What is Geodesy?1 Open GIS Consortium for a changing world. Spatial connectivity What is Geodesy ? Roel Nicolai senior consultant Geomatics and GIS Shell

What is Geodesy? 1

Open GIS ConsortiumOpen GIS Consortium

for a changing world.

Spatial connectivity

What is What is Geodesy Geodesy ??

Roel Nicolaisenior consultant Geomatics and GIS

Shell Exploration and Production Technology Application and Research

(SEPTAR)

Rijswijk - The Netherlands

What is Geodesy? 2




All right then – what is it ?All right then – what is it ?

"geo daisia" = "dividing the earth"

"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

What is Geodesy? 3




Shape of the EarthShape of the EarthŒflat large scale mapping street plans engineering surveys

spheresmall scale mapping, low accuracygeography

survey calculations (medium accuracy)

ellipsoid accurate (geodetic) mapping geodetic & survey calculations

geoid accurate heighting satellite orbitshigh accuracy geodetic calculations

What is Geodesy? 4




GeoidGeoidLocus (surface) of points with equal gravity equal gravity potentialpotential approximately at Mean Sea Level

must be measured (errors) difficult to describe mathematically even more difficult to calculate with

reduction of survey observations map projections

everyday heights are relative to geoid

(MSL) physically exists

What is Geodesy? 5




Geoid covering the USA (NGS96)Geoid covering the USA (NGS96)

7.2 m7.2 m

- 51.6 m- 51.6 m

Note: This image shows the height of the geoid above the US reference ellipsoidNote: This image shows the height of the geoid above the US reference ellipsoid

What is Geodesy? 6




What is Geodesy? 7




Ellipsoid ( = spheroid)Ellipsoid ( = spheroid)Approximate the geoid 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 many choices possible (~50)(~50) optimum local fit with geoid (sometimes global fit) rotation axis parallel to mean earth rotation axis based on surface geodetic observations

What is Geodesy? 8




The geoid and two ellipsoidsThe geoid and two ellipsoidsNN

EuropeN. America

S. AmericaAfrica

GeoidGeoid

typically several hundreds of metres

What is Geodesy? 9




Ellipsoids - examplesEllipsoids - examplesname semi-major axis

flattening

Bessel 1841 6377397 m1/299.15

WGS84 6378137 m1/298.26

Clarke 1866 6378206 m1/294.979

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.

What is Geodesy? 10




Many ellipsoids …. many datumsMany ellipsoids …. many datums

Approximate the local geoid with different ellipsoids ...

different origins different orientation of axes different shapes and sizes

differentdifferent GEODETIC DATUMS GEODETIC DATUMS

What is a ‘Geodetic Datum’?location (origin)orientation shapesize

of the ellipsoid in space

What is Geodesy? 11




Geographic coordinatesGeographic coordinates

P

X-axis

Y-axis

Z-axis

semi-major axis

H Greenwich meridiansemi-minor

axis

oblate at poles

= Geographic Latitude

= Geographic Longitude

H = Ellipsoidal height

What is Geodesy? 12




Latitude is not unique !Latitude is not unique !

1

2

nor is Longitudenor is Longitude

1

2

Due to different Geodetic Datums:

What is Geodesy? 13




President Ford’s secret Alaskan visitPresident Ford’s secret Alaskan visit

Why not the shorter route?

0 o

15 o N

30 o N

45 o N

60 o N

75 o N

80 o N

15 o S

30 o S

45 o S

60 o S

0 o 30 o E30 o W60 o W90 o W120 o W150 150 o o WW180 o 150 o E120 o E90 o E

Anchorage

Washington

Tokyo

?

What is Geodesy? 14




Washington to Tokyo - Orthographic ProjectionWashington to Tokyo - Orthographic Projection

TokyoAnchorage Washington

What is Geodesy? 15




Mercator projection

Globular projection

Orthographic projection

Stereographic projection

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

What is Geodesy? 16




Easting

Northing

LongitudeEast

Longitude West

equator

LatitudeNorth

LatitudeSouth

AA

Geographic and map coordinatesGeographic and map coordinates

(N,E) = F (Lat, Lon)

distortionsdistortions

What is Geodesy? 17




What errors can you expect?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

Coordinate Reference SystemCoordinate Reference System

What is Geodesy? 18




CRS data in the oil industryCRS data in the oil industry

0

5

10

15

20

25

30

35

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ge n

um

ber

of

CR

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acti

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urve

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ta

Fac

ility

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Pro

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Con

cept

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ata

Sch

edul

ing

data

Bud

get d

ata

Fin

ance

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Tra

nspo

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logi

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Mat

eria

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data

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ety

and

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spat

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eode

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OpC

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trol

Dat

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Data module Coordinate Reference System

data

What is Geodesy? 19




Metadata: Spatial ReferencingMetadata: Spatial Referencing

Prime Meridian Ellipsoid

Datum Coordinate System

Coordinate

Operation

ISO 19112

SRS-by-geographic-ID

by geographic identifier

Spatial Reference SystemSRS

Coordinate Reference System

by coordinatesCRS

Operation method

Operation parameters

What is Geodesy? 20




Coordinate SystemCoordinate 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

What is Geodesy? 21




Types of Coordinate (Ref) SystemTypes of Coordinate (Ref) SystemCoordinate Ref

SystemCoordinate

SystemCharacteristics

Geocentric Cartesian or spherical

Proper 3D spatial modeling;spatial applications

Geographic 3D ellipsoidal Locations described relative to ellipsoidal surface

Geographic 2D ellipsoidal Locations described on ellipsoidal surface; for large national/continental geodetic control networks

Projected Cartesian For national mapping; smaller area than Geographic 2D. Carefully controlled mapping distortions

Engineering various Earth curvature ignored; mostly flat-earth modelImage Cartesian or

oblique Cartesian

Distortions due to earth curvature determined by data acquisition characteristics

Vertical gravity-related,depth,barometri

c

Gravity-related means relative to geoid (~MSL)Depth: complex reference surfaces (tidal)

Eart

h c

urv

atu

re

mod

ellin

g

What is Geodesy? 22




EPSG Geodesy WGEPSG 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

What is Geodesy? 23




EPSG databaseEPSG 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

>1800 Coord Ref >1800 Coord Ref SystemsSystems

>1700 Coord >1700 Coord OperationsOperations

What is Geodesy? 24




EPSG data usageEPSG data usage

Coordinate Transformati

on Application(as delivered)

Geodetic data with

application

Geodetic data from user database

Geodetic data with application

replaced

Coordinate Transformati

on Application

(as used)Industry standard geodetic data

(EPSG)

User geodetic database

User proprietary geodetic data

What is Geodesy? 25




EPSG data deprecation policyEPSG 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

What is Geodesy? 26




Some myths about EPSGSome 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

What is Geodesy? 27




Geodetic skillsGeodetic skills Are definitely needed in any “spatial” business You need very little of it (like Vitamin ‘C’) Deficiency will lead to the dreaded disease:

Corporate GEO-SCURVY !!!Corporate GEO-SCURVY !!!

Documents

What is Geodesy?1 Open GIS Consortium for a changing world. Spatial connectivity What is Geodesy ? Roel Nicolai senior consultant Geomatics and GIS Shell