What is the difference between GDA and MGA?

GDA and MGA are the recipeand the product.  GDA is the mathematical ellipsoid which we use to measure position, as well as where we position the ellipsoid over Australia and it’s orientation (See figure).  MGA is the product we get when we use the GDA ellipsoid and orientation.  MGA is the flat map showing positions as Eastings and Northings which are determined using the mathematics behind GDA94. (See Recipe for a Datum on the Where am I Horizontally page).

 

How are MGA and AMG different?

MGA and AMG are both maps of Australia showing coordinates as Eastings and Northings via a UTM projection.  The difference is the model that they are based upon.  AMG is older.  It is based on an older datum called (See datums) AGD66.  The position of this datum is different to the position of others such as GDA.  MGA94 is based on GDA94.  (See historical list of datum and map grid pairs).

 

I have a plan in AMG.  What do I do?  Can I use it? 

Yes you can use it but be careful to consider how you use it.  A plan on AMG is based on the AGD66 datum so coordinates will be different to those on a plan using MGA.  It is best practice to create new data or plans/maps using MGA (therefore using the GDA94 datum), but if you are not creating anything new you can continue to use the plan in AMG as long as you are not trying to mix with MGA information.  It’s important to remember that the coordinates of a position will be different depending on if they are in reference to MGA or AMG or other.  It is easy to transform from one system to another using (these websites and resources).  If using a mix of plans or information on different datums transform everything to GDA to keep everything relative.  If you use plans in two different datums it’s like writing a book in 2 different languages.  (See AMG MGA experiment)