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

(SEPTAR)

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


Geoid

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


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

N

Europe

N. America

typically several hundreds of metres

Geoid

S. America

Africa

The geoid and two ellipsoids


Ellipsoids examples

Ellipsoids - examples

namesemi-major axisflattening

Bessel 18416377397m1/299.15

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.


Many ellipsoids many datums

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’?


  • Geographic coordinates

    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


    Latitude is not unique

    Due to different Geodetic Datums:

    f1 ¹ f2

    f1

    f2

    Latitude is not unique !

    nor is Longitude


    President ford s secret alaskan visit

    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

    ?


    Washington to tokyo orthographic projection

    Washington to Tokyo - Orthographic Projection

    Tokyo

    Washington

    Anchorage


    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

    A

    A

    Northing

    Latitude

    North

    Longitude

    West

    Longitude

    East

    Easting

    equator

    Latitude

    South

    • (N,E) = F (Lat, Lon)

    • distortions


    What errors can you expect

    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


    Crs data in the oil industry

    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


    Metadata spatial referencing

    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


    Coordinate system

    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


    Types of coordinate ref system

    Types of Coordinate (Ref) System

    Earth curvature modelling


    Epsg geodesy wg

    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


    Epsg database

    >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


    Epsg data usage

    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


    Epsg data deprecation policy

    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


    Some myths about epsg

    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


    Geodetic skills

    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:


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