What is geodesy
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What is Geodesy ?. Roel Nicolai senior consultant Geomatics and GIS Shell Exploration and Production Technology Application and Research (SEPTAR) Rijswijk - The Netherlands. All right then – what is it ?. "geo daisia" = "dividing the earth".

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What is geodesy
What is Geodesy ?

Roel Nicolai

senior consultant Geomatics and GIS

Shell Exploration and Production

Technology Application and Research


Rijswijk - The Netherlands

All right then what is it
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

Shape of the earth
Shape of the Earth

Πflat 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



  • everyday heights are relative to geoid (MSL)

  • physically exists


  • must be measured (ð errors)

  • difficult to describe mathematically

  • even more difficult to calculate with

    • reduction of survey observations

    • map projections


Locus (surface) of points with equal gravity potential approximately at Mean Sea Level

Geoid covering the usa ngs96
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

Ellipsoid spheroid
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

The geoid and two ellipsoids




N. America

typically several hundreds of metres


S. America


The geoid and two ellipsoids

Ellipsoids examples
Ellipsoids - examples

name semi-major axis flattening

Bessel 1841 6377397m1/299.15


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.

Many ellipsoids many datums

location (origin)




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’?

  • Geographic coordinates








    semi-major axis




    oblate at poles


    Geographic coordinates

    f= Geographic Latitude

    l= Geographic Longitude

    H= Ellipsoidal height

    Latitude is not unique

    Due to different Geodetic Datums:

    f1 ¹ f2



    Latitude is not unique !

    nor is Longitude

    President ford s secret alaskan visit

    80 o N

    75 o N


    60 o N

    Why not the shorter route?

    45o N


    30o N


    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


    A familiarly shaped continent in different map projections
    A familiarly shaped ‘continent’ in different map projections

    Orthographic projection

    Globular projection

    Mercator projection

    Stereographic projection

    Geographic and map coordinates
    Geographic and map coordinates projections














    • (N,E) = F (Lat, Lon)

    • distortions

    What errors can you expect

    Coordinate Reference System projections

    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

    Crs data in the oil industry

    Well data projections

    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





    Average number of CRUD actions per activity





    Coordinate Reference System data

    Data module

    Metadata spatial referencing

    SRS projections

    Spatial Reference System

    by geographic identifier




    by coordinates

    ISO 19112


    Coordinate Reference System


    Coordinate System

    Operation method

    Operation parameters

    Prime Meridian


    Metadata: Spatial Referencing

    Coordinate system
    Coordinate System projections

    • 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

    Types of coordinate ref system
    Types of Coordinate (Ref) System projections

    Earth curvature modelling

    Epsg geodesy wg
    EPSG Geodesy WG projections

    • 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

    Epsg database

    >1800 Coord Ref Systems projections

    >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

    Epsg data usage

    Coordinate Transformation Application projections

    (as delivered)

    Geodetic data with application replaced

    Geodetic data with application

    Coordinate Transformation Application

    (as used)

    Industrystandard geodetic data


    Geodetic data from user database

    User proprietary geodetic data

    User geodetic database

    EPSG data usage

    Epsg data deprecation policy
    EPSG data deprecation policy projections

    • 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

    Some myths about epsg
    Some myths about EPSG projections

    • “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

    Geodetic skills

    Corporate GEO-SCURVY !!! projections

    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: