"Simon Bennett" <>
Wed, 13 May 1998 08:27:00 +0000
There has been some discussion about apparent disparity between
UTM/AMG and latitude/longitude coordinates. The following may shed
To accurately locate a place on the earth you need to know not only
the x and y coordinates (either Latitude and Longitude or UTM/AMG
eastings and northings), but also what DATUM your GPS was set, or was
used to make the map.
The datum is a mathamatical approximation of the shape of the earth.
The same lat/long (e.g. 36 57 23S 145 22 57E) or UTM/AMG coordinate may
point to two different places depending on what datum was used.
This difference may be as much as 200 metres.
For a given map datum, both Latitude/Longitude or UTM/AMG co-ordinates
point to exactly the same place.
Present Australian 1:100,000 maps are based on the Australian
Geodetic Datum 1966 or 1984 (there is only 6 metres difference
GPS units default to a datum called WGS84, (but most can be
set to Australian 1966/84).
If you have your GPS set to WGS84 and are comparing the coordinates
it gives with a current Australian 1;100,000 maps then both
lat/long and UTM/AMG will differ by 200 metres. If you set your GPS
to Australian 1966/84 then the co-ordinates will concur (GPS/map
The WGS84 datum is quickly becoming the world standard and Australia
is following suit! Sometime in the next few years we will be switching to
the new Geodetic Datum of Australia (GDA). GDA is virtually
identical to WGS84.
When new 1:100,000 maps are published using GDA both the
latitide/logitude and AMG will differ from where the are on
present maps. The AMGs will in fact be called MGA (Map Grid of
The main thing for birdwatchers to do is if they are recording a precise
coordinate (either in Lat/long or UTM/AMG) then they should also
record what datum their GPS was set to; or if reading from a map,
make a note of the edition and publication year of the map.
GPO Box 1229,
Canberra ACT 2601
Phone: 02 6281 3941
Fax: 02 6281 7326
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