Surveying & Positioning Guidance note 10

Transcription

1 Surveying & Positioning Guidance note 10 Geodetic Transformations Offshore Norway Revision history Version Date Amendments 1.0 April 2001 First release 1 Background European Datum 1950 (ED50) is the de facto and de jure geographic coordinate reference system for oil industry licencing and operations offshore Norway. In 1981 the nations bordering the North Sea published technical recommendations for deriving ED50 coordinates from the coordinate reference systems used by the then current satellite navigation systems, WGS72 and WGS72BE 1. These recommendations were widely adopted by the oil industry. In some parts of the North Sea (Denmark, UK) their use has continued after modification for the difference between WGS72 and the WGS84 system used by current generation GPS navigation satellite positioning, but this usage is no longer appropriate in Norway. In 1990 the nations bordering the North Sea published technical recommendations for deriving ED50 coordinates from WGS84 2. These are the so-called North Sea formulae, formerly adopted for use in Norway south of 62º North. They use a two-step approach, first from WGS84 to ED87 using a position vector 7-parameter transformation, then from ED87 to ED50 through a 4 th -order reversible polynomial 3. In 1991 a Statens Kartverk note reconfirmed the adoption of the North Sea formulae for offshore Norway south of latitude 62º North 4. This same note detailed a position vector 7-parameter transformation for use offshore Norway north of latitude 65º North 5. The note also advocated linear interpolation between the results from this transformation and the North Sea formulae for the area between 65ºN and 62ºN. These various transformations were included in the Statens Kartverk program SLGTRF. (This program is no longer available). In 1997 Statens Kartverk published coordinates for a new Norwegian geodetic network, Stamnettet, based on ETRS89 realised through an adjustment called EUREF89 (see paragraph 10). A new transformation between ETRS89 and ED50 was defined for use offshore Norway north of latitude 65º North 6. This transformation was included in the Statens Kartverk coordinate conversion program 1 Report of an investigation into the use of doppler satellite positioning to provide co-ordinates on the European Datum 1950 in the area of the North Sea. Ordnance Survey of Great Britain Professional Paper New Series No. 30, The Transformation between ED50 and WGS84 for exploration purposes. Norwegian Mapping Authority. Geodesy Division. Geodetic publication Nr.1990:1 3 See EPSG geodetic parameter data set geodetic transformation ED50 to WGS 84 (14) (code 8046) or the reference in footnote 2 for details. 4 Norwegian Mapping Authority note of 13-Feb-1991 Om Transformasjon mellom Geodetiske Datum i Norge 5 From ED50 to WGS84: dx= dy= dz= metres; rx= ry= rz= microradians; ds= ppm. See EPSG geodetic parameter data set geodetic transformation ED50 to WGS 84 (15) (code 8047) or reference in footnote 4 for derivation. 6 Position Vector 7-parameter transformation. From ED50 to ETRS89: dx= dy= dz= metres; rx= ry= rz= microradians; ds= ppm. See EPSG geodetic parameter data set, code 1588.

2 WSKTRANS at this time. For offshore Norway, this 1997 transformation produces results which differ from the 1991 transformation by between approximately 5cm at 72º North to 70 cm at 65º North. The interpolation between 62º North and 65º North was maintained, but because the transformation north of 65º North had changed the results of the interpolation differ from previous interpolation results by 0cm at 62º North and 70 cm at 65º North. However this update to WSKTRANS was not widely known. Many practitioners using alternative coordinate transformation applications continued to use the 1991 data. Most offshore oil industry operators and their service providers have not included the North Sea formulae or the mid-norway interpolation into real-time positioning applications. Many different alternatives have been used as pragmatic approximations. Most of the data management software packages in use within the oil industry are off the shelf international products that do not allow for non-geocentric geodetic transformation methods. 2 The relationship between ETRS89 and WGS84 The International Earth Rotation Service (IERS) maintains a coordinate reference system for the international geodetic community. This system, the IERS Terrestrial Reference System or ITRS, represents the scientific community s best model of the Earth. It is realised through coordinates and coordinate velocities for a set of stations around the world and published periodically by IERS as the IERS Terrestrial Reference Frame, ITRFxx where xx represents the year of realisation 7. This is a global system. ETRS89 was defined to be coincident with the ITRS at 1/1/1989. But ETRS89 is not a global system: it is anchored to the stable parts of the EurAsian tectonic plate. This plate is moving at about 17mm per year relative to the ITRS. In just over a decade since 1989, ETRS89 has diverged from the current ITRS by about 25cm. The two continue to diverge. See also footnote 8. WGS84 is maintained by the US. Like the ITRS it is a global system. Initially it was coincident with ITRS to an accuracy of about 1.5 metres. It is currently maintained within 10cm of the ITRS. WGS84 is therefore not fully consistent with earlier published versions of ETRS89. But to an accuracy of a few decimetres the two can be considered to be approximately the same. This approximation will remain valid for several years. 7 In some international geodetic literature both the terms System (as in ITRS) and Frame (as in ITRF) have been adopted as synonomous to Datum. Strictly speaking these terms have slightly different meanings. 8 In a number of countries (including Norway), the term EUREF89 (also called ETRF89) has been used for the reference system realisation introduced following a European GPS campaign carried out in May To avoid confusion in the naming, the name EUREF89 has been retained and subsequent network adjustments have been referred back to the same epoch (1/1/1989). For oil industry practical purposes EUREF89 and ETRF89 can be taken to be aliases of ETRS89. 2

3 3 Recommendations With effect from 1 st May 2001 the following transformation is recommended for converting GPS WGS84 coordinates to ED50 coordinates offshore Norway north of 62º North: Transformation method: Position Vector 7-parameter transformation Transformation parameter values: dx = m dy = m dz = m rx = µrad = sec ry = µrad = sec rz = µrad = sec ds = ppm See annex 1 for examples of the application of this transformation. For transformations from ED50 to WGS84 the signs should be reversed 9. Adoption of this recommended transformation north of 62º North will create differences from previous practice of up to 4 metres. These differences are within the general accuracy of offshore positioning and coordinate transformation uncertainty. It is therefore recommended that ED50 coordinates from historic operations offshore Norway north of 62º North remain unchanged and be considered consistent with application of the 2001 recommendation. No action is required for management of historic data. Where users have a special need for exact consistency with past practice they should continue to use that practice for local areas. But for new projects the above transformation is recommended. South of 62º North the 1991 North Sea formulae (paragraph 2 above) remains the recommended transformation between ED50 and WGS84. See annex 1 for examples of the application of this transformation. If a simpler transformation method from WGS84 to ED50 is required in the Norwegian sector south of 62º North, then as an approximation to about 1 metre the following is recommended: Transformation method: Position Vector 7-parameter transformation Transformation parameter values: dx = m dy = m dz = m rx = µrad = sec ry = µrad = sec rz = µrad = sec ds = ppm For transformations from ED50 to WGS84 the signs should be reversed 10. See annex 2 for examples of the application of this approximation. Adoption of the recommendations in paragraphs 10 and 13 above will give rise to a discontinuity in the transformation along the parallel of 62º North of between 3.2m in the west to 4.5m in the east (see annex 1). It must be emphasized that there is no discontinuity in the ED50 graticule, nor of the ETRS89 or WGS84 graticules. Operations across the 62º North parallel will have great difficulty in applying two discontinuous transformations. It is therefore recommended that for any field operation straddling the 62º North parallel, the transformation applicable to the majority of the survey area be applied to the whole survey. This is similar to the procedure adopted in the case of a survey crossing a projection zone boundary. 9 This reversed transformation is code 1612 in the EPSG geodetic parameter database. 10 This reversed transformation is code 1613 in the EPSG geodetic parameter database. 3

4 4 Discussion It is noted that the Norwegian license-block numbering schema has a discontinuity at 62º North, and also that the area immediately adjacent to the parallel of 62º North is of lesser geological interest to the oil industry than most of offshore Norway. For offshore oil industry operations, once an absolute accuracy of a few metres has been achieved then relative accuracy and consistency become more important. Uniformity of application between operators and service providers and ease of application in real-time field operations and data management are also important. The recommendations above have taken consistency and ease of use to be more important than continuity of transformation. Users of the Statens Kartverk coordinate conversion program WSKTRANS requiring these 2001 transformation recommendations need to use v4.0 or later versions of the program. 5 Summary of recommendations South of 62º North the 1991 North Sea formulae continues to be the recommended transformation between ED50 and WGS84 (paragraph 2). However for data management applications and real-time field operations where a two-step transformation and/or a polynomial transformation are not available, an approximation accurate to 1 metre may be used (paragraph 14). North of 62º North the recommended transformation between ED50 and WGS84 is as given in paragraph 10. No action is required from data managers following this clarification of geodetic transformation. Historically-derived ED50 coordinates may be assumed to be consistent with those derived through these 2001 transformations. Any operations which cross the parallel of 62º North should use only one transformation for the full survey area. The transformation adopted should be that which applies to the majority of the survey area. 4

5 Annex 1 ED50 to WGS84 co-ordinate transformations along the parallel of 62 degrees North. ED50 WGS84 via North Sea formulae WGS84 via north of 62º transformation North Sea formulae - north of 62º tfm Blocks north of 62 N Latitude Longitude Latitude Longitude Latitude Longitude True Dist Azimuth Blocks south of 62 N UK-Norway boundary 33/2 NW corner 6201/11 SW corner N E N E N E 3.22m /3 NW corner 6201/12 SW corner N E N E N E 3.26m /1 NW corner 6202/10 SW corner N E N E N E 3.30m /2 NW corner 6202/11 SW corner N E N E N E 3.32m /3 NW corner 6202/12 SW corner N E N E N E 3.35m /1 NW corner 6203/10 SW corner N E N E N E 3.39m /2 NW corner 6203/11 SW corner N E N E N E 3.44m /3 NW corner 6203/12 SW corner N E N E N E 3.50m /1 NW corner 6204/10 SW corner N E N E N E 3.61m /2 NW corner 6204/11 SW corner N E N E N E 4.58m /3 NW corner 6204/12 SW corner N E N E N E 4.55m /3 NE corner 6204/12 SSE corner N E N E N E 4.42m Inshore boundary NB: The UK-Norway Boundary is taken as the intersection of the Geodesic between pts 9 and 10 of Treaty Series dated 31/1980 and the latitude 62deg N (both expressed in ED50 terms). The Norway Inshore boundary is taken as the intersection of the Geodesic between Norske grunnlinjepunkter 67 and 68 and the latitude 62deg N (both expressed in ED50 terms). Annex 2 Example ED50 to WGS84 co-ordinate transformations in the Norwegian sector of the North Sea. Location ED50 WGS84 via North Sea formulae WGS via North Sea approximation Differences Latitude Longitude Latitude Longitude Latitude Longitude True Dist Azimuth 33/2 NW corner N E N E N E /1 NE corner N E N E N E /2 NE corner N E N E N E Test point - 8/3 NE corner N E N E N E

6 Annex 3 Schematic diagram of EPSG Geodesy Database Transformations for Offshore Norway Past Future Year: EPSG CRS Name Code Comment ED50 to WGS84 (15) 8047 Codes , via ED87 Latitude 65 N ED50 to WGS84 (21) 8569 Codes , via ETR S89 ED50 to WGS84 (23) 1612 Bursa-Wolfe transformation, no more interpolation ED50 to WGS84 (20) 1450 Interpolation between 8047 and 8046 ED50 to WGS84 (22) 1590 Interpolation between 8569 and N ED50 to WGS84 (14) 8046 Concatenation of codes via ED87, including polynomial ED50 to WGS84 (23) 1613 Bursa-Wolfe transformation, approximates code Blackfriars Road London SE1 8NL United Kingdom Telephone: +44 (0) Fax: +44 (0) Bd du Souverain 4th Floor B-1160 Brussels, Belgium Telephone: +32 (0) Fax: +32 (0) Internet site: [email protected] Disclaimer Whilst every effort has been made to ensure the accuracy of the information contained in this publication, neither the OGP nor any of its members past present or future warrants its accuracy or will, regardless of its or their negligence, assume liability for any foreseeable or unforeseeable use made thereof, which liability is hereby excluded. Consequently, such use is at the recipient s own risk on the basis that any use by the recipient constitutes agreement to the terms of this disclaimer. The recipient is obliged to inform any subsequent recipient of such terms. Copyright OGP The contents of these pages are The International Association of Oil & Gas Producers All rights are reserved.