Chris, The national geodetic adjustment which produced the AGD84 coordinate set was performed using the same reference ellipsoid (Australian National Spheroid, ANS), the same latitude and longitude values for the coordinate origin, Johnston Geodetic Station, and the same semi-minor axis orientation as were used in the AGD66 adjustment.
The AGD84 coordinate set not only differed from the original AGD66 coordinate set as a function of position relative to the Johnston coordinate origin but, even more importantly, as a result of inhomogeneities in the data sets used in the 1966 adjustment. The vector differences between the AGD66 and AGD 84 coordinate sets amounted to a maximum of ~6 metres in the Northern Territory, and to ~3 metres in the Hammersley Ranges. These differences were generally the result of the use of substantially more, and significantly more reliable, data sets in the AGD84 adjustment (e.g. Transit Doppler, VLBI, satellite laser ranging, high precision Geodimeter traverses etc.) For both the AGD66 and AGD84 adjustments, Australian Map Grid (AMG) coordinates were derived from a Transverse Mercator projection of the geodetic coordinates (latitude and longitude) resulting in the AMG66 or AMG84 coordinate sets respectively. There is absolutely no difference between the geodetic datums used in the 1966 and 1984 national geodetic adjustments, or in the means by which projection coordinates were derived from either of the resulting data sets Hope this helps. Regards Brian Murphy ----- Original Message ----- From: "Brown, Christopher (HI)" <[EMAIL PROTECTED]> To: "MapInfo-L (E-mail)" <[EMAIL PROTECTED]> Sent: Wednesday, May 29, 2002 12:49 PM Subject: MI-L Australian Map Grid Datum Question? > I have been deliberating whether to post this to the list for some time but > decided to 'bite the bullet' and fire away! It is regarding the datum number > associated with Australian Geodetic 1966 (AGD66) and Australian Geodetic > 1984 (AGD84) being 12 and 13 respectively (see below copied from > mapinfow.prj file). > "--- Australian Map Grid (AGD 66) ---" > "AMG Zone 50 (AGD 66)\p20250", 8, 12, 7, 117, 0, 0.9996, 500000, 10000000 > "--- Australian Map Grid (AGD 84) ---" > "AMG Zone 50 (AGD 84)\p20350", 8, 13, 7, 117, 0, 0.9996, 500000, 10000000 > > AGD66 results from the network adjustment of geodetic observations to 1966, > based on the ANS. AGD84 corrects deficiencies shown to exist in the AGD66. > > What I cannot understand is, seeing that both these numbers use the same > Australian National spheroid (ANS) what would the difference be in the > parameters used in the projection statement and how can you find this out? > > If for instance you save an AMG Zone 50, AGD66 table of points to an AMG > Zone 50, AGD84 table, the adjusted coordinates show the small difference > between the coordinate systems but underlying all this is the fact that they > both use the same spheroid and all we are dealing with here is an adjustment > due to a better geodetic model brought in about 1982. > A work colleague of mine is adamant that MapInfo is wrong to apply two > different datum numbers due to the above and believes MapInfo deals with > this incorrectly. I do not understand his logic. > > Can anyone possibly clear my mind on this issue? > Regards > Chris > > Chris Brown - Cartographer > Resources Planning, Hamersley Iron P/L > Tel. +61 8 9327 2176 > Fax. +61 8 9327 2478 > mailto:[EMAIL PROTECTED] > > > > ---------------------------------------------- > This email (including all attachments) may be confidential. If you are not > the intended recipient, you must not use the information contained in it. > If you have received this email in error, please delete the email and notify > the sender. > > --------------------------------------------------------------------- > List hosting provided by Directions Magazine | www.directionsmag.com | > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > > --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
