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What is Geodesy ?

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Roel Nicolai

senior consultant Geomatics and GIS

Shell Exploration and Production

Technology Application and Research

(SEPTAR)

Rijswijk - The Netherlands

"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

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)

ellipsoidlaccurate (geodetic) mapping

l geodetic & survey calculations

geoidl accurate heighting

l satellite orbits

l high accuracy geodetic calculations

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

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

7.2 m

- 51.6 m

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

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

N

N

Europe

N. America

typically several hundreds of metres

Geoid

S. America

Africa

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.

location (origin)

orientation

shape

size

of the ellipsoid in space

Approximate the local geoid with different ellipsoids ...

- different origins
- different orientation of axes
- different shapes and sizes

What is a ‘Geodetic Datum’?

Z-axis

P

Greenwich

meridian

H

semi-minor

axis

semi-major axis

f

Y-axis

l

oblate at poles

X-axis

f= Geographic Latitude

l= Geographic Longitude

H= Ellipsoidal height

Due to different Geodetic Datums:

f1 ¹ f2

f1

f2

nor is Longitude

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

?

Tokyo

Washington

Anchorage

Orthographic projection

Globular projection

Mercator projection

Stereographic projection

A

A

Northing

Latitude

North

Longitude

West

Longitude

East

Easting

equator

Latitude

South

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

Coordinate Reference System

- 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

- entirely the wrong projection:
- No geodetic metadata coordinates cannot be interpreted
- datum
- ellipsoid
- prime meridian
- map projection

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

35

30

25

20

Average number of CRUD actions per activity

15

10

5

0

Coordinate Reference System data

Data module

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

- 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

Earth curvature modelling

- 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

>1800 Coord Ref Systems

>1700 Coord Operations

- 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

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

- 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

- “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

Corporate GEO-SCURVY !!!

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