Difference Between Wgs84 & Agm84

[nidrac]

Hey, Just a quick question, i'm reletively new to using GPS's and my unit uses the Datum WGS 84 but the chart i have uses AGM84. Just like to know what the difference is if any and how to convert them over

Cheers

[dangles]

Hey, Just a quick question, i'm reletively new to using GPS's and my unit uses the Datum WGS 84 but the chart i have uses AGM84. Just like to know what the difference is if any and how to convert them over

Cheers

try www.gpsaustralia.net for all gps questions

[netic]

Here is some info taken from a Diving site

Australian Geodetic Datum - AGD

Most maps and charts in Australia (as at 2001) were created using the Australian Geodetic Datum (AGD). Adopted in 1966, AGD is based on the Australian National Spheroid (ANS). Note some charts (eg the sea floor bottom charts for Sydney) are not based on this datum but it can be hard to tell as most do not have any reference to the datum used.

The following coordinate systems are based on AGD:

AGD66 - the 1966 coordinate system of geodetic Latitudes and Longitudes

AGD84 - the 1984 coordinate system of geodetic Latitudes and Longitudes used in QLD, SA & WA

AMG66 - the 1966 Australian Map Grid coordinate system of Eastings and Northings

AMG84 - the 1984 Australian Map Grid coordinate system of Eastings and Northings used in QLD, SA & WA

ISG66 - the 1966 Integrated Survey Grid coordinate system of Eastings and Northings used mainly in NSW

XYZ66 - the ANS cartesian coordinates

XYZ84 - the ANS cartesian coordinates

For AGD, most GPS units have map datums called AUS66 or AGD66 or AUS84. Some may use AMG66, AMG84 or AGD84. My Magellan 3000 portable GPS unit has AUS66 and AUS84. These are all basically the same and, for most purposes, any one of these can be used and the result will be the same.

Differences between GDA and AGD

The difference between identical longitudes and latitudes if used with AGD and GDA map datums (very confusing names I know) is just over 200 metres. Therefore, if you use one of the coordinate systems but should be using the other, there will be a straightline difference of about 200 metres (at least in NSW).

All of the GPS Readings used in my Web Site (except possibly some on the NSW Shipwreck Page) have been taken using AGD (AUS66) as the Map Datum. If you use any other datum on your GPS (eg one not for use in Australia), then you need to change to one of the correct ones for Australia. If you use GDA (GDA94 or WGS84), you will need to modify the readings on my web site for use with your GPS by either using AGD (AUS66) or by subtracting 5.6' from the given latitudes and adding 4.2' to the given longitudes (approximately in Sydney and NSW). A better way is to download a free piece of software called Geodetics Transformations (V3.42 as June 2006) or GEOD for short). This is available from the NSW Department of Lands Web Site. Go to their web site at GEODand download this small but excellent program. This will enable you to convert readings easily.

An example of what would occur if you use a GPS Reading off my web site but used the wrong datum is as follows. This is the GPS Reading for the wreck of the SS Tuggerah off southern Sydney:

Reading on my site using AUS66/AUS84 Actual location using GDA94/WGS84 Difference in minutes Where you will

be using GDA

Latitude 34° 08' 21"S 34° 08' 15.3"S -5.7' 105.1 metres south

Longitude 151° 09' 02"E 151° 09' 06.2"E +4.2" 190.9 metres west

If you used AGD as datum instead of GDA when using my GPS Readings for the Tuggerah, you would be 105.1 metres south and 190.9 metres to the west of the wreck. In a straight line, this would make you 218.2 metres south-west of the wreck. So you can see how important it is that you use the right datum when using GPS.

[nidrac]

Righto thanks, Thats everything i needed to know.

Cheers