know where u stand with gda

8/6/2019 Know Where U Stand With GDA

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Know Where

You StandWith GDA

G E O C E N T R I C D A T U M O F A U S T R A L I A


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There is a change occurring in the mapping

datum used in Australia, and that change will

affect everyone who uses mapping and spatial

data. The new datum, called GDA (Geocentric

Datum of Australia) will provide a single

standard for collecting, storing and applying

spatial data at all levels local, regional,

national and international.

As the words imply, geocentric datum has its

origin at the centre of the earth. The key

advantages of the GDA over Australias current

datum are that GDA is totally

compatible with satellite-based

navigation systems such as the

Global Positioning System

(GPS) and with major

international geographic systems.

In fact, the GDA will provide

very significant and substantial

benefits to those using satellite

positioning and/or operating

nationally or internationally.

GDA is being progressively introduced, with

all government mapping agencies and

authorities scheduled to have it adopted by

2000. Prior to 2000, most other private and

public organisations which produce or use

geographic information in any form, will need to

assess and prepare for the change, and whereappropriate, implement their own changes.

To assess both the impact and benefits of GDA,

organisations will need to understand both their

current and future operational requirements as

well as the broader financial, administrative and

managerial issues. In particular, they will need to

consider:

existing and future spatial data requirements

and use for example many organisations are

linking spatial and other business systems,

some of which may be accessed and

maintained by other organisations;

spatial data collection, storage and

management practices migration from

paper to electronic documents, access and

distribution of information via internet and

other online systems are examples;

operational needs for transformations

between datum for example, where either

clients or providers have not yet converted to

GDA or where dual coordinates are required

for a limited transition period; and

legal issues relating to the use of current or

GDA-based coordinates for example, where

coordinates are included in legislation such

as some boundaries and leases, etc.

The following sections outline the GDA in

more detail including how you can obtain more

information.

What Do I Need to Do ?Organisations need to plan for transition to

GDA. The key issues are:

raising the awareness of GDA internally and

promoting discussion about GDA with

clients, providers and industry bodies;

making an assessment of the urgency to

change to GDA, based on the implications

on operations, clients and providers;

determining the appropriate strategies and

timetable for any changes; and

make the strategies and timetables publiclyaccessible to aid others developing strategies.

Common Questions About GDA

What is the GDA ?A datum is a mathematical surface on which a

mapping and coordinate system is based. A

geocentric datum is a datum which has its

origin at the Earths centre of mass. The

advantage of the geocentric datum is that it is

directly compatible with satellite-based

navigation systems.

Therefore the GDA is based on a model which

is a best fit model for the whole Earth with the

GDAs centre being coincident with the Earths

centre of mass.

A C h a n g e i s O c c u r r i n g

The AGD and GDA are two

different mathematical

models of the shape of the

Earth. Each has a different

origin and subsequently a

point on the Earths surface

will have different coordinates

based on each datum.


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What are the benefits of a geocentricdatum and the GDA ?Adopting a geocentric datum allows for a single

standard for collecting, storing and using

geographic data. This will ensure compatibility

across various geographic systems at the local,

regional, national and global level. This is themain reason that the GDA will form the basis

for the Australian Spatial Data Infrastructure

(ASDI) the infrastructure to manage

Australias key spatial data sets.

A geocentric datum will also:

provide direct compatibility with Global

Positioning System (GPS) measurements and

mapping or Geographic Information Systems

(GIS) which are based on the geocentric

datum;

minimise the need for casual users to

understand datum transformations;

allow more efficient use of organisations

spacial data resources by reducing need for

duplication and unnecessary translations;

help promote wider use of spatial data

through one user friendly data environment ;

reduce the risk of confusion as GPS, GIS and

navigation systems become more widely used

in business and recreational activities.

Why is Australia adopting GDA?

The current Australian Geodetic Datum (AGD)is based on a mathematical surface of the earth

designed to fit the Australian region.

Continuing to work with AGD coordinates in

an international environment where

positioning, navigation and information systems

relate to a global earth model is becoming

increasingly inefficient and difficult.

The use of a globally compatible coordinate

system within Australia is now inevitable and

the longer we remain with AGD, the more

difficult and expensive it will be to convert to

GDA.

When will the changeover from AGD toGDA take place?The Inter-governmental Committee on

Surveying and Mapping (ICSM) has

recommended that Australia convert to GDA

by 2000 and has provided the new coordinate

framework for this to happen. Some authorities

have already begun converting to GDA, some

will convert very soon, and others will do so as

their business operations evolve. It is expected

that the migration will be managed in all

organisations as part of standard business

strategy and as such will be implemented

differently within each organisation.

When should I change to GDA ?No private organisation will be forced to

change. Individual state government policies

may require state agencies to comply with GDA

at least for the transfer of data. As always,

individual organisations will be able to work

with any project-based coordinate system they

require (AGD or whatever) . However, as others

change to GDA, it may become increasingly

inefficient to use project-based coordinate

systems where external data is required.

Can I continue to use AGD and justtransform coordinates as required ?Yes, transformation parameters have been

developed to allow users to convert data

This illustrates the difference

between latitude and longitude

coordinates generated from both

the AGD and GDA datum.While

features on the ground will not

change,their coordinates will

change by approximately 200m

in a north easterly direction.


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between various datum. However, convenience,

efficiency, compatibility and commercial

competitive pressures are influencing many

organisations to decide to change to globalcoordinate systems as soon as possible. This ends

the need for transformations which introduce

some additional uncertainty into coordinates.

What other datum are commonly usedin Australia ?Australia is using a number of datum which

have been developed prior to the extensive use

of satellite positioning technologies. These

datum include AGD and more recently

WGS84.

The AGD was based on a mathematical surfaceof the Earth which was designed to fit the

Australian region. While this is a very good

datum for local use, it has an origin

approximately 200m offset from the Earths

centre of mass; thus it is not a geocentric datum.

The WGS84 datum is the datum used in GPS

receivers and is an Earth-centred datum used

and developed by the US Department of

Defence, primarily for defence applications.

What is the difference between GDA

and AGD coordinates?GDA and AGD coordinates will vary by

about 200m in a north easterly

direction because of the different

models. The actual size and

orientation of the change will

vary slightly from place to place.

Its also important to note that

both grid coordinates (derived

from a projection) and latitude

and longitude coordinates

(geographical coordinates)

depend on the datum and bothsets of coordinates will vary

between datum. In other words a

feature on the ground will have

different sets of geographical or grid

coordinates depending on the datum adopted.

The Australian Height Datum (AHD) will not

be affected by the adoption of GDA.

What is the difference between GDAand WGS84 coordinates ?For all practical purposes, GDA and WGS84 are

the same. The difference is of the order of 10cm.

The GDA uses a more accurate model which is

endorsed by the International Association of

Geodesy (IAG) and to which WGS84 is being

aligned.

Can all data be converted to GDA?ICSM is working with software developers to

ensure that standard GIS and GPS productssupport transformations from existing datum to

GDA. Users are encouraged to check with their

GIS or GPS vendors about GDA

transformations.

It should be possible to convert existing digital

spatial data to GDA where the datum of the

original digital data is known. Raster or image

data such as remote sensing images may need to

be treated differently and the suppliers or

developers of this data should be consulted on

any transformation issues.

It is estimated that about 30 per cent of allgeographic information is still held as paper

maps. Much of this information may not

require conversion, or will not be economically

feasible to convert. All organisations will require

individual strategies for paper-based products.

Are the State and Territory governmentsadopting GDA ?Yes, all States and Territories are committed to

converting to GDA and are developing and

implementing state or territory-based plans for

GDA. Within the Commonwealth someagencies are already converting to a geocentric

datum and others are developing individual

strategies.

How will GDA affect legal boundaries?Legal boundaries are generally defined by

physical features or marks on the ground. These

boundaries will not change, though the

coordinates of them may. Some boundaries may

have been defined by coordinates in t erms of the

AGD (or another system). These boundaries too

will not change, although the coordinateswill be different when converted to

GDA. In some cases, boundaries

have been defined by coordinates

without reference to a coordinate

Approximate coordinate shift from

AGD to GDA 94 (in metres)

207207

206

207205

204

204

194

204

201

207207

206

207205

204

204

194

204

201


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system. In these instances, the physical

location has always been uncertain and this

will remain.

If there is a legal issue about adopting

GDA, it is recommended that independent

legal advice is sought.

What is the link between the GDAand the Australian Spatial DataInfrastructure (ASDI) ?The ASDI will be based on the GDA and

all data themes will be supplied to users in

GDA coordinates. This will allow users of

the ASDI to integrate the ASDI with

other information systems and

communication t echnologies such as GISand GPS. GDA is the datum to use to

minimise confusion for all users and

provide maximum accuracy where high

precision is required.

What is the difference between

GDA, GDA94 and MGA94?GDA is the datum, while GDA94 is the

geodetic coordinate (latitude and

longitude) computed in terms of the datum

at 1 January 1994.

Likewise, the Universal Transverse

Mercator coordinates (east, north and

zone) generated from GDA94 are known as

MGA94 (Map Grid of Australia 1994)

coordinates.

, , ,

, , ,

, , ,

The GDA results from an international movement to adopt a geocentric datum to make mapping and spatial data coordinates compatible with satellite-based coordinates. The

work of geodesists in Australia has contributed to developing the global geocentric framework.The GDA is based on this global geocentric framework and geodetic observations

are continuing to monitor the validity of the GDA coordinates.

The GDA provides coordinates that are appropriate for applications at all levels (local through to international) and allows the easy integration with other information

technologies such as GIS and satellite navigation systems.

The Australian Spatial Data Infrastructure initiative provides consistent national datasets to users on the same coordinate system the GDA.

The type and number of spatial data users is increasing with the development of information system technologies. Nearly every industry sector uses these technologies. The

GDA is important to Australian industry as it simplifies the capture and integration of spatial information.


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Coordinate ConundrumsQuestion: A fisherman records the coordinates of his

favourite fishing spot with his handheld GPS. Later

when he plots the coordinates on a topographic map

he notices that the coordinates plot about 200m away

from the creek where he was fishing.What might have

caused this problem?

Answer: The handheld GPSis probably using the default

WGS84 datum and the map the fisherman is using is

probably based on AGD datum. The fisherman would

need to apply a correction to the WGS84 GPS

coordinates to bring them onto the AGD datum. Or,

he could purchase a GDA map as soon as its available

for that particular area.

Question: An environmental scientist wants to record the

location of sample sites with a handheld GPS.He is

worried that because Australia is moving, thecoordinates he records will vary with t ime. How far

would Australia have to move before it would cause a

problem with his recorded locations?

Answer: Currently most GPSunits can only record

location to somewhere around +/- 30m. At the

current rate of continental drift, it would take

approximately 600 years before there is a problem

with his recordings. In any case the effect of

continental drift can also be removed by using

differential GPStechniques.

Question: An authority finds its data providers and some

data users are asking for spatial data in GDA. What

should the authority do?

Answer: If this happens the authority may not be

prepared for the introduction of GDA and would

need to take the following steps:

1. Assess how the authority is storing data currently.

2. In the short term translate the data.

3. Conduct an organisational assessment of data and

business requirements and develop an appropriate

GDA implementation strategy.

Note:the sooner an assessment is done,the cheaper

any implementation is likely to be.

Question: If an Australian Rules football umpire bounces a

ball at S37o

50 00, E 145o

00 00 (based on the

AGD) and the ruckman taps it to the rover at

S37o

49 54, E 145o

00 05 (based on the GDA),

how far did the ruckman have to run to receive the

ball?

Answer: No distance at all. Although the coordinates are

different, they refer to the same point. (See the

Melbourne Cricket Ground diagram)

What is being done to assist organisations to convert to GDA ?ICSM has initiated a number of strategies to assist organisations to convert

to GDA. These include:

Industry briefings: ICSM and other State and Territory representat ives will

provide briefings to industry at workshops and seminars up to 2000.

Educational material: ICSM will be publishing material on a regular basis in

popular industry publications and will be encouraging the education

industry in Australia to include GDA implementation as part of its

education programs.

Software development: ICSM is encouraging GIS and GPS software

companies to incorporate the necessary transformation routines in th eir

products. When purchasing GIS or GPS products, organisations are

encouraged to specify that the product must capture and or convert data to

GDA coordinates.

On-line support through the World Wide Web: ICSM has a web site at

http://www.anzlic.org.au/icsm/icsmmain.htm which contains detailed

technical information about GDA and links to other jurisdictional sites.

This brochure has been produced by the Inter-governmental Committee

on Surveying and Mapping to assist organisations plan for the introduction

of the GDA.

Contact Details:

ICSM Executive Office

Scrivener Building

Dunlop Court

BRUCE ACT 2617

PO BOX 2

BELCONNEN ACT 2616

Telephone 02 6201 4292

Facsimile 02 6201 4366

Email [email protected]

Internethttp://www.anzlic.org.au/icsm/icsmmain.htm

Map of MCG reproduced with permission of UBD.

Copyright Universal Press Pty Ltd JD10/97

Publication date: October 1997

GEOCENTRIC DATUM OF AUSTRALIA

know where u stand with gda

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