3 Replies Latest reply on Sep 29, 2010 1:38 PM by MKennedy-esristaff

    WGS84 to GDA94 conversion

    jmoroney
      I am importing GPS data into ArcInfo from a .csv file using Make XY Event Layer.  I then convert this to a feature class in a File Geodatabase. 

      The coordinates have been collected using WGS84.  When I input the Spatial Reference details I select Geographic Coordinate Systems > World > WGS 94.prj.

      I then attempt to Project to GDA94 - specifying the Output Coordinate System to be Geographic Coordinate Systems > Australia and New Zealand > Geocentric Datum of Australia 1994.prj

      It then requires a Geographic Transformation and only offers GDA_1994_To_WGS_1984 - I actually want the reverse.

      The result is the points have exactly the same coordinates (to the 8th decimapl place) and positions.  I believe there should be a small shift in coordinates if the projection was working correctly.

      Any assistance would be greatly appreciated.

      John
        • Re: WGS84 to GDA94 conversion
          AmyN
          Hi John,
          Pretty sure that the transformation is the same whether you're going GDA94 to WGS84 or back the other way.

          ArcMap also reprojects on the fly, so you might have two layers in ArcMap and one is GDA94 and the other WGS84 and they will appear in the same location. GDA94 and WGS84 both use geographic coords (like -34.1234, 150,1234).

          Not sure about whether GDA94 and WGS84 should have slightly different coords...
          • Re: WGS84 to GDA94 conversion
            jmoroney
            Thanks Amy,

            As you say both GDA94 and WGS84 use geographic coordinates.

            I expected at least some variation in 6th or 7th decimal place.

            A colleague has a tool that allows conversion from WGS84 and GDA94.  Using that conversion the separation betwwen points is close to 1 metre.  I don't have a lot of confidence in that software.  I would prefer to use the ESRI solution, but I need to know that it works and I am using it correctly.

            Regards,
            John
            • Re: WGS84 to GDA94 conversion
              MKennedy-esristaff
              Thanks Amy,

              As you say both GDA94 and WGS84 use geographic coordinates.

              I expected at least some variation in 6th or 7th decimal place.

              A colleague has a tool that allows conversion from WGS84 and GDA94.  Using that conversion the separation betwwen points is close to 1 metre.  I don't have a lot of confidence in that software.  I would prefer to use the ESRI solution, but I need to know that it works and I am using it correctly.

              Regards,
              John


              Hi John,

              The only transformation we have between GDA94 and WGS84 is the original "null" transformation. That is, the parameter values are zeroes.

              WGS84 is now tied to ITRF. I see that there's an ITRF-GDA converter available from ICSM and values here:

              http://www.ga.gov.au/image_cache/GA3795.pdf

              http://www.icsm.gov.au/icsm/gda/gdatm/index.html (RapidMap Ready Downloadable)

              You could do a custom transformation using the 7 parameters from PDF, but we aren't set up yet to handle time-dependent transformations.

              Melita