As per ToR specification and requirement, Physical Surveys will cover the following items:
Detailed methodologies of conducting physical surveys are described in the following sections below.
Survey manager along with survey & equipment experts, GPS and Total Station surveyors will be mobilized after the finalization and approval of the survey/work plan by the project authority of CLIENT. As per TOR, the Study area and project boundary shall be carried out by the fifth month and the overall survey work is expected to be completed by the end of 11th month.
A large number of advanced survey equipment including Real Time Kinematic Global Positioning System (RTK-GPS) and Differential Global Positioning System (DGPS), Total Station (TS), Optical Levels will be deployed for conducting topographic, physical feature & land-use surveys. A local area network (LAN) comprising numbers of Pentium-IV desktop computers, colored printer, plotter and scanner will be used for data processing and mapping purposes at field and Dhaka office. The equipment, computer & peripherals are listed in below.
GPS and Total Station based advanced survey technique will be used for conducting physical feature, topographic, physical infrastructure survey. GPS and Total Station survey techniques to be used for conducting all type of physical survey are presented as below:
The Global Positioning System (GPS) is worldwide all-weather radio-navigation and positioning system formed from a constellation of 24 satellites and their 5 nos. ground control & monitor stations. GPS receivers use these US Navigation Satellites for Timing and Ranging (NAVSTAR) to calculate positions accurate to mater of meters. GPS receives radio waves, modulated for positioning, transmitted by a maximum number of 24 satellites, which enables to work out the distance between satellite and observation points. By receiving radio waves from four satellites simultaneously it is possible to find out the three-dimensional co-ordinates and time (UTC) of the observation point with an accuracy level which cannot be conceived in traditional ground survey. The facility of GPS has been utilized in different kinds of ground surveys including geodetic, topographic and hydrographic survey in the recent times. GPS based survey with its computer based data storage and processing facility on and off the field offers immense flexibility in map production under a GIS environment. To ensure precision and accuracy in survey work and to facilitate georefence/digital map production by GIS software and finally to complete the whole work in a rather shortened time schedule, GPS technology was the best and logical approach to be followed.
Differential Global Positioning System (DGPS) and Real-Time Kinematic Global Positioning System (RTK-GPS) are different versions of GPS technology, each with its own range of applicability and accuracy level. DGPS option gives about ±one meter accuracy data and RTK option deliver ±centimetre position accuracy data. GPS based surveying has a number of advantages over conventional surveying methods. These are: Highly accurate, Very fast, Line of sight not required, Unified 3-dimentional global co-ordinate system (x,y,z) output and Digital/Computerized data storage, processing facility
1.1.3.2 Total Station Survey Technique
Electronic Total Station (TS) is combination of electronic theodolite, distance meter and leveling Machine with on-board computer having graphic icon menu with LCD display and built-in MS-Dos operating system. It can measure and store the positioning data of a target point in digital form. It consists of a microprocessor with special software for operation, data capture, storage & processing, transmission and receiving to/from a computer. The data can be stored in internal memory or in external memory card. It transmits ledger beam towards the target where a reflector (i.e. prism) is placed and receives the reflected beam by which calculate the distance, bearing and 3-D coordinate of that target point with respect to the reference points whose coordinates are known. Measurements to be done by a Total Station survey technique are as below:
· Distance measurement.
· 3-dimensional co-ordinate measurement (x, y, z).
· Traverse-style co-ordinate measurement.
· Resection.
· Offset measurement.
· Missing line measurement.
· Remote elevation measurement.
Mathematics of total station measurements to calculate the
parameters of target point is presented below:
Where, Instrument Position = N0, E0 & Z0, Target Position= N1, E1 & Z1, Slope Distance= S, Zenith Angle= qz, Azimuth Angle= qh Instrument Height= Mh Target Height= Ph
1.1.3.3 Establishment of Bench Marks (BM)
For GPS and Total Station Survey, establishment of adequate and uniformly distributed Bench Mark is very crucial. Since all the subsequent survey operations are dependent on and related to the bench mark, any error simply multiplies and compounds to a menacing total deviation. As such accuracy of Bench Mark coordinate values both along horizontal and vertical axes is of utmost importance.
Survey being a continuous and time-intensive operation, maintenance of the bench marks has many beneficial aspects. Since these bench marks will be used in physical feature, engineering infrastructure, land use and topographic survey of the project area for preparation of the detailed area plan as well as for the survey works that will be carried out in implementation of the planned works in future, preservation of the bench marks will ensure consistency and conformity of the past, present and future works. During selection of the bench mark locations, these considerations need to be considered.
Besides, as foliage of tall trees prevents satellite signals from reaching the receiving GPS sets, which interferes with data quality, availability of free open sky around will also be considered during selection of BM locations. These BM pillars will be used frequently during secondary/temporary BM establishment survey as well as subsequent other surveys. As such accessibility is another high consideration during the location selection process.
At least 20 nos. of Marks (BM)/Control Points (CP) having three dimensional co-ordinates (Northing, Easting and Reduce Level in mPWD/mMSL) will be established covering the entire project area to carry out the total survey activities. Establishment of BMs comprises the following item of works:
· Construction and Installation of BM pillars.
· Establishment of Co-ordinate of BM Pillars (x, y, z i.e Northing, Easting & RL in mMSL).
The BM pillars will be constructed and installed before the survey work start. The construction design and specification of such BM pillars shall be finalized as per suggestion of Client. The BMs will be established with uniformly distributed grid covering the total project area. However in consideration of the criteria to be followed, listed below, some adjustments in positioning of the pillars will be necessary.
· Accessibility at all times.
· Availability of open sky for good satellite signals.
· Secure place for long term preservation.
· Local resistance to installation of pillars at private lands.
The location and the number of BM pillars will be selected by a comprehensive Joint (client-consultant) reconnaissance survey of the project area. In case of any difficulties of installation of BM pillars, the control point of that particular location will be established and marked on an existing permanent structure such as bridge, culvert, monument etc. After establishing the co-ordinates of the BM, the corresponding surveyed values (x, y z) will be marked on the face of each BM pillars as shown below (Figure-2).
Figure-2: Sample of the BM pillar /ground point
Establishment of co-ordinates {x, y, z i.e latitude/northing, longitude/easting & Reduce Level (RL) in Mean Sea Level (MSL)}of BM Pillars needs extensive GPS survey, BM carry and data processing work. The total work comprises the following items:
· Selection of reference BM (x, y, z)
· Baseline survey by RTK-GPS Static Method.
· Network Adjustment
· Development of Geoid Model.
Selection of existing reference BM inside or around the project area is essential for establishment of new BM network for the project area. Reference BM provides georeference (x, y) and elevation (z) with respect to a datum i.e. the co-ordinates of the BM pillars. For establishing co-ordinates of the new BMs the available SoB BMs inside or periphery of the project area suggested by the client will be used as reference BM. The BM information will be collected from SoB by the consultant before survey starts. SoB BMs are geo referenced National Bench Marks/Control Points established by JICA (Japan International Cooperative Agency) in association with Survey of Bangladesh (SOB). These BMs are very precisely surveyed and established both in horizontal and vertical datum. In the period of 1992 to 1995, a total of 141 numbers of Bench Marks/Control Points in an approximate grid of 30km were installed and established by JICA. Those 141 Bench Mark covers most of the areas of Bangladesh except the coastal region. Later on numbers of BMs were also established covering the country. Some of these are also available inside and periphery of the project area. Information of these BMs will be collected from SoB and used as reference BM for establishing new BM network for the project area.
The map showing the location and the detail list of the SoB-JICA BMs covering the entire Bangladesh is enclosed as Figure-3
Figure-3: Location Map of SoB-JICA BMs
Baseline Survey by RTK-GPS Static Method
The Baseline survey is the simultaneous data collection in static mode at two or more fixed points using two or more duel frequency GPS receivers. The measurement network for RTK-GPS baseline survey will be planned by connecting the BM/Control Points to be established and the selected SoB reference BM points (known Latitude, longitude and ellipsoidal height) available inside and around the project area. A line connecting two measurement points is known as baseline. It is important to emphasis that the configuration of network was based on practical considerations rather than requirements of an ideal network.
The GPS measurements consists a simultaneous static
measurement with dual frequency GPS receivers at the ends of a baseline
concerned. Measurement or logging time for a session is usually one hour. During the measurements the GPS receivers at
the two points record the information or data (Latitude, Longitude, Ellipsoidal
Height) on the configuration of available satellite at the time, which at the
end of day’s work will be processed using Trimble Geomatic Office software. If
results from the field measurements found unacceptable, measurements will be
repeated.
Figure-4: RTK Static Survey set-up for Baseline Survey
The verified results of each baseline will be stored for the subsequent network adjustment. After completing the baseline survey, network adjustment will be done with respect to the known values (Latitude, Longitude, and Ellipsoidal Height) of selected SoB reference BMs available inside and around the project area. The adjustment module of Trimble Geomatic Office software will be used for network adjustment. After network adjustment the precise co-ordinates (Latitude/Northing, Longitude/Easting, and Ellipsoidal Height) of each BM will be obtained.
With the output co-ordinates of the BMs in latitude/nothing, longitude/easting and ellipsoidal height from the network adjustment result, a network of geoid points will be established for the project area.
Database Format of BM
The output point shape file format for Bench Marks in GIS database would be as follows:
Shape File Name: R_BM [Benchmark Pillar], R represents RangpurA Geoid is a representation of the earth surface over which the earth’s gravity is constant. If the value of earth gravity on the geoid is the value of the gravity on the average sea level, then the geoid represent the mean sea level of the corresponding area.
With the output co-ordinates of the BMs in latitude, longitude and ellipsoidal height from the network adjustment result, a network of geoid points was established for the project area. However, to be a precise geoid model the ellipsoidal heights were converted to the corresponding geoidal heights. Furthermore, in order to develop the Geoid Model of the project area in terms of a functional datum i.e. mPWD/mMSL, it was required to determine the heights or levels of the BMs in mPWD/mMSL datum.
As in Figure-5 in the context of Bangladesh, the SoB datum or height is equivalent to orthometric height and could be calculated from the corresponding geoidal height and ellipsoidal height. The corresponding mathematical relation is shown below:
N = H + h
Where, N = Geoidal height, H = Orthometric height, h = Ellipsoidal height
Figure-5: Earth Surfaces (Topographic, Ellipsoidal & Geoid)
Some scientist measured the earth’s gravity in a grid of several kilometres all over the world and then made a mathematical model, which is known as “World Geoid Model”.
To establish a relationship between the ellipsoidal height (h) & geoidal height (N) of an area, the relationship between the world geoid model and local land topographic (undulation) characteristics of that area was required. The following procedure was applied for that purpose:
Using the network adjustment data of baseline survey as input the “World Geoid Model” the geoid separation or geoidal height (N) for each of the surveyed points (BM point) was obtained. Then the orthometric height (H) was calculated by using the above equation. The combination of “World Geoid Model” and the surveyed data made a new model known as “Local Geoid model”.
Secondly it was needed to make a relationship between the orthometric height (H) and the local datum (mPWD/mMSL). To do so, with the output co-ordinates of the BMs in Latitude, Longitude and Ellipsoidal Height from baseline survey results, the vertical heights will be established with respect to MSL datum To do so, few numbers of BMs those covers the boundary conditions of Local Geoid Model were needed to be surveyed with respect to national datum (mPWD/mMSL) from one or more SoB reference BM available inside or around the project area using BM carry survey by auto level machine as shown in Figure 6. Then an input data file for the “local geoid model” which consists of Latitude, Longitude, Ellipsoidal height, and mPWD/mMSL height will be prepared. Incorporating this input file to the Local Geoid Model a relationship data file will be established between the orthometric height (H) and local datum (mPWD/mMSL). This data file is known as “local model input data file”.
Finally, if the Latitude, Longitude, and Ellipsoidal height of a particular point (whose local datum i.e. mPWD/mMSL datum is unknown) was used as input to this “Local Geoid Model” in association with the local model input data file, then the mPWD/mMSL height of that unknown point will be calculated by the model.
Figure-6: 1st Order BM Carry/Level Survey
Database Format of BM
The output point shape file format for Bench Marks in GIS database would be as follows:
Shape File Name: R_BM [Benchmark Pillar], R represents Rangpur.
1.1.3.4 Physical Features Survey
Physical Features shall be surveyed using Total Station (TS) survey technique. Location and dimension of all existing structures and installations along with types in respect of use, construction and storied will be surveyed. Names of structures, type of construction, uses and storey etc. will also be recorded during physical feature survey. Survey will also cover location of all existing exposed light/electric, telephone posts and twers, gas, water, sewerage line, roads etc. Physical feature survey will be conducted using RTK-GPS supported Total Station (TS) survey technique. Data will be recorded in the TS memory with separate ID or code number for each feature (as Line, Point and Polygon). Later on the TS data will be transfer directly to the GIS database where the feature will be kept in separate layer wise as per specified code or ID. Physical feature survey information will be presented on RS/CS Mouza maps.
For supporting the Total Station Survey, huge numbers of Secondary Control Point (SCP) will be established using RTK fast static survey technique. These SCPs will be used by the total station groups as reference points (Station and Back Points) for physical feature, topographic and landuse survey.
1.1.3.5 Topographic Survey
Topographic and Physical Infrastructures will be surveyed using both Total Station (TS) and DGPS survey technique. Land levels/spot levels and cross-sections and alignments of drains will be surveyed by Total Station. Alignment and closed boundaries like Road, River, Khal, Marshland, Homestead, Large Water bodies etc. will be surveyed by DGPS. Where DGPS survey could not be possible for weak satellite signal due to obstruction, TS survey technique will be applied for those particular areas. As per ToR the items to be covered by Topographic survey is presented below:
The Total Station survey groups/teams will be responsible for measurement of spot levels (Northing, Easting, Elevation or RL) for contour generation. In general the spot levels on the land will be taken at minimum 10m interval. In addition, most of the physical infrastructures will also be survey by the TS team. The utility pole and alignment of utility lines will be surveyed using both TS & DGPS. The secondary BMs established by RTK-GPS will be used by the Total Station Groups as reference (Station and Back Point). 0.3 m interval contour map (Topographic Map) will be prepared at 1”=165' or 1: 1980 scale. DGPS will be used for surveying the location/alignment of all roads, flood embankments and other drainage divides as well as closed boundary/outline of homestead, water bodies, swamps, forest etc.
Figure-7: Topographic (Spot Level) Survey by Total Station
DGPS group will measure and store the alignment in x and y co-ordinates of roads, embankment and other line features roughly at 1 to 3m intervals. The point and closed boundary features also surveyed by the DGPS groups. The optical teams will pic-up the crest level of the road at not exceeding 50m intervals. DGPS group also responsible for taking the position and the information of the structures (hydraulic structures, bridges and culverts etc.). At the end of day’s survey, the DGPS data will be downloaded and post-processed in the office using Pathfinder Office software and stored into GIS database. Names of settlements, village, rivers, khals, roads, markers, etc. will be also presented on the topographic map.
Figure-8: ProXR DGPS Survey
(Road Alignment Survey as Line Feature)
Figure-9: Road Level Survey Using Optical Level
1.1.3.6 Land Use Survey
Land use survey basically records the use of land by its functional activity such as residential, industrial or commercial etc. Total Station and DGPS based topographic and physical features survey data will be used for land use survey. During Topographic and physical feature survey, each survey feature/structure will be recorded with individual ID or code. Later on land use features will be extracted/identified and classified using the recorded code and separated in different layers during data processing stage, from where the category wise land use map will be drawn using the identification layers of each land use features. Later on the landuse map will be updated through field checking and verification. The land use map will be prepared indicating the board categories of land use described in ToR. It should be mentioned here that the detailed land use categories may further need to be prepared by the joint team of Client and the experts of consulting team before start the landuse survey at field level.
As stated before, utilizing the physical features survey overlay on cadastral survey map the land use map shall be prepared indication the categories of land use indicated below (as mentioned in the format of landuse survey in ToR). The characteristics of each land use area will be described in the survey report. The Land Use Map will be prepared on CS/SA. map at 1" = 165' or 1:1980 scale.
1.1.4 Data Processing of Physical Surveys
GPS data will be stored in WGS84 format (latitude, longitude, ellipsoidal height in meter) and later on will be projected and stored in BTM (Northing, Easting, ellipsoidal height in meter) projection system. As, Total Station data will be stored in BTM projection system therefore TS data will be transfer to GIS database directly.
All spatial information or data from different survey such as line and point features, structures dimensions etc. will be processed and stored under a comprehensive GIS database (ESRI Shape/ Geodata). Geographic information System (GIS) software such as Arc GIS 9.3.1 or higher version will be used for processing & analyzing of survey data and preparing map. The well known Triangulated Irregular Network (TIN) and Grid method will be applied to draw the contours of the land surface. All data will be provided in digital format (x, y, z) as well as hard copy.
1.1.5 Preparation of Physical Survey Maps
Geographic information System (GIS) software such as Arc GIS 9.3.1 will be used for processing of physical survey data. As there is no mention in the ToR regarding the legend, layout and other specification of physical survey maps (layout, size etc.) will be finalized in consultation with the project authority of Client during map preparation process, The well known Triangulated Irregular Network (TIN) method will be applied to draw contour lines. AS per ToR the consultants will prepare the following physical survey maps incorporating both RS/CS Mouza maps and the features/items mentioned in the Survey Formats (Physical Feature Survey Format, Topographical and Physical Infrastructure Survey Format, Land-use Survey Format) described in the above sections as in the ToR. All maps will be printed colored on 120gm off-set paper. Both soft and hard format will be supplied.
Format for Base Map, Physical Feature, Landuse and others maps as mentioned in the ToR are presented below:
1.1.6 Field Verification of Physical Survey Maps
After preparation of physical survey maps, one set of coloured maps (topographic and physical infrastructure, physical feature and land use) will be plotted in 1:1980 scale for field level verification. Numbers of survey team will be engaged for field level checking of topography, physical features, infrastructures and land use categories. As the effective device for physical survey map checking, the consultant will also use QUICK BIRD or IKONOS Image to be supplied by the client. This field level checking will be supervised and monitored by the joint team of Client and consultant.
1.1.7 Updating and Finalization of Physical Survey Maps
Based on field verification, the necessary updating of physical survey maps will be done and final map layout will be produced for submission to Client. If necessary, some of the areas/features may be needed for further re-survey. Before preparing final map layout, necessary approval on draft map layout will be taken from Client on Title, Legend and Size of the Maps.