Standard for the Australian Survey Control Network

Transcription

1 Standard for the Australian Survey Control Network Special Publication 1 Intergovernmental Committee on Survey and Mapping (ICSM) Geodesy Technical Sub-Committee (GTSC) 30 March 2012

2 Table of contents Table of contents...ii List of Tables...ii List of Figures...ii Terms and definitions...iii Terms and definitions...iii 1 About this Standard Introduction Purpose of this Standard Brief history of this Standard Maintenance of this Standard Informative comment in this Standard References Scope Intended use of this Standard Connection to datum National geodetic datum National height datum Conducting control surveys Quantifying survey quality Adjustment and testing of survey control Circular expression of uncertainty Classifying and testing survey uncertainty...7 List of Tables Table 1 Orders of uncertainty...7 Table 2 Order classifications for maximum allowable levelling misclosure...8 List of Figures Figure 1 Relationship between the standard error ellipse and the horizontal circular confidence region (not to scale)...7 Intergovernmental Committee on Survey and Mapping ii

3 Terms and definitions For the purpose of this Standard, the following terms and definitions shall apply. Term/Acronym Accuracy AFN AHD71 AHD TAS83 AUSGeoid09 Constrained adjustment. Fully Constrained adjustment. Minimally CORS Datum Error Error. Gross Definition The level of closeness of measurements of a quantity to its true value. The greater the difference between a measurement, or computed value, and the true value means a lower level of accuracy. Australian Fiducial Network. Network of eight Continuously Operating Reference Stations around Australia that were used to define the Geocentric datum of Australia 1994 (GDA94). The Australian Height Datum 1971 is the NGRS orthometric height datum for mainland Australia. The Australian Height Datum (Tasmania) 1983 is the NGRS orthometric height datum for Tasmania. The national geoid model for converting ellipsoidal heights to AHD heights. An adjustment which has a sufficient number of constrained coordinates to optimally propagate datum and uncertainty throughout the survey control mark network. An adjustment which has the minimum number of constrained coordinates required to calculate all dimensions of the network s reference frame (1D, 2D or 3D). Continuously Operating Reference Station. A survey control mark hosting a permanent GNSS station. An official, fully-defined, spatial reference system to which measurements and/or coordinates upon the Earth may be defined in one, two, three or four dimensions. The difference between the true value and a measured or computed value. Error may be attributed to one of three types random, systematic or gross. Since error is never precisely known, uncertainty is used to indicate the likely error after all known systematic and gross errors have been removed. The result of a definite mistake during the process of measurement. Intergovernmental Committee on Survey and Mapping iii

4 Error. Random Error. Systematic GDA94 GNSS GRS80 GTSC ICSM ITRF Measurement MGA94 MSL NGRS Order Measurement is a process that is subject to random influences. Fluctuations in these influences are circumstantial and dependent upon chance. The variation between any one measurement and the average (or mean) of a set of measurements is known as random error. Error due to a consistent influence on the measurement process. As a consequence, this type of error effects measurements in a systematic way and tends to be consistently positive or consistently negative. Geocentric Datum of Australia Realised by the derived coordinates of the Australian Fiducial Network (AFN) geodetic stations, referenced to the GRS80 ellipsoid and determined with respect to ITRF92 at epoch Global Navigation Satellite System a generic term for satellite based positioning systems. Geodetic Reference System 1980 reference ellipsoid, where a = m, f = 1 / Geodesy Technical Sub-Committee Intergovernmental Committee on Surveying and Mapping International Terrestrial Reference Frame - a realisation of the International Terrestrial Reference System produced by the International Earth Rotation and Reference Systems Service (IERS). A measurement is an observed value, the outcome of a repeated set of observations, or the result of processing such observations. Map Grid of Australia Universal Transverse Mercator projection of the Geocentric Datum of Australia Mean Sea Level A tidal datum derived from the arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years). Australia s National Geospatial Reference System - a combination of infrastructure, data, software and knowledge. It includes all aspects of a coordinate datum, along with tools, utilities and the standards and guidelines that facilitate its use. A classification of control survey uncertainty which reflects the adopted survey techniques and measurement quality. Intergovernmental Committee on Survey and Mapping iv

5 Precision The Standard Survey control mark Uncertainty A term used to quantify the variability (or random error) of a measurement or computed value. If several measurements are taken repeatedly to represent the same quantity, precision is used to refer to the degree of closeness or conformity of those measurements to each other. For both measurements and computed values, precision is measured by uncertainty. Standard for the Australian Survey Control Network Special Publication 1 A monument that provides a physical realisation of one or more datums. The range within which the value of a measurement lies. It is a measure of random error and is expressed in terms of precision according to the 95% confidence level. Used to quantify the level of precision, not accuracy. Uncertainty. Positional The uncertainty of the horizontal and/or vertical coordinates of a survey control mark, measured in metres, at the 95% confidence level with respect to the defined reference frame arising from a fully constrained least squares adjustment. Uncertainty. Relative Survey UTM The uncertainty of the horizontal and/or vertical coordinates of a survey control mark, measured in metres, at the 95% confidence level arising from a minimally constrained least squares adjustment. That is, the uncertainty of a coordinate relative to the survey it was observed in, irrespective of datum connection. Universal Transverse Mercator projection. Intergovernmental Committee on Survey and Mapping v

6 1 About this Standard 1.1 Introduction The development of Australia depends upon the availability of a reliable, high accuracy spatial referencing system. The authoritative referencing system in Australia is the National Geospatial Reference System (NGRS). NGRS is one of Australia s fundamental positioning infrastructure assets. It is connected to and contributes to the global reference frame, is used to establish Australia s geodetic and height datums, provides for the integration of all spatial information, and underpins a vast array of public, government and commercial positioning and navigation applications. NGRS is comprised of an integrated network of Continuously Operating Reference Stations (CORS) and survey control marks, information about those survey control marks, systems for the management and delivery of survey control mark information, and various policies, standards and guidelines. Australia s survey control network comprises the survey control marks within the Australian mainland and Tasmania, Australia's islands and its External Territories. The External Territories include the Ashmore and Cartier Islands, Christmas Island, Cocos (Keeling) Islands, the Coral Sea Islands, Heard and McDonald Islands, Norfolk Island and the Australian Antarctic Territory. The Intergovernmental Committee on Surveying and Mapping (ICSM) through its Geodesy Technical Sub-Committee (GTSC) administers national geodesy policies for the coordination of geodetic activities within Australia. This Standard promotes the adoption of uniform procedures to achieve the highest level of rigour and integrity in the delivery and maintenance of Australia s survey control mark network. 1.2 Purpose of this Standard The purpose of this Standard is to specify the minimum requirements for the determination of one-, two- or three-dimensional position and associated uncertainty of Australia s survey control marks relative to Australia s NGRS datums. 1.3 Brief history of this Standard This document, and its associated technical manuals and guidelines, supersedes: ICSM (2007), Standards and Practices for Control Surveys Special Publication 1 (version 1.7), Intergovernmental Committee on Surveying and Mapping, Canberra, Australia Intergovernmental Committee on Survey and Mapping 1

7 1.4 Maintenance of this Standard The maintenance of this Standard and its associated guidelines is the responsibility of the GTSC. 1.5 Informative comment in this Standard This Standard incorporates commentary on some of the sections. The commentary directly follows the relevant section and is printed in italics in a shaded panel. 1.6 References The following guidelines and ICSM technical manuals may have relevance to the application of this Standard. International Guidelines JCGM 100:2008, Evaluation of Measurement Data Guide to the Expression of Uncertainty in Measurement, Joint Committee for Guides in Metrology Bureau International des Poids et Mesures, Paris, France. SP1 Guidelines ICSM (2012), Guideline for the Adjustment, Testing and Classification of Survey Control, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM (2012), Guideline for Control Surveys by Differential Levelling, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM (2012), Guideline for Control Surveys by GNSS, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM (2012), Guideline for Control Surveys by Total Station, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM (2012), Guideline for the Installation and Management of Survey Control Marks, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM (2012), Guideline for Continuously Operating Reference Stations, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. ICSM Technical Manuals ICSM (2006), Geocentric Datum of Australia Technical Manual, Intergovernmental Committee on Surveying and Mapping, Canberra, Australia. Intergovernmental Committee on Survey and Mapping 2

8 ICSM (2007), Australian Tides Manual Special Publication 9, Intergovernmental Committee on Surveying and Mapping, Wollongong, Australia. 1.7 Scope This Standard and its associated guidelines prescribe ICSM's requirements for: Connection of control surveys to Australia s NGRS datums Conducting control surveys Adjustment of survey control Testing of measurement and network reliability Quantifying, testing and classifying uncertainty 1.8 Intended use of this Standard Government and private sector organisations shall comply with this Standard when their surveys are intended for the development and maintenance of Australia s survey control mark network. This Standard may also be used to guide the development and maintenance of private survey control marks and networks used for other purposes. Such survey control marks may not be formally included in Australia s NGRS survey control mark network, but may be specified to be consistent with it. Information on how to use this Standard, together with additional technical detail and commentary, is provided in the technical manuals and guidelines listed in the reference section. Intergovernmental Committee on Survey and Mapping 3

9 2 Connection to datum Survey control marks in Australia s NGRS shall be coordinated relative to the NGRS datums defined in the NGRS as set out below. 2.1 National geodetic datum The NGRS geodetic datum for Australia is the Geocentric Datum of Australia 1994 (GDA94). GDA94 is a three-dimensional, static coordinate system based on International Terrestrial Reference Frame (ITRF) 1992, held fixed at the reference epoch of 1 January Coordinates of survey control marks on the Australian mainland, Tasmania, Australian islands and Australian External Territories, shall be computed in terms of GDA94. The Universal Transverse Mercator (UTM) projection of GDA94 (using the GRS80 ellipsoid) is known as the Map Grid of Australia 1994 (MGA94), and is the officially recognised grid coordinate realisation of GDA94. In the absence of GDA94 survey control marks, coordinates of survey control marks on Australian External Territories may be computed in terms of ITRF. Where ITRF coordinates are used, the definition and epoch of the ITRF need to be clearly specified. Note: At the time GDA94 was produced, GDA94 and ITRF were coincident with WGS84. However, due to the tectonic motion of the Australian plate (approximately 70 mm per year in a NNE direction) and refinements to ITRF, both ITRF and WGS84 realisations have diverged from GDA94. Consequently, GDA94 cannot be considered equal to WGS84 or the latest realisation of ITRF. 2.2 National height datum The NGRS orthometric height datum for Australia is the Australian Height Datum (AHD). Orthometric heights for survey control marks on the Australian mainland shall be referred to the Australian Height Datum 1971 (AHD71). Orthometric heights for survey control marks on Tasmania shall be referred to the Australian Height Datum (Tasmania) 1983 (AHD-TAS83). In the absence of AHD survey control marks, orthometric heights for Australia's islands and External Territories shall be specified to refer to at least one of the following: (a) A locally recognised height datum. (b) Locally-determined Mean Sea Level (MSL). (c) Derived AHD using an accepted geoid model and either a differential GNSS connection to AHD, MSL or ellipsoidal height. Intergovernmental Committee on Survey and Mapping 4

10 3 Conducting control surveys When observing a survey control mark network there are a variety of measurement and processing techniques available to the surveyor. The surveyor should select the techniques which best suit the requirements of the project, keeping in mind the limitations of each technique. In association with this document, a selection of survey control guidelines have been created to outline the recommended survey practice procedures required to achieve certain levels of control survey Order. The survey control guidelines are listed in Section 1.6. Note: All survey instruments used to conduct control surveys shall be tested and calibrated where appropriate, to minimise the effects of instrument error on the survey observations. 4 Quantifying survey quality The quality of the horizontal and/or vertical coordinates of a survey control mark shall be quantified in terms of Relative Survey Uncertainty (RSU) and/or Positional Uncertainty (PU). RSU is the uncertainty of the horizontal and/or vertical coordinates of a survey control mark arising from a minimally constrained least squares adjustment. RSU is free from the influence of any imprecision or inaccuracy in the underlying datum and therefore reflects only the uncertainty resulting from survey measurement error, measurement precision and network geometry. RSU represents the quality of the survey itself. PU is the uncertainty of the horizontal and/or vertical coordinates of a survey control mark with respect to the defined reference frame obtained from a fully constrained least squares adjustment. PU reflects the uncertainty of the datum to which a survey control mark is connected, and includes uncertainty resulting from survey measurement error, supplied measurement precision and network geometry. PU represents the combined uncertainty of the datum and the quality of the survey. Recognising that PU cannot be reliably quantified for AHD, orthometric height control surveys using differential levelling techniques shall be specified and assessed in terms of survey misclosure (see Section 4.3). 4.1 Adjustment and testing of survey control The horizontal and/or vertical coordination of survey control marks relative to the NGRS shall be adjusted in a rigorous least squares adjustment process. Throughout and following least squares adjustment, survey control projects shall be tested to demonstrate the reliability of the survey and computed coordinate uncertainty. Intergovernmental Committee on Survey and Mapping 5

11 The specification and testing of reliability shall be expressed in terms of PU and RSU using the 95% confidence region. Refer to the Guideline for the Adjustment, Testing and Classification of Survey Control for ICSM s recommended practices for the adjustment of survey control, testing of survey measurements and network adjustment, and for computing and testing uncertainty. 4.2 Circular expression of uncertainty Uncertainty is generally expressed in the form of standard error ellipses. However, ICSM has adopted a circular confidence region as a simplified method for expressing uncertainty. For the horizontal component, the circular uncertainty is the radius, in metres, of a circular 95% confidence region. The radius (r) of the circular confidence region shall be computed in metres by: Horizontal Circular Uncertainty Radius (r) = a x K (4.1) where: where: a = the semi-major axis of the standard error ellipse (m) K = q 0 + q 1 H + q 2 H 2 + q 3 H 3 H = b/a b = the semi-minor axis of the standard error ellipse (m) q 0 = q 1 = q 2 = q 3 = Values for a and b shall be derived from the full a posteriori variance matrix obtained from the network adjustment. Figure 1 illustrates the relationship between the standard error ellipse and the circular form. For the vertical component, the radius (r) of the circular uncertainty is also the 95% confidence region and is obtained by: Vertical Circular Uncertainty Radius (r) = sd x 1.96 (4.2) where: sd = standard deviation of the adjusted height (m) Intergovernmental Committee on Survey and Mapping 6

12 95% Circular confidence region r a b Standard error ellipse Figure 1 Relationship between the standard error ellipse and the horizontal circular confidence region (not to scale) 4.3 Classifying and testing survey uncertainty Table 1 classifies the circular expression of uncertainty as defined in Section 4.2 into discrete ranges of uncertainty (Order) that shall be used to specify a desired outcome for a control survey. PU and RSU can be compared against the uncertainty ranges in Table 1. Order Uncertainty (m) 1st nd > rd > th > th > 0.25 Table 1 Orders of uncertainty Testing whether the quality of coordinates and heights meets a particular Order requires a direct comparison of the computed uncertainty with the specified uncertainty value. Table 2 classifies the maximum allowable misclosure into levels of Order for orthometric height control surveys using differential levelling techniques. Allocation of Order for new survey control cannot be better than the Order of the AHD heights upon which the survey is based. Intergovernmental Committee on Survey and Mapping 7

13 Order 1st 2nd 3rd 4th 5th Maximum allowable misclosure (mm) 2 x distance (km) 4 x distance (km) 12 x distance (km) 20 x distance (km) 30 x distance (km) Table 2 Order classifications for maximum allowable levelling misclosure More details on the assessment of vertical uncertainty are provided in the Guideline for Control Surveys by Differential Levelling. Note: The assigned level of Order should be commensurate with the Order corresponding to the adopted measurement technique as described in the relevant survey guidelines. In addition, the assigned PU Order cannot be lower than that of the survey control marks to which measurements have been made from, irrespective of the quality of the measurements. For example, if a project specifies that coordinates be produced at the third Order level with respect to datum, this implies that third Order survey techniques were undertaken, and that the PU for all marks arising from a constrained adjustment will not exceed five centimetres at the 95% confidence region (see Table 1). Alternatively, if a project specifies that coordinates be produced at the first Order level irrespective of datum, this implies that survey techniques corresponding to first Order were undertaken, and that the RSU for all marks arising from a minimally constrained adjustment do not exceed two millimetres at the 95% confidence region (see Table 1). Intergovernmental Committee on Survey and Mapping 8