Datums

JamesWat · Post by **JamesWat** » 08 November 08 4:41 pm

Hi everybody,
ive got a question which probably has a simple answer but ive had a quick search of google without alot of luck so thought if i came here you could help me out.
 
Im in tassie and most of the tasmaps (1:25000) use AGD 66 datum or if they are newer (published after 94) GDA94. My question is, how do i convert from normal gps datum WGS84 to these?
 
I know my gps does it quickly and easily but id also like to know how to do it on paper. (I know i know curiosity killed the cat and all that but hey, thats me

)
 
If there is a site that explains it could you post a link? or just a quick explanation would be good too. Thanks so much 
JamesWat

Gwennie1984 · Post by **Gwennie1984** » 08 November 08 5:35 pm

If you are using GDA94 you can use this website for conversions to WGS:

 http://www.ga.gov.au/geodesy/datums/red ... to_geo.jsp

 There is also a link that converts WGS84 to GDA94. And there are is a speadsheet here that allows use of Redfearn's forumla offline.

 By pulling apart the spreadsheet you can begin to understand the conversion process. The links on the site also have some brilliant information explaining both grid and geographic systems.
 Hope this helps. It certainly has helped me with my uni topics on this subject =D

Map Monkey · Post by **Map Monkey** » 08 November 08 8:03 pm

Gwennie1984 wrote:If you are using GDA94 you can use this website for conversions to WGS:

 http://www.ga.gov.au/geodesy/datums/red ... to_geo.jsp

 There is also a link that converts WGS84 to GDA94. And there are is a speadsheet here that allows use of Redfearn's forumla offline.

 By pulling apart the spreadsheet you can begin to understand the conversion process. The links on the site also have some brilliant information explaining both grid and geographic systems.
 Hope this helps. It certainly has helped me with my uni topics on this subject =D

ummm, errr......

Just to clarify:

- GDA94 is close enough to WGS84 for all consumer GPSr purposes (less than 1m).

- The achievable resolution on a 1:25000 topo is considered around 25m (ie. 1mm on the map), therefore it will not matter if you use WGS84 or GDA94 in any case.

- IF you really want to know how to do the conversion using pen and paper, then download the GDA technical manual from here ===> http://www.icsm.gov.au/icsm/gda/gdatm/

- Various calculations (Redfearn formula, Molodensky's etc) are embedded in the excel spreadsheets here ===> http://www.ga.gov.au/geodesy/datums/calcs.jsp

- BTW For a simple program to convert or test your answers, try this java one ===> http://home.hiwaay.net/~taylorc/toolbox ... atumtrans/ .

mm

JamesWat · Post by **JamesWat** » 08 November 08 8:08 pm

Thanks, very helpfull. 
Think im happy with that nice java version. Was more interested in the 66 datum as thats what most of my (now probably out of date) maps are.
Thanks for your help, its greatly appreciated!

Guest · Post by **Guest** » 08 November 08 8:51 pm

Map Monkey wrote:- The achievable resolution on a 1:25000 topo is considered around 25m (ie. 1mm on the map), therefore it will not matter if you use WGS84 or GDA94 in any case.

AGD66 -> GDA94 is ~200m north east iirc, which at 1mm = 25m would be about 8mm...

Gwennie1984 · Post by **Gwennie1984** » 08 November 08 9:04 pm

Map Monkey wrote:ummm, errr......

Just to clarify:

- GDA94 is close enough to WGS84 for all consumer GPSr purposes (less than 1m).

- The achievable resolution on a 1:25000 topo is considered around 25m (ie. 1mm on the map), therefore it will not matter if you use WGS84 or GDA94 in any case.

- IF you really want to know how to do the conversion using pen and paper, then download the GDA technical manual from here ===> http://www.icsm.gov.au/icsm/gda/gdatm/

- Various calculations (Redfearn formula, Molodensky's etc) are embedded in the excel spreadsheets here ===> http://www.ga.gov.au/geodesy/datums/calcs.jsp

- BTW For a simple program to convert or test your answers, try this java one ===> http://home.hiwaay.net/~taylorc/toolbox ... atumtrans/ .

mm

Thanks for that. I was rather tired when I wrote that and misread what the post said. Sorry all. However, Geoscience does have some great stuff for MGA conversions (which can be handy with some maps that use grid projections).

Kerry · Post by **Kerry** » 09 November 08 9:50 pm

Conversion GDA94 <> AGD66 is really quite simple using freeware such as GDAit or GDAy etc and the appropriate NTv2 distortion grid.