Week 6-MGA94 & AHD71-2018-v1 PDF

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MGA94 & AHD71 The National Datums

Week 6 NEC 2104 Engineering Surveying Stephen Larsen LS

MGA94 & AHD71  Mapping & Survey Control  Geodetic Datum of Australia 1994  Ausgeoid 2009  Mapping Grid of Australia 1994  Australian Height Datum 1971  Terminology (6)  Quiz  References

Mapping & Survey Control  Who is the map for?  Public  Architect  Planning Department  Construction Engineer/Contractor  What is the map for?  Overview/publicity/Public consultation  Preliminary design  Construction detail  As built details  Specifications

Example Map

Example Map – Metadata

Mapping – North (Reference Meridian)  Maps include a north point diagram which shows

the direction of true north, grid north and magnetic north at the centre of the map.  This graphic also shows the actual grid-magnetic angle for the centre of the map.  True north (TN) is the direction to the Earth’s geographic North Pole.  Grid north (GN) is the direction of the vertical grid lines on a topographic map.

Mapping – North (Reference Meridian) Magnetic north (MN) is the direction from any point on the surface of the earth towards the earth’s north magnetic pole. The angular difference between TN and MN is known as magnetic declination. As GN is used in preference to TN for map reading purposes, it is more useful to know the difference between GN and MN. This is known as the Grid/Magnetic angle. As the position of the north magnetic pole moves slightly from year to year, the grid/magnetic angle and magnetic declination will vary by a small amount each year. In using a map for accurate navigation, magnetic variation can be important, particularly if the map is several years old.

Mapping – Cross Section

AHD

Gravitational

GDA94 / WGS84

MAP

It is important to recognise that the relationship between these four surfaces is not always the same. Rather, as this diagram indicates, they 'wobble' around each other

Geodetic Datum  A Geodetic Datum approximates the shape of the Earth  Position defined by Latitudes & Longitudes  Spherical  However, the Earth is not evenly round - it is in fact wider

around the Equator than it is between the North and South Poles.  Ellipsoid / Spheroid  However, this is also a very simplistic concept. The Earth in reality is a very misshapen object.  Geoid  The Earth's surface which is complex to accurately describe mathematically.  The Earth's Geoid approximates Mean Sea Level or  AHD RL = 0.000

Geodetic Datum – Ellipsoid (Spheroid)

Geodetic Datum – Latitude & Longitude

Equator

Geodetic Datum  Mapping and coordinate systems are based on a datum,

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which is a mathematical surface that best fits the shape of the Earth. AGD66: Australia’s previous datum, the Australian Geodetic Datum was defined in 1966 and best fitted the shape of the Earth in the Australian region only. WGS84: Used for the GPS system operated by the U.S. Department of Defence. AGD84: An updated version of AGD66 was adopted by some Australian States in 1984. AGD84 coordinates are based on the same datum as AGD66 and for map reading and navigation purposes can be regarded as being the same. GDA94 is the current datum that was adopted in 2000 and is practically the same as WGS84

Australian Geodetic Datum of 1966

Local or Regional Datums This diagram represents the Australian Geodetic Datum which was created in 1984 (AGD84 or AGD 1966). • Note how the Ellipsoid/Spheroid has been 'placed' over Australia to best identify longitude and latitude. If you used this datum in the northern hemisphere longitude and latitude would be very poorly identified on the surface of the Earth.

AUSTRALIA

Geodetic Datum of Australia 1994  ITRF: International Terrestrial Reference Frame 1992

(ITRF92) at epoch 1994.0. ITRF is a global network of accurate coordinates (and their velocities) maintained by the International Earth Rotation Service (IERS)  GRS80: The Geodetic Reference System of 1980 is an internationally recognised ellipsoid which has been adopted for GDA.  WGS84:The World Geodetic System is the ellipsoid used by the GPS system operated by the U.S. Department of Defence.  GDA94: The Geocentric Datum of Australia. This earth centred Datum, totally compatible with the GPS satellite system, is realised by the position values for the AFN and ANN stations. This datum was accepted by ICSM in November, 1994.

GDA94  AFN: The Australian Fiducial Network. A network of 8

permanently tracking GPS stations around and within Australia which provide constant monitoring of the position of the Australian land mass upon which is founded GDA94.  ANN: The Australian National Network which consists of some 80 stations distributed across Australia which were observed over multiple-day sessions by GPS receivers and adjusted contemporaneously with the permanently tracking AFN stations to provide the basis for GDA94.  ARGN: Australian Regional GNSS Network. 100 stations throughout the region monitoring Crustal Dynamics & sea level rise

GDA94 Geocentric Datums This diagram represents the Geodetic Datum of Australia which was created in 1994 (GDA94 or GDA). • This datum is part of a world-wide datum which is compatible with the USA Global Position System (GPS). • Note how the agreement with the Geoid is more even over the whole of the Earth and that the centre of the Earth is also the centre of the Ellipsoid/Spheroid. This is an essential part of a Geocentric Datum because they are designed for use with Global Position Systems; and their satellites which orbit around the Earth's central mass.

GDA94  From the year 2000, all Australian mapping authorities

are using the Geocentric Datum of Australia (GDA). This datum was defined in 1994, and is based on a mathematical surface that best fits the shape of the Earth as a whole, with its origin at the Earth’s centre of mass, hence the term ‘geocentric’.  The primary reason for this change is the widespread use of satellite-based navigation systems such as the Global Positioning System (GPS), which is based on a geocentric datum known as the World Geocentric System 1984 (WGS84). 

Note: Ublox a manufacturer of GPS engines has a free Android app that will demonstrate all the available information coming from your phone on-board GPS engine.

Ausgeoid09: Specifications  Gravimetric – geometric quasigeoid.  The gravimetric component was developed using:  degree-2160 spherical harmonic expansion of the EGM2008 global

gravity model (Pavlis, Holmes et al. 2008)  approximately 1.4 million land gravity anomalies from the Australian national gravity database  the 9” x 9” GEODATA-DEM9S digital elevation model of Australia, and  altimeter-derived marine gravity anomalies from the DNSC2008GRA grid (Andersen, Knudsen et al. 2010)  The geometric component (Brown et al. 2011) was developed

using two datasets:  The primary dataset of 2638 co-located GNSS-AHD heights

provided by the State and Territory surveying authorities.  A secondary dataset of 4233 Australian National Levelling Network Junction Points to provide a higher-resolution definition of the offset between the AHD and gravimetric quasigeoid.

Ausgeoid09: Geoid – Ellipsoid Separation (N) • N: Vertical difference between the ellipsoid and the

geoid known as the Geoid/Ellipsoid separation. • Ideally the two surfaces should be coincident, however the ellipsoid is a mathematically smoothed surface whereas the geoid is shaped by physical laws affecting the earth. • Obviously the quantity will vary according to the ellipsoid adopted in the area in question. The designation of N should be referenced to the relevant ellipsoid and its date. • The geoid, usually taken as mean sea level, may be thought of as a constant ellipsoidal surface which may however be affected by local gravity influences and hence display undulations.

Ausgeoid09: Geoid – Ellipsoid Separation  Elevation View

N: Vertical difference between the ellipsoid and the geoid known as the Geoid/Ellipsoid separation.

Ausgeoid09: Geoid – Ellipsoid Separation (N) X’

X

Ausgeoid09: Geoid – Ellipsoid Separation (N)

AUSTRALIA

Section X – X’  Cross Section  Elevation View

Map Grid Australia- Datum GDA94  The Universal Transverse Mercator (UTM) map projection of

the national grid on the GDA ellipsoid.  For most practical purposes, WGS84 and GDA coordinates are the same. Note that the GDA coordinates, both latitudes and longitudes, and Eastings and Northings, differ from their AGD predecessors by approximately 200 metres.  Parameters  UTM Projection  60 zones, 6°wide,  Central Meridian – Line of Longitude (Zone 55 = 147°E)  False Origin – Intersection of Equator & Central Meridian

has the coordinates 300,000E & 10,000,000N  Scale Factor at the Central Meridian = 0.9996

Mapping Grid of Australia 1994 – UTM Projection Transverse Mercator Projection Geodetic coordinates (latitude and longitude) are represented on a map or chart, by mathematically "projecting" them onto a surface, which can be laid flat. The Transverse Mercator system projects geodetic coordinates onto a concentric cylinder which is tangent to the equator and makes contact along one meridian.

MGA94 – Projection Zones

MGA – Projection Zones for Victoria

MGA94 UTM Projection

Field Measurements Reduced to the Ellipsoid

 Cross Section  Elevation View

Mapping Coordinates GDA94 coordinates are shown as geographical coordinates, given in degrees, minutes and seconds as: Latitude: 037o 48’ 49’’ S Longitude: 144o 57’ 47’’ E

-37.8136111 144.9630556

MGA94 coordinates are shown as rectangular coordinates given in metres as: Easting: 320700.564 Northing: 5812910.381 Zone 55  Coordinates of the Melbourne GPO

Australian Height Datum 1971  The Australian Height Datum (AHD) is the official height

datum for Australia.  On 5 May 1971, Geoscience Australia, on behalf of the

National Mapping Council of Australia, carried out a simultaneous adjustment of 97 230 kilometres of two-way levelling. Mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges around the coast of the Australian continent.

 http://www.ga.gov.au/earth-monitoring/geodesy/geodetic-datums.html

AHD71  The resulting datum surface, with minor modifications in

two metropolitan areas, has been termed the Australian Height Datum (AHD71) and was adopted by the National Mapping Council at its twenty-ninth meeting in May 1971 as the datum to which all vertical control for mapping is to be referred.  AHD: The datum surface is that which passes through mean sea level at the thirty tide gauges and through points at zero AHD RL throughout Australia  http://www.ga.gov.au/earth-monitoring/geodesy/geodetic-datums.html

MGA – AHD : PM64, Building D

PSM

Terminology (6)  ITRF: International Terrestrial Reference Framework  GRS80: Geodetic Reference Surface 1980  WGS84: World Geodetic System 1984  GDA94: Geodetic Datum of Australia 1994  AFN: Australian Fiducial Network  ARGN: Australian Regional GNSS Network  ANN: Australian National Network  MGA94: Mapping Grid of Australia 1994  UTM: Universal Transverse Mercator Projection  Central Meridian – Line of Longitude defining a Zone

Quick Quiz 1. What is a geodetic datum? 2. What is the geoid ? 3. Describe what AHD is? 4. Explain Ellipsoid – Geoid separation 5. Name the reference ellipsoid used by the GPS

system

Quick Quiz Answers 1. A mathematical reference ellipsoid (spheroid) 2. The Earths surface (Approximately AHD RL

0.000) 3. The datum surface which passes through mean sea level at the thirty tide gauges and through points at zero AHD RL throughout Australia 4. N: Vertical difference between the ellipsoid and the geoid 5. WGS84, practically the same as GDA94

References  VU Collaborate  Textbook (Available online at the Library)  Engineering Surveying  6th Edition, W.Schofield & M.Breach  Chapter 8 – Position  ICSM  Geocentric Datum of Australia Technical Manual v2.4  Standard for the Australian Survey Control Network-

Special Publication 1  Survey Practice Handbook  Part 2, Section 12.9...