What is Geodesy ?

1. What is Geodesy ? Roel Nicolai senior consultant Geomatics and GIS Shell Exploration and Production Technology Application and Research (SEPTAR) Rijswijk - The Netherlands

2. All right then – what is it ? "geo daisia" = "dividing the earth" Aim: determination of the figure of the earth or, more practically: determination of the relative positions on or close to the surface of the earth. • oldest profession on earth but one • geodesy, not geodetics

3. Shape of the Earth Œ flat l large scale mapping • l street plans • l engineering surveys spherel small scale mapping, low accuracy l geography l survey calculations (medium accuracy) Žellipsoidlaccurate (geodetic) mapping l geodetic & survey calculations geoidl accurate heighting l satellite orbits l high accuracy geodetic calculations

4. C • everyday heights are relative to geoid (MSL) • physically exists D • must be measured (ð errors) • difficult to describe mathematically • even more difficult to calculate with • reduction of survey observations • map projections Geoid Locus (surface) of points with equal gravity potential approximately at Mean Sea Level

5. Geoid covering the USA (NGS96) 7.2 m - 51.6 m Note: This image shows the height of the geoid above the US reference ellipsoid

7. Ellipsoid ( = spheroid) Approximate the geoid (not the earth's surface) • good approximation possible • variations ~10-5 (±60 m over earth radius ~ 6,400,000 m) • ellipsoidal calculus is feasible • can be defined exactly: semi-major axis (size) and flattening(shape) • computational aid only; no physical reality • ellipsoidal heights are not practical • many choices possible (~50) • optimum local fit with geoid (sometimes global fit) • rotation axis parallel to mean earth rotation axis • based on surface geodetic observations

8. N N Europe N. America typically several hundreds of metres Geoid S. America Africa The geoid and two ellipsoids

9. Ellipsoids - examples name semi-major axis flattening Bessel 1841 6377397m1/299.15 WGS846378137m1/298.26 Clarke 18666378206m1/294.979 Bessel 1841: usage: • Europe (German influence sphere), Namibia, Indonesia, Japan, Korea • National control network and mapping. WGS84: usage: • the entire world • the GPS system in conjunction with a datum of the same name. Clarke 1866: usage: • USA except Michigan, Canada, Central America, Philippines, Mozambique • National control network and mapping.

10. location (origin) orientation shape size of the ellipsoid in space Many ellipsoids …. many datums Approximate the local geoid with different ellipsoids ... • different origins • different orientation of axes • different shapes and sizes • different GEODETIC DATUMS What is a ‘Geodetic Datum’?

11. Z-axis P Greenwich meridian H semi-minor axis semi-major axis f Y-axis l oblate at poles X-axis Geographic coordinates f= Geographic Latitude l= Geographic Longitude H= Ellipsoidal height

12. Due to different Geodetic Datums: f1 ¹ f2 f1 f2 Latitude is not unique ! nor is Longitude

13. 80 o N 75 o N Anchorage 60 o N Why not the shorter route? 45o N Washington 30o N Tokyo 15o N 0 o 15 o S 30o S 45o S 60o S 90 o E 120 o E 150o E 180 o 150 o W 120o W 90o W 60 o W 30o W 30o E 0 o President Ford’s secret Alaskan visit ?

14. Washington to Tokyo - Orthographic Projection Tokyo Washington Anchorage

15. A familiarly shaped ‘continent’ in different map projections Orthographic projection Globular projection Mercator projection Stereographic projection

16. Geographic and map coordinates A A Northing Latitude North Longitude West Longitude East Easting equator Latitude South • (N,E) = F (Lat, Lon) • distortions

17. Coordinate Reference System What errors can you expect? • Wrong geodetic datum: • several hundreds of metres • Incorrect ellipsoid: • horizontally: several tens of metres • height: not effected, or tens to several hundred metres • Wrong map projection: • entirely the wrong projection: hundreds, even thousands of kilometres (at least easy to spot!) • partly wrong (i.e. one or more parameters are wrong): several metres to many hundreds of kilometres • No geodetic metadata  coordinates cannot be interpreted • datum • ellipsoid • prime meridian • map projection

18. Well data Legal data Facility data Budget data Finance data Materials data Reservoir data IC specific data Production data Scheduling data Conceptual data Field survey data Human Resource data Safety and environm. data Transportation/logistics data Geospatial and geodetic data OpCo Managem't Control Data CRS data in the oil industry 35 30 25 20 Average number of CRUD actions per activity 15 10 5 0 Coordinate Reference System data Data module

19. SRS Spatial Reference System by geographic identifier SRS-by-geographic-ID Coordinate Operation by coordinates ISO 19112 CRS Coordinate Reference System Datum Coordinate System Operation method Operation parameters Prime Meridian Ellipsoid Metadata: Spatial Referencing

20. Coordinate System • Link between the geodetic metadata and coordinates: • Defines the coordinate axes • Names, abbreviations • Angles between the axes (& properties of coord.space) • Units of measure • Order of the coordinates (and axes) • Determined by local custom: • Many parts of Europe: X (= northing), Y (= easting) • Elsewhere: X (= easting), Y (= northing) • Everywhere: Latitude, Longitude

21. Types of Coordinate (Ref) System Earth curvature modelling

22. EPSG Geodesy WG • Working Group of EPSG • European Petroleum Survey Group • Members: • Roger Lott – BP/Amoco (chair) • Roel Nicolai – Shell • Jim Cain – geodetic consultant, formerly Western Atlas • Bruno Ravanas – TotalElfFina • Alan Faichney – Society of Exploration Geophysicists • Geir Simensen – Statoil (Norway) • Approached by POSC in 1992 • Geodetic advice on EPICENTRE data modelling • Suppliers of geodetic reference parameters • Website: www.epsg.org (hosted by IHS Energy) • US sister established in 1999: APSG

23. >1800 Coord Ref Systems >1700 Coord Operations EPSG database • Documenting usage • Official CRS’s and CT’s • Commonly used CRS’s and CT’s • Initially oil industry focussed • Later extended to cover GIS users interests • Excluded: • Dubious and/or incomplete data • User-specific, often Local (engineering) systems • History: • Initially MS Excel files (1996) • MS-Access database • Version 4 – internal non-standard data model • Version 5 – EPICENTRE compliant • Version 6 – ISO 19111 (& OpenGIS) compliant • Algorithms for transformations • Guidance Note 7 • Version 1 commissioned by POSC • Later additions: EPSG

24. Coordinate Transformation Application (as delivered) Geodetic data with application replaced Geodetic data with application Coordinate Transformation Application (as used) Industrystandard geodetic data (EPSG) Geodetic data from user database User proprietary geodetic data User geodetic database EPSG data usage

25. EPSG data deprecation policy • New policy w.e.f. July 2001 • No data will be deleted • ISO and SEG requirement • Critical data in error • i.e. numerical integrity of coordinate data affected by error • new record(s) created with correct data • incorrect record marked as deprecated (+date and reason) • replacement path will be recorded • dependent records also deprecated and replaced • Change recorded explaining actions • Non-critical data in error • correction of data in record; revision date changed • Change recorded • Superseded data (in usage) • data stays in; comment added

26. Some myths about EPSG • “Reference to an EPSG number is all I need to solve all my spatial referencing problems” • EPSG cannot hope to record all CRS and CT data • You need to check whether data is still valid • “EPSG precribe how they believe CRS data should be used” • EPSG record common usage and established practice • “EPSG is liable for any errors in their database” • EPSG makes every effort to minimize the number of errors • EPSG have made a disclaimer on liability resulting from errors • “ISO 19127 is going to make the EPSG database obsolete” • ISO 19127 only collects ‘official’ parameters • ISO 19127 data will be included in the EPSG dataset

27. Corporate GEO-SCURVY !!! Geodetic skills • Are definitely needed in any “spatial” business • You need very little of it (like Vitamin ‘C’) • Deficiency will lead to the dreaded disease: