Section 12 - Guidelines for Cadastral Surveying using Global Navigation Satellite Systems (GNSS) 12.0 Introduction

Transcription

1 Section 12 - Guidelines for Cadastral Surveying using Global Navigation Satellite Systems (GNSS) 12.0 Introduction The Surveyor-General of Victoria is responsible for setting and monitoring cadastral survey standards and practices in Victoria under the Surveying Act The Surveying Act 2004 also sets out the role and functions of the Surveyors Registration Board of Victoria which is primarily concerned with the regulation of the training and Registration of Surveyors in the State. For cadastral surveys in Victoria, the Surveying (Cadastral Surveys) Regulations 2005 and Survey Co-ordination Regulations 2004 contain much of the specific detail to which surveyors must conform. The Survey Co-ordination Regulations 2004 makes reference to the Intergovernmental Committee for Surveying and Mapping Standards and Practices for Control Surveys (Special Publication 1), known as SP1. This section assumes that the surveyor has a solid understanding of the concepts and specifics of these regulations and SP1. The use of Global Navigation Satellite Systems (GNSS) (predominantly GPS carrier phase-based positioning) has been adopted and used by the surveying profession. Traditionally, GPS has been used for high precision geodetic survey, engineering and topographical surveys (via post processing and real time techniques). Use by practitioners for cadastral surveys has been limited by equipment costs, unfamiliar operational procedures and the current accuracy requirements of State legislation. However, licensed surveyors are expected to be capable of deciding: whether GPS can be effectively used to achieve the cadastral standards required by Victorian legislation, and the appropriate techniques to achieve this required accuracy. These guidelines outline recommended procedures in the relation to the use of GPS for cadastral surveying in accordance with the Surveying (Cadastral Surveys) Regulations It should be noted that the use of GPS might not be suitable to some areas such as built up areas where satellite visibility is poor. It is also necessary for cadastral practitioners to be well trained and educated in the use of carrier phase-based GPS techniques, the testing and certification of equipment plus the appropriate field/office procedures associated with it. However, the use of GNSS does not differ from conventional surveying techniques in that quality assurance processes must be utilised on a routine basis. This is essential to ensure that satisfactory accuracy specifications can be, and are being, met. In reality, GPS is just another surveying tool and as such may be used in conjunction with traditional methods to provide sufficient information to fix boundaries, marks and occupations. Therefore, no matter what instrumentation is employed, it is the responsibility of the professional surveyor to know and understand the following to obtain the accuracies required of the survey:

2 the limitations of the equipment to be used the observational procedures the processing techniques geodetic and map projection reductions for the MGA, suitable practices to ensure measurement redundancies and basic statistical analysis 12.1 Validation of Equipment Legislation It is the prerogative of the Licensed Surveyor to determine the equipment to be used in carrying out a cadastral survey. The Surveying (Cadastral Surveys) Regulations 2005 require that licensed surveyors are required to maintain and compare survey equipment used for cadastral surveys. Survey equipment must be capable of achieving the levels of precision set out in Part B of the ICSM Standards and Practices for Control Surveys (Special Publication 1), referred to as 'SP1'. These requirements are in addition to those specified in the National Measurement Act 1960 and National Measurement Regulations Licensed Surveyors are to retain records of all calibration and standardisation, records may be inspected on request by the Surveyor- General. Thus, it is the surveyor s duty of care, under State legislation, to ensure that the equipment and methods used are capable of meeting the accuracy requirements. Also, from time to time the Surveyor General circulates practice directives that provide instructions for Licensed Surveyors in accordance with the Regulations and inform them of changes to the requirements of the Surveyors Board of Victoria. These directives include issues such as the frequency of equipment validation. Surveyor-General Victoria provides survey instrument calibration services, refer to for the calibration of EDMs, Tapes and Bands, and Staves. However, there are no guidelines for any other equipment, such as theodolites (optical or digital), gyro theodolites or GPS, which may be used for cadastral surveys. The basic vector used in cadastral surveying is the bearing and the horizontal ground distance of a line. It is only the distance that legislation currently attempts to calibrate. A differential GPS determination of the same line is a 3D vector which can then be reduced, like any other surveying method, into the same bearing and horizontal distance. There is still much uncertainty about the use of GPS and legal traceability based on the National Measurement Act A sub committee of the ICSM has been working to address this issue for many years and it is expected that it would recommend specific procedures for GPS validation. SP1 includes the Best Practice Guidelines for the use of GPS for survey applications. Fortunately, the Regulations state that surveyors need only ensure that the process and basis of comparisons with a standard is adequate for the legislated accuracies. A pragmatic approach to equipment verification, e.g. GPS, can therefore be adopted. The

3 Surveyor-General of Victoria has left this procedure to the professional discretion of the surveyor. Therefore, philosophically, any verification/testing process can be devised so long as it can be established that the measurements: have been compared to the appropriate standards, and can be made within the standards of accuracy as specified in the regulations. GPS manufacturers quote specifications for their receivers and processing software, which have been developed from extensive programmes of research and development. The Federal Geodetic Control Subcommittee in the USA (FGCS) test all GPS surveying receivers (single and dual frequency instruments) released onto the market. Manufacturers can test and verify quoted accuracy specifications for their products. However, it is the responsibility of the professional surveyor to validate all equipment purchased and used in practice Validation Methods The GPS validation process should test: equipment measurement techniques together with processing, and transformation and heighting methodologies. Successful validation will also demonstrate the competence of the surveyor in using GNSS technology to achieve the required accuracy. The surveyor should retain the results of validation to comply with Regulations 5 and 14 (2c) where necessary. Ideally, GPS validation would consist of a combination of various methodologies i.e. zero baseline test, a coordinated network, an RTK test site and a coordinated EDM calibration baseline. The combinations would be dependent on the GPS available to a surveyor e.g. static technique only, RTK or a combination. The frequency of validation would be consistent with the current Survey Practice Directive from the Surveyor General. However, it would be advisable for the surveyor to undertake validation as often as deemed necessary to satisfy professional due care, best practice and competence. A Zero Baseline test (All receivers) This can be carried out to check the correct operation of a pair of GPS receivers, associated antennas/cabling, and data processing software. As the name implies, a zero baseline test involves connecting two GPS receivers to the same antenna via an antenna splitter (as recommended by the manufacturer). The computed baseline should be theoretically equal to zero and any variation will represent a vector of receiver errors (usually results should give sub millimetre results). This is a very simple and inexpensive process which: verifies the precision of the GPS receiver measurements, proves that the receiver is operating correctly, and also validates the data processing software N.B. A zero baseline test does not examine satellite ephemeris, time or atmospheric errors.

4 However, making measurements and processing data over known baselines or a network of coordinated points can achieve this. correct modelling of the atmosphere as well as geoid determination to obtain AHD values A High Accuracy GPS Test Network (Static Techniques) This can be undertaken to ensure that the operation of GPS receivers, associated antennas/ cabling, and data processing software, give high accuracy baseline/coordinate results. Satellite ephemeris errors, clock biases and atmospheric effects must be removed or minimized during baseline processing. Network validation allows GPS equipment to be tested under realistic field conditions which includes the dynamic nature of the satellite constellation and the atmosphere. Mission planning (finding what time of day gives acceptable GDOP for observations) is as essential for GPS validation as it is for real survey applications. The test network should: consist of extremely stable ground marks with almost perfect sky visibility be of a very high precision e.g. first or second order have stations which are ALL coordinated in both the local geodetic system (AGD/GDA φ, λ, h or x, y, z) and plane projection (MGA E, N and AHD elevations) have a variety of baseline lengths and directions consist of points with varying elevations to check for the Example of GPS Test Network Network validation is suitable predominantly for static/ rapid static surveys because the baselines are generally longer than for kinematic surveys. However, RTK GPS equipment/firmware can be checked on a network to validate the equipment and the procedures used to obtain acceptable final results. After observing the network, the surveyor can process the data to produce a network of vectors. These vectors can then be reviewed, adjusted and analysed following conventional methods: a) Independent vectors can be used to determine loop closures and precision b) By holding the values of one of

5 these sites fixed, coordinates for all other sites are derived using the GPS observations initially using a minimally constrained least squares adjustment. The ensuing statistics can be reviewed and assessed. Any flagged outliers can be noted and examined. c) A subsequent adjustment can then be undertaken holding multiple stations fixed to calculate 3D final coordinates of all stations. This ensures that the adjustment has attempted to solve the transformation parameters of the local area. Final coordinates and baseline vectors can be compared to the known values. d) AHD elevations may be required and can determined using two methods. During the final least squares adjustment process using a geoid model (such as AUSGEOID98) within the software package, OR Manually, after the adjustment, by calculating height differences from known benchmark AHD values and comparing with corresponding ellipsoidal heights differences. A geoid model for the local area can be interpolated from this data for all other points. A Coordinated RTK/Kinematic Test Site Kinematic surveys are generally restricted to baselines of less than 10km and involve occupying points for a short period of time e.g. less than one minute. A test site, designed for techniques such as RTK (and possibly as a simplistic rapid static check), can be established that is an array of points to be coordinated from a fixed base station. At the end of the observation session final coordinates can be compared to a set of known values (E, N, MGA and AHD elevations). The inclusion of obstructions such as trees could be planned into the array to test the accuracy of the re-initialisation processes of the OTF hardware and firmware. The surveyor should re-observe the array under different satellite configurations to ascertain possible precision under varying conditions i.e. for horizontal coordinates and AHD determination. Example of RTK Test Array

6 An EDM baseline test (Static and RTK Techniques) A pair of GPS receivers (plus ancillary equipment) can be tested over the various pillars of a validated EDM baseline. Measurements would involve setting up one receiver on the start pillar and simultaneous observation would be made to the other one on each pillar along the baseline. EDM calibration baselines have been established throughout Victoria to service the requirements of the surveyors under the Surveying (Cadastral Survey) Regulations Because these baselines are certified annually as subsidiary standards of length, surveyors can then make a comparison of known lengths with EDM measurements. Similarly GPS-derived distances can be compared to the standard measurements. Example of a Combined EDM/GPS Coordinated Baseline baselines are used to validate the distance component of a measurement and not the vector as a whole. EDM baselines are rarely longer than one kilometre (i.e. well short of the operating range of GPS) and therefore only comparatively short distances can be checked. Finally, if the reduced GPS measurements can verify the known distances between the markers on the pillars of the EDM baseline, it can be considered that: the equipment is in good working order, competency has been proved for the observations technique and reduction processes undertaken, and the GPS receivers are capable of delivering baseline solutions that are within specification. This method of GPS validation would be useful for post processed and real time techniques e.g. static GPS and RTK respectively. It is important that the surveyor is well trained in GPS methodology and has a full understanding of the achievable accuracy of each technique so that baseline comparisons are realistic. One useful addition to an EDM calibration baseline for GPS validation would be for the end points to be coordinated in MGA to a high accuracy i.e. via connection to first/second order marks in a surrounding observed network. Therefore, GPS validation could include a coordinated network followed by a baseline comparison test. This combination would verify final 3D vectors and GPS derived distances. GPS can be used to measure the 3D geodetic vector of a baseline e.g. Δx, Δy, Δz. This can then be reduced to ground distance for comparison purposes. Traditionally, EDM

7 Additional Validation Considerations on GPS surveys. This is important for total quality management. All GPS equipment, software and procedures should be tested before general usage. Unlike EDM equipment, GPS receivers cannot be calibrated for scale because the definition of scale is inherent in the satellites and orbit data. However, Antennae should be checked for centring errors. These should not generally be significant if geodetic quality equipment is used for cadastral surveys. Antenna offsets may also be present when mixing different antenna types. Measuring a line of a few metres with GPS and comparing the results with a direct EDM or taped measurement can easily test this. Tribrachs should be regularly checked and if necessary adjusted to minimise plumbing and levelling errors. The validation of survey equipment is an attempt to ensure the quality of measurements. However, in line with good survey practice, it is recommended that multiple field checks be used throughout a GPS cadastral survey. If any significant modifications or upgrades are made to the GPS receiver or the postprocessing software, then the validation must be repeated. To avoid additional fieldwork for every software upgrade, reprocess the original validation raw data with the new version and check for any changes in the results. Another advantage of the validation process is that it allows the surveyor to train and evaluate the competency of staff employed 12.2 Surveys Determination of Survey Datum A licensed surveyor making a cadastral survey must adopt and verify a datum with a previous cadastral survey/plan and if practical bring the datum onto the MGA Traditionally a surveyor connects to various existing survey marks, checks their reliability and then establishes the survey orientation and scale. GPS observations can also be made directly between appropriate existing survey marks to set up the datum in the same way. Therefore the surveyor undertakes the same procedure of comparisons whatever means of measurement has been adopted. Where a GPS base station is used outside the area of the survey, appropriate existing surveys marks in the area of the survey still need to be connected. Note that a transformation of the GPS data to the local coordinate system of the origin marks may be required. The transformed data must then be used to prove the datum of the survey in terms of the Surveying (Cadastral Surveys) Regulations 2005 e.g. by calculating the GPS vectors between the origin marks and comparing with the bearings and distances between datum marks.

8 Field Survey GPS is only another instrumentation option for the practising surveyor. It provides the ability to operate over greater distances than with conventional equipment. Often base stations outside the area of the survey can be employed. All GPS surveys must be undertaken in accordance with accepted good survey practice such as: 1. GPS observation procedures should be designed to detect and eliminate: ambiguity initialisation errors the effects of multipath interference from electrical interference such as substations, microwave or other spurious radio signals poor satellite geometry due to satellite configuration and/or sky coverage obstructions 2. Observation networks and reduction procedures should be designed to ensure measurements are independent e.g. a multi-baseline static GPS survey observed for only one session provides some dependent baselines which may create uncertainty with the results. 3. Permanent Marks (PMs), Primary Cadastral Marks (PCMs) and reference marks (RMs) placed and measured using GPS should be intervisible, where possible, for ease of subsequent use by all suitable surveying techniques. 4. GPS observations for boundary definition are to be checked by independent observations from another base station. The checks may be made using any suitable instrumentation. The only exceptions to this would be ties to natural boundaries using techniques such as kinematic or RTK i.e. similar to traditional observations. 5. GPS observations from an independent base station can be used to connect survey or boundary marks to PMs, PCMs and RMs. The reference vector connection can be calculated from the independent GPS observations. Such observations must be independently checked to ensure compliance with the Surveying (Cadastral Survey) Regulations Sufficient observations are made to fix boundaries, marks and occupations using traditional best practice concepts for cadastral surveys i.e. not technology dependent. 7. Any boundaries marked using GPS techniques must conform to the accuracy standards of Regulation 7 of the Surveying (Cadastral Surveys) Regulations Connection of Cadastral Surveys to MGA The Surveyor-General of Victoria publishes Practice Directives from time to time to provide surveyors specific practice instructions and interpretation of regulations. Practice Directives are published on The Surveyor-General s requirements for

9 appropriate cadastral surveys to be connected to MGA94 came into effect on 1st July The Surveying (Cadastral Surveying) Regulations 2005 require that a licensed surveyor making a cadastral survey must adopt and verify a datum in accordance with a previous cadastral survey or plan and bring the datum onto the Map Grid of Australia 1994 (MGA94) in a manner specified in Regulation 14(2) of the Survey Co-ordination Regulations Co-ordinate values for marks contained in SMES with values specified as 3rd order or above result directly from a network adjustment and provide a more homogenous system than previously available with AMG66. Therefore, co-ordinate information is to be presented in terms of MGA94 in cadastral surveys commenced after 1 July 2005, where co-ordinate information is required in support of documentation to be lodged with either Titles Registration Services or the Surveyor-General. Surveys commenced before 1 July 2005 that are current and connected to AMG66 will be regarded as complying with the requirements of the relevant legislation and directives of the Surveyor-General. Generally, cadastral surveyors are requested to connect to coordinated marks. However, if these marks are not within 3 set-ups (traditional surveys), then the Surveyor General will arrange the provision of coordinated marks within the vicinity of the survey. Refer to Surveyor-General Practice Directives and Regulation 11 of the Surveying (Cadastral Surveys) Regulations Victorian surveyors have two possible GNSS services available to support the connection requirements of the Surveying (Cadastral Survey) Regulations 2005 and the Survey Coordination Act These are: 1. GPSnet A permanent GPS Base Station Network which records, distributes and archives GPS satellite correction data for post-processed accurate position determination in Victoria. Land Victoria, working in cooperation with Industry, has established public access, dual frequency base station infrastructure to support GPS users across the state. The surveyor can download the RINEX data from the base stations records and then differentially post-process with their own single/dual frequency receiver data to achieve accurate and reliable GDA/MGA positions. 2. AUSPOS a free online GPS Processing Service operated by Geoscience Australia which: provides users with the facility to submit dual frequency, geodetic quality, GPS RINEX data observed in a 'static' mode, to the GPS processing system and then receive rapid turn-around GDA and ITRF coordinates, and takes advantage of both the IGS product range and the IGS GPS network and works with data collected anywhere on Earth Note: Both these techniques allow precise GDA/MGA coordinates to be determined for GPS stations in the survey area. GPS receivers

10 can be placed directly onto PCMs or RMs without the need for multiple traverse setups to a coordinated mark. This is very useful for the coordination of new marks or for checking purposes Additional Survey Considerations 1. GPS is another measurement technique and as such should only be used if it is the most efficient and cost effective method of survey available 2. Use the receiver/antenna configurations recommended by the manufacturer but avoid mixing different types of receivers in a survey. 3. GPS allows the surveyor to place new stations exactly where required without the intervisibility requirements of traditional surveys 4. Mission planning is the first phase of managing any static or real time GPS survey. This is necessary to define significant aspects of the survey so that it can be performed effectively and efficiently under foreseeable conditions. Commercial planning software is available and by using the latest satellite almanac the surveyor can: visualise and predict satellite availability via graphs and tables, simulate field conditions with respect to satellite selection, time zones, site visibility obstructions, and elevation masks, and determine the best time of day for observation sessions, given necessary constraints on PDOP and sky view obstacles.

11 5. In order to minimise post-processing errors and biases, calculation of baselines should start from a mark based on known geodetic coordinates i.e. in a datum such GDA. The accuracy of these values should be better than 20 metres both horizontally and vertically. This is because an uncertainty of start position of 20 metres adds a systematic 1 ppm error into baseline results. 6. Take time to plan the baselines to be measured. Where possible the following concepts on GPS baselines should be considered. Connect GPS baselines to build up a network which increases the redundancies in the survey Similarly keep GPS loops small GPS traversing is acceptable between coordinated marks or can be checked via loop closure Unlike conventional surveys, the shape of a GPS network is not significant in the final accuracy. 7. It is vital that all GPS antennae heights are meticulously measured and checked. GPS baselines are required as ground-toground vectors. However, GPS vectors are observed from antenna phase centre to antenna phase centre in the field. Baseline processing software reduces these vectors based on antenna phase centre information supplied. Any errors in the measured antennae heights will affect the final reduced baselines i.e. affect both horizontal and vertical components of the vector. 8. Processed baselines can be used as input into network adjustment software but it is important to have the appropriate statistical input. GPS baselines that have been observed simultaneously in the same session are correlated (or linearly dependent). The misclose of any loop within the session would theoretically be zero. As a result there is no independent check or redundant observations for that session and any dependent vector is referred to as a trivial baseline. Combining Static and Kinematic GPS Surveys Measure between adjacent sites keeping baselines short i.e. baseline length affects accuracy For example, if stations A, B, C are observed in the same GPS session, then the baselines are correlated and vector A- C is a dependent or trivial baseline.

12 N.B. For n simultaneous GPS stations, there are n-1 independent baselines Independent checks, using GPS data, must come from additional observing sessions i.e. observations recorded at a different time. When using multiple receivers it is good survey practice to link sessions by re-observing some common baselines i.e. through pivot stations that are common to two or more sessions. These independent baseline sets (primarily Static techniques) can then be used to build up a network of control points. The established network can be checked for a series of independent loop closures, and then subsequently adjusted without an artificial sense of redundancy. Baselines from different sessions can be added together to form closure checks Linking GPS Session with a Common Baseline 9. In certain environments, the GPS antenna may receive multiple signals which have been reflected off nearby objects and surfaces e.g. large water surfaces, buildings and vehicles. Urban environments are the most likely to have multipathing problems. As a result of multipath, baseline vectors are altered

13 and as such the final position of the receiver is in error. Multipath errors are not constant but change rapidly over time due to the dynamic nature of satellite geometry. Therefore, these errors are particularly hard to detect and eliminate. The surveyor can minimise multipath effects by: making GPS observations from stations that are totally clear of objects and surfaces that may introduce signal reflection, or taking a second measurement after a suitable time period has elapsed (after about 30 minutes apart) plus making independent check from another station. 10. All high precision surveying applications require differential carrier phase observations. The processing software/firmware must be capable of determining the integer ambiguities for post processed static surveys (this may be only possible for short lines), prost processed kinematic and real time positioning techniques short line static and real time methods such as RTK. However, sometimes ambiguity initialisation can be incorrect even though the recommended techniques and statistical tests are followed. It is vital that the surveyor adopts procedures (whether real time or post processed techniques) to ensure the correct resolution of ambiguities. These procedures could include: Re-initialisation of real time GPS receivers e.g. OTF initialisation of the RTK rover - the receiver is re-started or by turning the antenna upside down so that lock is lost to all satellites. After re-initialisation, some marks could be checked for a second time. Re-occupation of the same base station at a later time the ambiguities resolution is random and points can then be checked. If the reoccupation is after a suitable time interval then this also provides a check on multipath errors. Occupying and observing from a second base station at later time this provides the most reliable check on ambiguity resolution as well as an independent check on base station coordinates and multipath errors. 11. Baseline processing provides the surveyor with a series of vectors plus an overall quality of the GPS measurements, but good surveying practice should include a network adjustment of all observations. The subsequent least squares adjustment will provide final results and an analysis of the consistency of the observed baselines within the network. Even RTK observations may be added to a network if the necessary field files are also logged (as recommended by the manufacturer) and then imported into the software for subsequent recomputation and adjustment.

14 12.3 Measurements and Dimensions Conventional Bearings and Lengths In line with the Surveying (Cadastral Surveys) Regulations 2005, GPS vectors and boundary lines determined via GPS are to be supplied as bearings, and horizontal distances (either ground distances on a plan or plane distance on MGA). Using two GPS receivers, the relative position or baseline between two station marks is determined i.e. a cartesian vector ΔX,ΔY, ΔZ in a geodetic datum such as WGS84. Post processing usually occurs in the WGS84 datum and then various transformations can be implemented to bring the vector into the required datum of GDA as a cartesian vector (or if necessary a geodetic azimuth and ellipsoidal distance). This is usually done within the proprietary GPS software. Once this has been done the geodetic vector can be projected onto the UTM to produce the MGA vector (ΔE,ΔN) and then into final coordinates (E, N plus Zone) if start values are known. N.B. 1. The GPS vector (the final bearing and distance) between two points determined from simultaneous GPS observations at those points is regarded as the measured dimension. 2. GPS observations on a plan shall be shown as the two dimensional polar (horizontal) vector between survey marks, e.g., as a bearing and reduced horizontal ground distance. GPS Coordinates [regulation 10] Where coordinates derived from GPS observations are being shown, they shall be provided as MGA coordinates (i.e. E, N plus Zone), and not as geocentric cartesian coordinates (e.g., X, Y, Z) or geographic coordinates (e.g., φ, λ, h) in GDA Heights Where heights are to be shown on the plan, GPS ellipsoidal heights (h) in GDA, must be transformed to the Australian Height Datum (AHD). This requires knowledge of the geoidellipsoid separation (N) for that particular geodetic datum. The N value can be determined by observations onto Benchmarks (i.e. AHD ellipsoidal height comparison) or by adopting a geoid model such as AUSGEOID Classification and Accuracy of Surveys The standards of accuracy for GPS data must comply with the Surveying (Cadastral Surveys) Regulations Accuracy applies to traditional cadastral survey techniques as well as the indirect measurements from various GPS methods such as static, kinematic and RTK. It is

15 always the responsibility of the licensed surveyor to use the appropriate instrumentation and procedures to achieve the accuracy of the Regulations. It should be noted that the bearing and distance of a measured line, is a vector that can be determined from conventional surveying or by GNSS. These vectors can then be manipulated in the usual way e.g. traversing, radiations, resections, intersections, and networks. GPS has certain advantages for the cadastral surveyor. Flexibility in designing surveys i.e. GPS can be used at all times of day (and night) and not significantly affected by poor weather conditions. The system is also global and can be used in any location. The two receivers, required for differential operation do not require line of sight intervisibility. This enables surveyors to coordinate marks to survey accuracy over distances which previously may have required several days of traverse measurements. It is this feature that makes GPS so attractive for survey work. Marks do not need to be placed for traditional traversing (i.e. line of sight such as on top of hills), but can be placed directly where they are needed. The system requires only a clear, unobstructed view of the sky above a selected elevation. Obviously this restricts the use of GPS in urban areas and densely forested areas. Differential GPS methods allow a high degree of precision to be obtained over distances from metres to thousands of kilometres. Thus the notion of a "traverse closure" is not always appropriate to GPS surveys as traversing is not necessarily the most efficient GPS observation procedure. GPS loop closures are applicable if each line is from an independent observation session. Therefore, the surveyor must design procedures to analyse results similar to those already employed by the profession e.g. statistics, least squares Independent Checks The need to perform independent checks on measurements is specified in the Surveying (Cadastral Surveys) Regulations This requirement applies, particularly, to the measurements used to locate and determine survey boundaries. Licensed surveyors are familiar with best survey practices required for checking conventional surveys. Independent checks on GPS surveys should be treated in a similar way to any traditional survey. The reliability of observations can be safeguarded by way of additional or redundant observations (such as traversing, radiations, intersections, distance and offset and distance measurement between radiations). It should be noted that differential GPS measurements between two receivers gives a vector for that observing session. That vector can be re-determined independently by observations made at a different time (at least 30 minutes after the first observation) to enable satellite geometry to change and thus ensure that any multipath errors will be detected.

16 Unfortunately, because this vector is not connected to the whole survey then multiple observations of the same vector cannot be accepted as an independent check for cadastral surveys i.e. not good survey practice. However additional observations are always useful to increase the redundancies in the survey. Checks may be made by GPS and/or traditional methods. A few examples include: GPS traversing by using two receivers simultaneously a single vector (bearing and distance) is obtained between stations. By an observation sequence of leap frogging receivers, a consecutive series of vectors analogous to a traverse is obtained. Each traverse line is then independent and conventional loop or traverse closures can be adopted. GPS network incorporating important marks into network observations involving numerous sessions of multiple receivers that are being moved to ensure sufficient redundancies. A minimally constrained least squares adjustment is then carried out and results can be analysed for precision and possible outliers detected. Observations from two or more base stations - using a GPS base station and fixing each mark via a rover receiver and then checked by an additional measurement from at least a second different base station (or alternatively multiple base stations to increase redundancy). This method will check for correct ambiguity initialisation plus multipath errors and also provide an independent check on the base station coordinates. Traditional terrestrial measurements - any three marks placed by GPS may be checked by terrestrial measurements i.e. the three inter distances or two distances and the included angle. Also, conventional survey techniques need to be used when GPS observations are impractical due to vegetation and buildings. Distances Only Angle and Distances 12.6 Reporting Under the cadastral regulations, a licensed surveyor must prepare a detailed survey report when lodging an abstract of field records with the Surveyor-General or the Registrar of Titles. However, the regulations stipulate that if a cadastral survey has been performed by methods other than a direct determination of directions and distances, then such methods

17 must be described. Therefore, the use of GPS for a cadastral survey would necessitate an additional section to the survey report required. The additional section would provide the following information to show that suitable GPS observations and reduction procedures were employed for the cadastral survey: 1. List of Equipment Used - the type and model of equipment used. Also, information on any base station service that has been used. 2. The date and validation methods of GPS equipment validation 3. Description of the GPS methods employed - a description of the methods could include: the method of survey used e.g., static, rapid static, stop and go, kinematic, or real time kinematic (RTK) the expected precision from the method of survey used. This may be provided by manufacturers, software providers, other survey literature or the surveyor s experience description of any specific parameters programmed into the receiver or used in processing that would be likely to affect the result of the survey, e.g. use of tropospheric models for static observations, an indication of observation, session times and ephemeredes used in the pots processing i.e. broadcast or precise the mode of operation e.g., single/dual frequency observations, carrier phase, differential pseudorange, or carrier phase smooth DGPS a tabulation of the observations used from any base stations a description of the GPS reduction techniques used including the software used 4. Assessment of GPS data quality the following would help prove the appropriateness of the methodology used for the survey. the repeatability of observations e.g. the maximum difference or standard deviation of repeated observations on each line a comparison of GPS observations with underlying work such as comparisons with traditionally determined vectors summary of independent checks to verify quality assessment e.g. loop closures or network analysis

18 12.7 Additional Information Annex 1, Background to GPS Surveying has been added so that the licensed surveyor has an overview on the general use of GPS for all types of surveys. Similarly, Annex 2, Geocentric Datum Of Australia, has been added to provide the surveyor with sufficient reference material to calculate GDA/MGA coordinates for their control networks and final cadastral corners. Finally, additional GPS information can be found in Surveying Using Global Navigation Satellite Systems document provided by the Surveyor-General of Victoria.

19 Annex 1 Background to Surveying Using Global Navigation Satellite Systems 1.0 GNSS Surveying History The term Global Navigation Satellite System or GNSS is now frequently used to describe a number of different satellite positioning systems utilised around the world. Satellite Positioning systems have been well exploited in Australia over the past 30 years. The U.S. Transit Doppler Navigation System was used for establishing geodetic control across the country. Transit Doppler has been supplanted by the far more versatile Global Positioning System (GPS). GPS was developed by the United States Department of Defence to satisfy military and civil positioning and timing needs. The Russian, Global Orbiting Navigation Satellite System (GLONASS) can also satisfy civil positioning requirements. The European Union is already planning an independent satellite-based radionavigation system referred to as GALILEO. Satellite-based techniques will continue to become an integral part of surveying. 2.0 GPS Basics The Global Positioning System consists of a constellation of 28 satellites that provide continuous instantaneous position and time information to users around the world. GPS is suitable for a broad range of surveying applications including: cadastral/ engineering setout, topographic mapping, and geodetic control. There are some important differences between satellite and conventional methods that need to be recognised, and where possible utitlised: Satellite techniques have the advantage of not requiring inter-visibility between observing stations and can be used at any location around the world at all times of the day. The relative position of two stations spaced 50m, 50km, or even 500km apart, is readily achieved with GPS. Satellite receiving stations require a relatively clear view of the sky. GPS derived positions are related to a mathematical representation of the earth (WGS84 datum), rather than the physical earth (mean sea level). Care therefore needs to be exercised when merging GPS and conventional data, in particular how GPS heights are handled. 3.0 Aims This appendix aims to provide a brief review of GPS surveying fundamentals and includes a guide on GPS surveying in Victoria. GPS technology does not replace good survey practice and the need for verifying results is emphasised throughout. The Internet provides a wealth of information to GPS users and links are given to some of the most important websites. 4.0 Positioning Basics 4.1 Point Positioning (Absolute Positioning) The basic GPS positioning technique relies on a distance resection computation. A receiver on the earth tracks signals transmitted from orbiting satellites. Using these signals, the range, or distance, to each satellite is determined by multiplying the measured transit time of the satellite signal by the speed of light. In practice, at least four satellites must be

20 observed to estimate the users location and the receiver clock offset. GPS has been designed so that generally there will be 5 to 10 satellites available above a user s horizon at any point on the earth. The location of each satellite must be known for the instant the satellite signal was transmitted. Each GPS satellite broadcasts its location in terms of an ephemeris (the broadcast ephemeris), which is generally accurate to better than 10 metres. A single GPS receiver, irrespective of its quality can now estimate its location to 10m 15m horizontally and 30m vertically. Previously, GPS accuracy was controlled by a U.S. policy of Selective Availability (SA). SA limited the accuracy achievable when using a single GPS receiver to 100m horizontally and 150m vertically, however it was removed in April Differential Positioning (Relative Positioning) By using two GPS receivers tracking the same satellites simultaneously, it is possible to remove many systematic errors and determine their relative difference in position to metre- or millimetre-level. The relative difference in coordinates between the receivers can be determined using a number of techniques which include codebased Differential GPS (DGPS) or carrier phase based differencing. More information on relative positioning is available in the Surveying Using Global Navigation Satellite Systems document available: GPS Surveying GPS Surveying equipment is capable of tracking the carrier phase signals. The GPS L1 and L2 bands have wavelengths of approximately 19 and 24 centimetres respectively. Given that the carrier waves can be tracked to within a few percent of the wavelength, this means that millimetre-level positioning is possible. Unfortunately, carrier phase measurements contain a cycle ambiguity term that must be resolved to obtain accurate results. At the start of the day, and every time signal tracking is interrupted, the cycle ambiguities must be resolved. Modern surveying equipment and processing software automatically handle carrier phase ambiguity resolution. 5.1 Single and Dual-Frequency Equipment GPS Receivers that are capable of tracking only the L1 carrier wave are termed singlefrequency. Dual frequency equipment is able to track both the L1 and L2 carriers. Dualfrequency receivers are more expensive than single-frequency, however they are more versatile. Dual-frequency observations can be used to correct for the ionospheric error which becomes significant on baselines longer than 10km. Ambiguity resolution is faster and more reliable with dual frequency equipment and therefore improves productivity over singlefrequency receivers. 5.2 Error Sources GPS equipment is relatively insensitive to systematic and random error sources. Gross errors caused by incorrect antenna height measurement should be guarded against at all times always check antenna heights.

21 Carrier phase multipath is caused by signal reflection from around the antenna. Antenna ground planes and careful siting of stations away from buildings can help reduce carrier phase multipath. The maximum carrier phase multipath error is 5cm. Observing longer sessions at a site that is suspected of extreme multipath will help reduce the impact. An elevation mask rejects satellites that are low on the horizon and therefore helps minimise the incidence of multipath. Atmospheric errors tend to increase linearly as baseline length increases and only reach decimetre-level when baseline lengths exceed 20km and where receiver height differences are greater than 200m. An elevation mask also helps to reduce atmospheric errors. Atmospheric errors, particularly ionospheric errors, can be reduced using relative positioning techniques. Satellite ephemeris errors are generally insignificant up to baseline lengths of 100km or more. Antenna phase centre variation is the apparent movement of the electrical centre of the antenna away from the physical centre of the antenna. Phase centre errors are on the order of 1-2mm. The impact of phase centre variation is minimised by using matched antennas and aligning them in the same direction heights must be accurately determined to relate measured baseline vectors to the ground marks. More information on GPS is available in the Surveying Using Global Navigation Satellite Systems document available: The surveyor ultimately must use his or her professional judgement to select an appropriate observation session length. 7.0 Kinematic Surveying The continuous-kinematic and stop-and-gokinematic surveying techniques are used to improve productivity in open areas where many points need to be located. A reference receiver is established in an open area at a fixed location. One or more rover receivers are then moved to various points of interest. With continuous kinematic positioning, the relative location of the rover antenna is determined at each epoch. Whereas in the stop-and-go technique, the antenna location is only required when it is stationary over various marks. At least 5 satellites are required to initialise the carrier phase ambiguities. At least 4 satellites and preferably 5 or more must be observed during kinematic surveys. In many cases the satellites are tracked continuously whist the rover receiver is in motion between points of interest. 8.0 Post-Processing and Real-Time Kinematic Satellite measurements collected at sites in the field can be stored then combined in a computer for post-processing. Results are only obtained after the fieldwork is complete. In contrast, Real-Time Kinematic (RTK) methods use a datalink, usually in the form of a radio, to transfer reference station measurements to the rover receiver(s). Real-Time Kinematic techniques provide results with very little delay and therefore are suited to set-out applications.

22 Normally Real-Time Kinematic methods are limited in terms of the range that the rover can be from the reference. Furthermore, results are only obtained where there is radio coverage from the reference station. More information on post-processing and real time kinematic GPS can be obtained in the Surveying Using The Global Positioning System document available from Land Victoria. 9.0 Accuracy Standards 9.1 Cadastral and Engineering The accuracy standards for cadastral surveys are described in the Surveying (Cadastral Surveys) Regulations 2005 and reference made to SP1. Note that GPS measurements are not currently considered legally traceable under the National Measurement Act The ICSM and National Standards Commission are considering the means for achieving legal traceability of GPS measurements. GPS surveys should be connected to existing survey marks to provide a source of independent verification. The use of GPS for cadastral surveying has been already covered in the main Section 12 document. Engineering surveys can be very efficiently carried out by GPS techniques provided ambiguity resolution and multipath issues are addressed. 9.2 Geodetic The ICSM SP1 Publication contains nationally accepted technical standards and specifications for horizontal and vertical control surveys within Australia. Practitioners should adhere to these guidelines whenever appropriate. 9.3 Mapping Although there are no formal accuracy specifications for mapping purposes, these are normally specified in contracts. In many cases, code-based differential GPS techniques (DGPS) are sufficient for obtaining metre level results Datums and Networks 10.1 General All surveys are performed in some coordinate system. The framework for a state or national coordinate system is referred to as a geodetic datum. In Australia several different models have been used to represent the earth. These models are periodically refined to incorporate more precise measurements made available due to the advancement of measurement technologies Australian Fiducial Network, Australian National Network and Local Control In 1992 a large amount of GPS data was collected across Australia and formed the basis of a national adjustment to derive the Geocentric Datum of Australia. The primary framework is termed the Australian Fiducial Network (AFN) and consists of 10 highly stable pillars, with relative displacements known to better than 5cm. The Australian Fiducial Network is further subdivided into the Australian National Network (ANN), which includes 78 stations. At a practical level, local control is connected to the Australian National Network via high precision GPS surveys.