SPECIFICATIONS BOOK DOMIN PDF

SPECIFICATIONS FOR GEODETIC SURVEYS IN NIGERIA. BY SURVEYORS COUNCIL OF NIGERIA (SURCON) ISBN 978-066-933-7  FOREWORD Large Scale Mapping and Surveys needed to be controlled in scale and direction by being tied to Geodetic Controls. Geodetic contr...

SPECIFICATIONS FOR GEODETIC SURVEYS IN NIGERIA. BY SURVEYORS COUNCIL OF NIGERIA (SURCON) ISBN 978-066-933-7  FOREWORD Large Scale Mapping and Surveys needed to be controlled in scale and direction by being tied to Geodetic Controls. Geodetic controls therefore need to be established and coordinated to very high standards hence the need for Specifications for Geodetic Controls in Nigeria. The Specifications include Accuracy Standards, Types, Instrumentations, Observations, Geometry and Design of Controls. Tie Surveyors Council of Nigeria one of whose Statutory duties in section 4(d) of the Council's Enabling Act is "regulating and controlling the practice of the profession in all its ramifications" has seen the necessity of publishing such Specifications. Though, this is only a review of the existing Specification, it has enjoyed the inputs and experiences of major stakeholders in the profession. It is therefore imperative that reference be made to the contents of this booklet when Geodetic jobs are being executed. SURV. (Prof) R.N. ASOEGWU, fnis President SURCON  PREFACE The Surveyors Council of Nigeria (SURCON) was established by CAP 425 Laws of the Federation of Nigeria 1990. One of the Functions as stipulated in section 4 (d) is "regulating and controlling the practice ofthe profession in all its ramifications. " In pursuant to this provision the 4th Council mandated its Survey Laws and Regulations Committee to republish all previous SURCON publications with the necessary ISBN/ISSN numbers. The Committee therefore used the opportunity to review and update the existing SPECIFICATIONS FOR GEODETIC SURVEYS IN NIGERIA. The purpose of this handbook is to guide the practice of Geodetic Surveys in Nigeria in line with modern technological developments. These Specifications will provide information on the level of accuracy, maximum allowable errors and general procedures to be followed in carrying out geodetic surveys in Nigeria. Credit and gratitude must go to the resource persons who helped the Committee in this exercise. Any comment or issue needing further explanation should be referred to the Registrar for clarification. Surv. Col. Nwabichie, fnis, JP Chairman, Survey Laws and Regulations Committee.· i 55 54 ii  LIST OF TABLES Page Ta ble 1 National Horizontal Control Accuracy Standard 6 Ta ble 2 Specifications for Triangulation 10- 13 Table 3 Specifications for Trilateration 16- 17 Table 4 Specifications for Traverse 27- 28 Table 5 Vertical Control Standards 31 Table 6 Specification for Geodetic Leveling 32- 33 Table 7 Gravity Control Standards 36 Tabl e 8 Specification for Gravity Surveys 37 Table 9 Specifications for Doppler Observations 41 Table 10 Specifications for GPS Observations 44- 46 iii 53  CHAPTER 1 INTRODUCTION 1.0 GENERAL For National Defence, administration and general development of the. country, the government of Nigeria has since the colonial period embarked on country-wide surveys to produce maps, plans and other survey information required to support the conduct of public and private business. In general, surveys are required for the exploitation of mineral resources, the planning and development of urban areas, the planning of agriculture, the execution of engineering projects, the control of the environment, the settlement of disputes over land and the establishment of administrative and international boundaries, to mention a few. In the past, to co-ordinate the surveys and consolidate the practice, the government published from time to time, regulations and specifications which provided information on the level of accuracy expected from a Survey, the maximum allowable errors of measurements and the general procedures to be followed in different Surveys. As development in methodology and instrumentation grows, it has become necessary to review the provisions in these regulations and specifications and prepare an up-to-date and comprehensive document with sufficient authority to direct the establishment and extension of control points for all surveys in the country. It is for this purpose that the Surveyors Council of Nigeria (SURCON) created by Decree 44 of 1989 (Cap 425 LEN), ordered the preparation of the present booklet with the title "Specifications for Geodetic Surveys in Nigeria". The fundamental principle of carrying out asurvey is to go "from whole to part". This means that networks of horizontal, vertical and gravity control points are first established, before regional and local surveys are made. Horizontal controls are usually represented as geodetic latitudes and 52 1 fieldwork procedures, office computations and analysis of results. Where it is convenient in this text specifications are provided numerically as maximum or minimum tolerable values, for example in first order design of a triangulation network, the minimum size of a distance angle is pegged at 30°, while the maximum length of line between two stations is set at 65kms. In other cases, specifications are given as textual instructions. The Horizontal Control specifications for triangulation, trilateration and traverse, are set out in Chapter 2. In each of the three procedures, tolerances and instructions are specified under the Geometry and Design, Instrumentation, Observational procedures and office computations and analysis. Details are given in Tables 2,3 and 4. Chapter 3 contains corresponding specifications for vertical control viz for planning and execution of geodetic leveling and the required gravity information in vertical control. Table 6 provides the details. Gravity network control specifications are found in Chapter 4 with details in Table 8, while, Chapter 5 contains the relevant directives with regard to satellite methods viz Doppler and GPS systems. Chapter 6 is devoted to the description and presentation of monument types for all types of survey. 2 51  CHAPTER 2 HORIZONTAL CONTROLS Horizontal controls in Nigeria are represented as geodetic latitude and longitudes of selected points with reference to the Minna Datum. The Clarke 1880 ellipsoid was adopted as the reference surface for computations. Table 1, contains the various measurement techniques in use. They include the classical methods of triangulation, trilateration. and traversing. Specifications for these are presented in sections 2.1, 2.2. and2.3. With the advent of satellite surveying methods, the WGS72 ellipsoid was used as reference for Doppler surveys, while the WGS84 ellipsoid is currently the reference surface for GPS observations in Nigeria. The inter-conversion formulae for co-ordinates of points on pairs of these reference surfaces are available, even though some of them are also subject of further research. Whichever reference surface is used, the accuracy standards for horizontal control are the same. 2.0 ACCURACY STANDARDS AND CLASSIFICATION Accuracy is defined as the degree of perfection attained in the determination of a quality. In the establishment of horizontal control networks, it is convenient and sufficient to define the minimum accuracy of a line as the ratio of the standard error of that line (after an appropriate least squares adjustment) to the length of the line. Thus, if the length of the line is 1, and the standard error is S then, denoting the accuracy by 1/p, we write 1/p = s/1 For each line of the network, there is one value for Pin equation (1) above. By adopting a system of accuracy classification for all control points, we take a range of values from (1) as belonging to one order of work. For this purpose, the highest value of P in each range will not lie below the value adopted for the order. This is the basis for the classification of all horizontal control points in the country given in Table 1. For convenience of reference, Table 1 also contains information on 50 3 the principle uses of various standards, the recommended methods of realizing the standards and the recommended movement types to be used in each case. From the Table, we see that Nigeria has four orders of horizontal control points: the zero, the first, the second and the third orders. The second and third orders are each subdivided into two classes: class 1 and class 2, to satisfy user needs. The accuracy standards are presented in two forms. First in fractions and then in parts per million enclosed in brackets. For example the first-order is given as 1/100,000 and as (10ppm). The two mean the same thing. The identifiable principal uses correspond to the classification orders. The uses of the zero-order require the highest possible accuracy and they include surveys to study geodynamic phenomena and other crustal movements. These can be realized by specially designed GPS observation campaigns and supplemented by Satellite Laser Ranging (SLR) and Very Long Base Line (VLBL) observations if they are available. Zero-order standard is meant to provide the network for the whole country forming the basis for all other surveys; thus providing the "whole" required in the "whole to part" principle. The second order class 1 surveys provide the first breakdown of the basic network and thus widen the "whole" and release points needed to connect local urban and rural control surveys whose accuracies are not lower than 1: 1 0,000 i.e. third order class 1. 4 49 4. Azimuth Mark Beacons (Type B4} The beacons are to be used for the azimuth marks for the Primary beacons as listed in B(1) above. They should be sited, at least 500 metres from their main stations. 5. Witness Mark Beacons (Type BS). The beacons are also to be used as witness marks for the Primary beacons listedinA(1) above. They should be sited within 10 - 30 metres from the station. 2.2. VERTICAL CONTROLS 1 Fundamental Bench Mark (FBM)jType B6) Fundamental Bench Marks are established as primary vertical Control points. They are usually sited on firm live rocks and located at end points of primary geodetic levelliries. 2. Standard Bench Mark (SBMl(Type B7} Standard Bench Marks are established also as primary vertical control points. They are sited on solid soil, and located at end points of geodetic level lines. 3. Other Bench Marks i) TypeA(TYPEB8} These are emplaced in solid soil. ii) Type B (TYPE B91 These are emplaced on live rock below the ground surface. iii) TypeC(TYPEB10} These are emplaced on live surface rock. 48 5  6.1.3 IDENTIFICATION: The second order class 2 is specifically meant to include the existing a) Numbering: Each beacon should carry a specific and appropriate inter-state cadastral framework traverses; and to provide the accuracy identification number. standard of ground control points for medium and large scale map series b) Reference Marks/Objects: At least four reference marks sited at covering the entire country. It is therefore the appropriate order of work suitable geometric angles, should be provided for all primary beacons. for control supplementation and extension to areas not covered by the Permanent reference objects should b~ identified and recorded. second order class 1 scheme. Measurements from the main beacon to the reference objects should also be recorded. The third order class 2 standard is designed to control ordinary cadastral and other surveys in extensive rural districts with large, areas of farm 6.1.4 DESCRIPTION land, grazing land and forests. (Semi-rural areas growing with All stations should be described using appropriate description sheets. settlements as are rampant in most of the southern parts of the country, The description should include a sketch of location based on 1/50,000 should be controlled by the third order class 1 standard). topd map where available, and a sketch showing immediate environ. Bearing and distance measurements to all reference marks and objects The method of realizing the standards other than zero order, are clearly should be recorded and shown on the sketch. stated in the Table and need not be described further. As much as possible, where a precise EDM traverse is recommended, electro-optical 6.2 SECTION II: TYPES OF BEACONS and laser instruments should be used. 6.2.1. HORIZONTAL CONTROLS It should be emphasized that efforts put in control surveys are valuable only if the monuments are conserved. As a deliberate effort to secure the 1. First Class Beacons (Type B1) monuments, appropriate construction and selection of suitable sites These beacons are to be used as primary control points for any of should always be adhered to. the following types of surveys: i) International and Interstate Boundary Survey; 2.1 SPECIFICATIONS FOR TRIANGULATION ii) Primary GPS Survey; Triangulation is a "measurement system of joined or overlapping triangles iii) Primary Triangulation Survey. of angular observations supported by occasional distance and astronomical observations 2. Second Class Beacons (Type B2) 2.1.1. GEOMETRY AND DESIGN I) Cadastral Framework Survey; For a network to fulfill its basic goal as a strong and reliable reference ii) For Primary Traverse Survey; framework, it must be homogenous, with a reasonable number of iii)As Intermediate beacons between Primary GPS beacons redundancies and the individual figures should be well shaped. Stations should be evenly spaced out as much as possible and all adjacent pairs 3. Third Class Beacons (Type B331 of stations in the network should preferably be connected by direct stage. These are to be used for secondary and tertiary GPS, traverse and Trianqulation surveys. 6 47  CHAPTER 6 The following conditions should as much as possible be fulfilled by the network: MONUMENTATION: SURVEY BEACONS 6.0 GENERAL a) A Distance Angle is the angle opposite the side through which the Survey beacons are very important and precious inputs to all survey distance is propagated (computed) works. Hence, special attention should be paid to the construction, b) The strength of a Triangulation Figure is given by quality and preservation of the beacons. Section I deals with properties D-C common to almost all the beacons, while Section II deals with types of R= D beacons recommended for Geodetic survey controls. Where A is angle opposite the required side in a triangle. 6.1 SECTION I. PROPERTIES AND BEACONS S is angle opposite the known side is sixth place difference in logsine corresponding to one second variation in angle. 6.1.1 LOCATION D = No. of observed directions, It is found by summing all the directions a) On stable base like rock or solid ground. observed for the figure and subtracting the two belonging to the starting line which is considered fixed. b) Raised ground or hill top (triangulation), flat surface (traversing and C = No. of geometric conditions determining the strength of figure. It is vertical control) where inevitable. Steep slopes should be avoided found from where possible. C = 2L L' 3+ Su +4 where c) Flood area of river banks should be avoided where possible. L 1 = total no. of lines Where such areas could not be avoided, beacons should be L = total no. of lines observed one way constructed appropriately as described in paragraph 2 (b). S = total no. of stations Su = no. of unoccupied stations 6.1.2 QUALITY a) The concrete mixture of beacons should be in the ratio of 3.2.1 of A new survey should be tied to at least four network control points well sharp sand, crushed stone and cement. spaced out and the network points should have datum values equivalent to or better than the intended order and class of the new survey. b) Certain beacons should be reinforced with iron pipes, rods or angle irons where applicable. Where a beacon is to be emplaced 2.1.2 INSTRUMENTATION in soft or swampy soil, an iron pipe of appropriate diameter should b e The theodolites used for directions azimuths and for the determination of driven to the ground until it reached solid layer or bedrock. The latitudes and longitudes for triangulation should be of high quality and beacon should then be built in situ in accordance with specification. properly maintained. c) The center mark of any beacon should be clearly identifiable and of metal materials such as brass bolt, nail or iron rod. 46 7  With the fully developed constellation, any user, at any time, anywhere in the world is expected to have a minimum of four visible satellites for use. As at 2007, Nigeria has a four-satellite window covering 18 hours of the day. As these satellites move in their orbits, they transmit continuously two carrier waves L1 (1227.6MHz) and L2 (1575.42 MHz). The L1 frequency carries CIA (coarse acquisition) code and P(Precision) code while L2 frequency carries only that code. Observations of these satellite siqnals with adequate Geodetic GPS receivers can be used to derive the 3D positions of the antenna phase centers of the receivers. 5.2.2. INSTRUMENTATION GPS geodetic receivers are many and are of varied form. Some are designed to receive only one carrier frequency, some receive both carrier frequencies. Dual frequency receivers are required for more precise surveys involving medium to longer baselines, to correct the effect of ionospheric errors which ranges between 1 pm and 10pm. Geodetic receivers are made up of three major parts: antenna, receiver/processor and recording unit. Data gathered by these receivers include phase data, receiver time and signal strength. In some cases, the reference oscillator in the receiver require some warm-up time for stability of the clock which in turn dictates the accuracy of the survey. The required stability of the receiver clock depends on the receiver design, sophistication of the processing software. Users are therefore strongly advised to familiarize themselves with their receivers' instruction manuals. Users of GPS cod eless receivers should ensure that they obtain necessary time standards and get themselves well familiarized with how their receivers can be initially synchronized and the relative drift rate of the clock be maintained with the manufacturer's specifications CALIBRATION Field calibration is necessary for the elimination of systematic error and to ensure confidence in the system. The GPS instrumentation and the associated post processing software should be used on a 3D test net. 8 45 44 9 10 43  The quality of theodolites to be used are as follows: a) First Order: For Latitudes and Longitude determination Wild T4, DKM 3A or their equivalents shall be used while for directions and azimuths, Wild T3, DKM3 or the digital equivalent shall be used b) Second Order (Class I): Wild T 4, DKM 3A or an equivalent for latitude and longitude determination and Wild T3, DKM3 or digital equivalent for observing directions and azimuths. c) Second Order (Class II): Wild T3, DKM3 or digital equivalent. d) Third Order (Class 1):Wild T2, kern DKM 2A or DKM 2AE or equivalent.. e) Third Order (Class II): Wild T2, Kern DKM 2A or DKM 2AE or equivalent. The targets used should be precisely marked and have a clearly defined center resolvable at the minimum control spacing. They should be well mounted on tripods or supported towers. Optical pluments and collimators are required to ensure that the theodolites and targets are centred overthe station marks to within 1 mm. Electro optical and infrared distance measuring equipment shall be used to measure the base lines. Micro wave distancers are not sufficiently accurate to be used. 2.1.3. CALIBRATION Theodolites should be serviced every year or when the collimation error is found to be more than 15". On the other hand, the bubble and cross hairs should be adjusted whenever their misadjustments affect the readings by an amount more than the least count of the theodolite. All EDM accessories including the reflectors should be serviced and tested on base lines of known distances regularly. The EDM instruments should be calibrated annually and frequency checks carried out biannually. 42 11  5.1.4 OFFICE COMPUTATIONS Priorto final post processing, the following should be embarked upon: 1. Satellite passes with Doppler count of 20 or less should be rejected. 2. Cut off angle for both data points and passes should be 7.5 degrees. 3. Inspect data to ensure that rejected counts other than those % resulting from the cut-off angle are not more than 10 , else observations should be rejected. 4. Standard deviation of the range residuals should range between 5cm and 20cm depending on the number of passes. Good reduction software should be able to tackle most of the data clearing operations listed under 1-4 automatically. The software should be able to perform the following four major computational tasks for Doppler data post processing: MAJORITYVOTING: To condense and edit raw data PREPROCESSING: To edit out geometrically unacceptable passes and prepare adjustment input data. SINGLE STATION SOLUTION: To edit out statistically unacceptable pages and prepare edited data file. MULTISTATION.SOLUTION: To provide the required 3-D co- ordinates. 5.2 GLOBAL POSITIONING SYSTEM (GPSl 5.2.1 GPS SYSTEM The GPS is a space based satellite navigation system capable of yieldinc very high positioning precisions. The system has at present constellatior of at least twenty-four block I and block II satellites. When the constellation becomes fully developed, it would eventually consist of 1 C block II (operational) satellites with three spares. 12 41  2.1.4 OBSERVATIONAL PROCEDURES A. FIRST ORDER I. ANGULAR OBSERVATIONS Horizontal angles shall be observed in the night while reciprocal vertical angles should be observed at times of best atmospheric conditions, that is with minimum vertical refraction effect (between noon and late afternoon). The horizontal angular observations should be on minimum of 8 zeros well spread out (22, apart) containing 16sets in 2 nights. Aset is made up of a face left and face right observation.. The closure.on the horizon should be within 1".5 while the spread of the 16 sets should not be greater than 5".0. The maximum difference of the mean of two (2) nights is 1 ".0. Other specifications are contained in Table2. II. AZIMUTH OBSERVATIONS Bearings should be controlled by Astronomic azimuth determination taken at every 6-8 stations. A set of 16 observations will be carried out on 2 different nights. A set consists of a West and East star observations. III. LINEAR MEASUREMENTS Base lines shall be measured with Electro 'Optical and Infra-Red distance equipment. Wet and Dry temperatures to + 1°C and barometric pressures to + 5 mm of mercury should be taken at both ends of the line. IV. HEIGHTING Normally shall be by spirit leveling. Trigonometric Heights give provisional values. B. SECOND ORDER CLASS I The specifications are as in 1 st order but the angular observations shall be carried out for only one night. Other specifications are as 40 13  nd rd C. For 2 Order Class Hand 3 Order Classes I & II refer to table 2. 2.1.5 OFFICE COMPUTATIONSANDANALYSIS A minimally constrained least squares adjustment will be checked for· blunders by examining the normalized residuals. The observation weights will be checked by inspecting the. post adjustment estimate of the variance of unit weight. Distance standard errors computed by error propagation in this correctly weighted least squares adjustment will indicate the provisional accuracy classification. A survey variant factor ratio will be computed to check for systematic errors. The least squares adjustment will use modes which account for corrections for deviation of the vertical, geodesic and skew normals. 2.2. SPECIFICATIONS FOR TRILATERATION Trilateration is the method of surveying in which only the lengths of sides of the triangles formed in a network are measured. With the advent of Electronic Distance Meter this method of control establishment has become very important. It is very much in use in Nigeria today. Table 3 presents the relevant specifications for Trilateration. 14 39  CHAPTERS SATELLITE POSITIONING 5.1 SATELLITE DOPPLER POSITIONING 5.1.0 INTRODUCTION Positions of points on the earth surface can be determined in 3 dimensions by observing the Doppler shifts of radio signals from the Navy Navigation Satellite System otherwise referred to as TRANSIT. TRANSIT received measurement is made up of Doppler counts receiver clock time and signal strength level. These measurements are then related to the co-ordinates of the observing stations through some mathematical model which also cater for biases caused by relativity; refraction, instrumentation imperfection, etc. There are several Satellite Doppler observing and processing modes such as point positioning, Translocation, short Arc and Semi short Arc modes. Point positioning, for geodetic applications, requires that the Doppler data be reduced with the precise ephemeris, which can only be obtained through the Defence Mapping Agency of America. The ADOS points in Nigeria were processed through this mode. The other three modes favour relative positioning. Relative positioning is possible when two or more receivers are operated together in a survey project area. Since most satellite Doppler positioning errors are correlated between stations simultaneously tracking a satellite pass; advantage of these correlation are generally taken to improve the accuracy of relative position. The semi shortArc and the sh6rtArc allow up to five and six degrees of freedom in the satellite ephemeredes and thus allow the use of broadcast ephemeris for improved accuracy instead of precise ephemeris. The undergoing specifications for relative positioning are valid for data reduced using the broadcast ephemeris in the short-Arc or Semi short Arc mode. Since precise ephemeredes are not easily available, the specifications presented are concerned with the procedures using broadcast ephemeris. 38 15  4.1 GEOMETRY AND DESIGN (RELATIVE MEASUREMENTS). A relative measurement of gravity determines the difference in the magnitude of gravity between two stations. Design requires the selection of stations which are gravimetrically stable so that repeated measurements over a given term of years will reflect the true changes in gravity. However, for convenience of use, there should be a point in every 20' x 20' block for First Order, 10' x 10' block for second order and 5' x 5' block for the Third Order. This means each point will be about 10km away from another, making linear interpolation accurate and satisfactory. 4.2 INSTRUMENTATION The dial readings taken during a gravimeter survey are not in units of gravity. A calibration process is required to establish a model relating the differences in readings between stations to differences in gravity values. This should be done to an accuracy of one hundredth of a milligal (0.01 mgal). Again, the internal accuracy of a gravimeter should be established by making a survey over a calibration line. The internal error should be one order of magnitude below the accuracy standard sought. 4.3 OBSERVATIONAL PROCEDURES During a survey, temporal variation (drift) unavoidably arise in the readings of a gravimeter. These drift errors are due to many sources including the ageing of the spring, uncompensated temperature, fluctuations, elasticity errors due to locking and unlocking of the instrument and vibrations and shocks during instrument transportation. These errors are eliminated or reduced by repeated measurements using one of the following methods: a) Profile b) Star c) Step and d) the line The profile method takes readings at each station twice; one on forward and the other on backward journey. The star method takes readings at four stations in sequence as: from 1 to 2 to 1 to 3 to 1 to 4 to 1. thus each station reading is compared directly with the reading of the base 1. The step method goes from 1 to 2 to 3 to 2 to 4 to 3 to 4 to 5 to 4 to 5 to 6, etc. The line method begins at a point and ends at a different point 16 37  2.2.1 GEOMETRY AND DESIGN To maintain the strength of figures, quadrilaterals should approximate a square and should seldom contain angles less than 25° for first and second order controls. In no case however should any angle be less than 20° for the lower order control su rveys. 2.2.2 INSTRUMENTATION Only electro-optical Distance meter should be employed for first order control surveys, while for second and lower order surveys, infra-red instruments could be employed. Microwave Electronic Distance Measurement equipment are only suitable for third order control surveys. All EDM devices and reflectors should be serviced regularly and checked frequently over lines of known distances. Wild T3, DKM 3/3A theodolites should be used for azimuth observations in first and second order Trilateration controls while Wild T2 types theodolites will be used for lower order. 2.2.3 OBSERVATIONAL PROCEDURES For all classes of trilateration, distances shall be measured by Electronic Distance Measurement Equipment. Electronic Distance Measurements need a record at both ends of the line, of wet and dry bulb temperature to + 1 ° C and barometric pressure to + 5 mm of mercury. 36 17 18 35 The closure specifications should not be confused with the accuracy achieved. The latter are obtained after appropriate least squares adjustment and using equation (2) above. OFFICE PROCEDURES 1. Algebraic sum check of all the sections' misclosures of a leveling line should be carried out to ensure that they are within the limit specified. Loop misclosures should be similarly checked. 2. Gravity corrections should be applied for first order leveling only. 3. A minimally constrained adjustment will be carried out to check for blunders through the normalized residuals. The post adjustment estimate of variance of unit weight should be inspected to confirm proper weighting forthe observations. Standard error of elevations computed with a least squares adjustment under proper weighting conditions will indicate the provisional accuracy specification. 34 19 2.2.4 OFFICE COMPUTATION AND ANALYSIS Information on the principal uses of the various standards of accuracy, The following corrections should be applied to measured EDM distance. the recommended methods of realizing the standards and the I. Instrument constant recommended movement types to be used in each case. ii. Reflector constant iii. Slope correction 3.1 SPECIFICATIONS iv. Sea level correction v. Arc chord correction Specifications for geodetic leveling are discussed under the sub-heads, geometry and design, instrumentation and office procedures and are In computing the trilateration network, a minimally constrained least given in sequence. The directives for adequate gravity measurements to squares adjustment will be checked for blunders by examining the support heighting is presented in Chapter 4. normalized residuals. The observation weight will be checked by inspecting the post adjustment estimate of the variance of unit weight. 3.2 SPECIFICATIONS FOR GEODETIC LEVELING Distance standard error computed by error propagation in a correctly Table 6 contains the parameters to be specified to enable adequate weighted least squares adjustment will use models which account for the leveling data to be collected for various orders of vertical control corrections mentioned above. standard. 2.3 SPECIFICATIONS FOR TRAVERSES The geometry and design parameters including the spacing of leveling 2.3.1 PRINCIPAL USES lines and the spacing of Benchmarks along lines (i.e. length of sections), Various classes and order of traverses are used for various purposes, are specified in the first two rows of the Table. It should be noted that level depending on the accuracy standards. lines normally follow existing roads for convenience and use. These a) FIRST ORDER TRAVERSES requirements should be taken into consideration while enforcing the These are used as First order control network in lieu oftriangulation above specifications. Sometimes, it may be unavoidable to include a where the cost of using triangulation methods will be excessive in section which is less than 1 km. when this is the case, this abnormal line view of low relief and in areas with thick forest. They are also used shall be combined with an adjacent line for the purpose of closure in special scientific investigation and precise engineering survey calculation, since closure formulae are not applicable to sections less project.. than 1 km. b) SECOND ORDER (CLASS I) These are first breakdown of the 1 sl order control network of Instrumental specifications are presented in the following rows. Geodetic Traverses, Triangulations or Trilaterations. They form the main levels with 0.1 mm least count include the Zeiss and the Wild N-3. These network of controls for large Urban Centre and also for precise are suitable for First order work. The geodetic levels of 1 mm resolution engineering survey projects. are the Ni-2, wild NA-2, and the Kern NK-3 instruments. They may be nd used for 2 and lower order work. c) SECOND ORDER (CLASS II} They act as supplementary controls for third order surveys. They are used as controls for cadastral and township mapping projects. The field procedure specifications are clearly not a substitute for the detailed instrument manual 20 33 32 21  stable tripods or supported towers should be used. The target should have a clearly defined center resolvable by the instrument used. Optical plummets or collimators will be required to ensure that theodolites and targets are centred over the marks. Infra-red Distance Meters should be employed for linear measurements. Theodolites to be used for Angular measurement are as follows: a) First Order Wild T3, Kern DKM 3, and 3Aordigital equivalent. b) Second Order (Class I) Wild T3, Kern DKM 3, and 3Aor digital equivalent. c) Second Order(Class II) Wild T3, Kern DKM 3, ordigital equivalent. d) Third Order(Class I) Wild T2, Kern DKM 2A and 2AE ordigital equivalent.. e) Third Order (Class II) Wild T2, Kern DKM 2A and 2AE or digital equivalent. Latitude and Longitude Observations Wild T 4 and DKM 3A 2.3.4. OBSERVATIONAL PROCEDURES A) FIRST ORDER TRAVERSES 1) Angular Observations Horizontal Angles will be measured with a geodetic theodolite measuring 'direct to 0:1" or less. All angles will be observed on face left and face right equally spaced round the circle. The minimum number of positions should be 16. observations should be made by night. 2) Azimuth Observations Bearing should be controlled by azimuth observations taken every 5 -6 stations. A set of observations will be observed on 2 different nights. A set consists of a west and an east star. Azimuth closure at azimuth check point must not exceed 1".0 per station and the standard error of 22 31  the mean should not exceed +0.'13. Aim ata spread notgreaterthan 1".0 3) Heighting Every traverse station should be heighted by First Order spirit leveling. 4) Linear Measurements Distance shall be measured with EDM equipment. Electronic distance measurements need a record at both of the line of wet and dry bulb temperature to +1 °C and barometric pressure to +5mm of mercury. The theodolite, EDM and target should be centred to within 1 mm over the survey mark or eccentric point. B) SECOND ORDER CLASS I C) Angular Observations 51 Observational procedures and instrumentation are as in 1 Order control surveys. II) Azimuth Observations Bearing should be controlled by Azimuth observations taken every 10 - 12 stations. A set of 8 stars will be observed on 2 different nights. Azimuth closure at azimuth check points must not exceed 1".5 per station and the standard error of the mean should not exceed +0".45. Aim at spread not greater than 2".0· III) Heighting Every second order traverse station should be heighten by second order class I spirit leveling. IV) Linear Measurements (As for first order) V) Latitude and Longitude Observations Not applicable. C) SECOND ORDER CLASS II I) Angular Observations 30 23  Observational procedures and instrumentation are as in 151 Order CHAPTER 3 control surveys. VERTICAL CONTROL II) Azimuth Observations Bearing should be controlled by Azimuth observations taken every 3.0 ACCURACY STANDARDS AND CLASSIFICATION 12 - 15 stations. A set of 6 observations will be observed on 2 Vertical control is usually accomplished by geodetic leveling. In addition, different nights. Azimuth closure at azimuth cheek points must not gravity values are measured at selected benchmarks so that accurate exceed 2".0 per station and the standard error of the mean should orthometric heights may be obtained. not exceed +0".60. Aim at spread not greaterthan 3".0 The minimum accuracy of the elevation difference between two benchmarks is calculated from the ratio of the standard error of elevation III) Heighting difference in millimeters, (after an appropriate least squares adjustment) Every second order traverse station should be heighted by second to the square root of the distance in kilometers between the benchmarks. order class II spirit leveling. Thus if the inter-benchmark distance is L, km and the standard error is T, mm then denoting the accuracy by i/q, we write IV) Linear Measurements I/q = tlL (As for first order) (2) V) Latitude and Longitude Observations Not applicable. For each inter-benchmark distance there is one value for q in equation (2). By adopting a system of classification of accuracy standards for D) THIRD ORDER CLASS I vertical control points, we take a range of values from (2) as belonging to I) Angular Measurements one order of vertical control. For this purpose, the highest value of q in Third order class I horizontal angles should be measured by a each range, will not lie below the value. adopted for the order. Table 5 geodetic theodolite measuring direct to 6".0. All angles will be gives the accuracy standards of elevation differences between directly observed on face left face right equally spaced round the circle. linked benchmarks for the three orders of work adopted for Nigeria. The The minimum number of pointings should be 4. unit is in millimeters per kilometer raised to power half. For convenience of reference, Table 5 also contains II) Azimuth Observations Bearing should be controlled by Azimuth observations taken every 20 - 25 stations. A set of 8 stars will be observed on 2 different nights. Azimuth closure at azimuth check points must not exceed 8".0 per station and the standard error of the mean should not exceed +3".0. 24 29 i) Instrument constants Aim at spread not greaterthan 10".0 ii) Refractive Index Correction III) Heighting iii) Slope correction Every traverse station should be heighted by third order leveling. iv) Sea level correction v) Arc to Chord correction IV) Linear Measurements Electronic Distance Measurement equipment. b) COMPUTATIONS E) THIRD ORDER CLASS II A minimally constrained least squares adjustment will be checked for I) \ Angular Measurements blunder by examining the normalized residuals. The observation weights As in 3rd order class I but minimum number of pointings should will be checked by inspecting the post adjustment estimate of the be2. variance of unit weight. Distance standard error computed by error propagation in a correctly weighted least squares adjustment will indicate the provisional accuracy classification. II) Azimuth Observations As for third order class I. Azimuth closure at azimuth check points must not exceed 10".0 A survey variance factor ratio will be computed to check for systematic per station and the standard error of the mean should not exceed errors +8".0. Aim at spread not greaterthan 20".0 III) Heightinq Every traverse station should be heighted by third order leveling. IV) Linear Measurements Electronic Distance Measurement or Taped distances may be used, for linear measurements in third order class II control surveys. The taped distances shall conform with the specifications. 2.3.5. OFFICE COMPUTATIONSANDADJUSTMENTS (a) Corrections to measured values. 1. FOR EDM Observed Distances: 28 25 26 27