.      Also you can Study the Aerial photographs and remote sensing imagery of the area (if available) they are great source of information on topography, terrain, drainage etc.

      2     Reconnaissance Survey:-

             

     After desk study, Reconnaissance Survey is next, it start with a field inspection by walk or driving  or both and necessary information should be collected.it can be conducted in the following  

procedures:

                                 i.            Detail of route vis-à-vis topography of the area, whether its hilly or rolling or plain

                               ii.            Length of the road along various alternatives

                              iii.            Bridging requirements (number, length etc.)

                             iv.            Geometrics: 

·         Gradients that are feasible, specify the extent of deviation if any,

·         Curves and hair pin bends, etc.

                               v.            Right of way available

                             vi.            Existing means of communication i.e. car track, path, mule path etc.

                            vii.            Terrains and soil conditions i.e.

·         Geology of the area

·         Drainage characteristics of the area including area susceptible to flooding

·         General elevation of the road

·         Location of sand dunes.

·         Type and extent of vegetation, etc.

                          viii.            Climatic condition

                             ix.            Facilities/Resources

         3.      Preliminary Survey:

This is a relatively large scale investigation conducted to collect all physical information       which affects the proposed location of the new highway. During this phase of survey, the topographic   features and other features like houses, places of worships, etc. are tied to the transverse line. Longitudinal sections and cross-sections are taken and bench marks established. This data form the basis for determination of final center line of the road.

Procedures:

                                 i.            It starts with the running of a traverse along the selected route, adhering as far as possible to the probable final center line of the road, in difficult situations, a secondary transverse may be introduced.

N.B: Transverse consists of a series of straight lines with their distances and intermediate angles measured very carefully.

                               ii.            Transverse should be done with theodolite and all angles measured with double reversal method. The distance along the transverse line should be measured with metallic tape or chain.

                              iii.            Physical features like buildings, monuments, place of worships, schools rail line etc. that are likely to be affected by the project should be located by means of offsets measured from the transverse line.

                             iv.            Leveling work should be kept to the minimum. Generally, fly levels at 50m intervals, and at all intermediate breaks in ground should be taken along the transverse line.

      4.     Final Survey (Location Survey)
 

The purpose is to fix the center line of the road or selected alignment in the field and to collect additional data for the preparation of the drawings. if preliminary survey is properly done,final survey data collection will be limited

Procedure:

 

         i.            Staking (pegging) out the final center line of the road by means of a continuous transit survey

·         Process whereby the center line of the road as determined in the design office, is translated on the ground by means of a continuous transit survey and staking or pegging of the center line as survey proceed, fixed at 50m intervals in plain and rolling terrain, hilly, 20m intervals. All angles measured with theodolite

       ii.            Detailing leveling

Lesson Note On Establishing a site master benchmark

Establishing a site master benchmark 

One of your fist tasks as a site engineer on a new site is to establish height control. 

It is usual to establish a site Master Bench Mark (MBM) from nearby Ordnance Bench marks and the following lists the usual procedure 

a) The Master Bench Mark must be constructed in apart of the site clear of any construction activity and must be of very stable construction. 

A typical MBM comprises a steel pin firmly driven into the ground and surrounded by concrete. Provide protection by surrounding it with a fence and the reduced level of the bench mark should be written on that fence as well being recorded in the field book. 

b) The reduced level of the MBM is found by flying leveling from a nearby Ordnance Bench Mark (OBM) using the MBM as a change point (intermediate sights are not permitted) and then closing the circuit onto a second bench mark, if such exists. This not only provides the closing error but also checks the OBM for any possible movement.

 c) The leveling operation must be carried out a minimum of three times, ensuring that a good closing error is obtained each time. The range of results must not exceed 5 mm, if it does then further leveling must be carried out until such a range is achieved. The mean value can then be adopted as the MBM level. 

d) Having established the MBM, any number of temporary bench marks (TBM) can be located and leveled where and as required, working to the same order of accuracy as above. 

The ideal situation regarding TBMs is to locate them in such positions that a level can be set up anywhere on the site and be within sighting distance from it, thus avoiding the need to fly level to the area of operation.