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Aussie grids

 
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PegSneggle
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Joined: 30/04/2003 13:25:55
Posts: 10
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Aussie grids Reply with quote

Older Australian maps use either the AGD66 or AGD84 datum. These two datums differ by only about 5 metres. NSW, Victoria, Tasmania, and the NT use AGD66. SA, Qld, and WA use AGD84.



New Australian maps use GDA94 (Geocentric Datum of Australia). This new datum was introduced to bring our maps in line with the World Grid System (WGS84) so that they are more easily used with GPS units.



AGD66/AGD84 positions and GDA94 positions differ by about 200 metres.



New Zealand uses its own map datum, NZMG. This is not compatible with the Australian map datums. However, New Zealand plans to introduce a geocentric datum similar to Australia's new datum in the future.



Hope this helps.



Mike

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Ante
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Joined: 10/07/2003 20:17:58
Posts: 6
Location: Sweden

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Aussie grids Reply with quote

Thanks Mike!



If I happened to get hold of an AGD66 or AGD84 map what grid would be best to use if I don't want to use degrees and minutes? With GDA I'd certainly use UTM/UPS but I'm not sure about the other two. Do you know of any "user grid" that would suffice?



Cheers.



Ante
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Ante
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Joined: 10/07/2003 20:17:58
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Location: Sweden

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Aussie grids Reply with quote

Thinking of taking a trip down under along with my trusty eTrex. Does anyone know what Position Format (grid) I should choose for the various Australian Datums AGD 66, AGD 86 and GDA? These are all listed in eTrex datum setup but I'm not sure what corresponding grid to use.

Thanks in advance for all the help I can get.
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PegSneggle
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Joined: 30/04/2003 13:25:55
Posts: 10
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Aussie grids Reply with quote

The usual way to quote a map position is to name the map and give a 6 digit grid reference (GR). e.g. Six Brothers GR 725233. In this example, "Six Brothers" is the name of the map, 725 is the easting part of the GR, and 233 is the northing part of the GR. (Note: The full name of this 1:25000 map is actually "Six Brothers 9031-IV-S".



To determine the grid reference, look for the bold numbers on the edges of the map. The eastings are along the bottom and top, while the northings are along the left and right.



The first 2 digits of the easting are determined from the large bold numbers at bottom of the map. The 3rd digit is determined by measuring the distance of the feature from the left edge of the grid cell which contains it, dividing it by the width of a cell, and then multiplying it by 10. e.g. if the feature is 32 mm from the left edge of the cell, and the cells are 40mm wide, then the last digit will be 32/40*10 = 8. For 1:25000 maps, a simpler way is to just measure the distance in mm and divide it by 4.



The digits of the northing are determined in a similar way.



If you want to determine the full easting and northing from the map, look for the small digits which appear in front of some of the bold numbers on the edge of the map. In the above example, they are 2 for the easting, and 63 for the northing, giving full coordinates of 272500 metres East, 6323300 metres North.

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Ante
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Joined: 10/07/2003 20:17:58
Posts: 6
Location: Sweden

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Aussie grids Reply with quote

Thanks again,



I think maybe that I haven't expressed myself clearly. Here in Sweden I can choose in my eTrex a datum named RT90 and a grid (Position format) called "Swedish grid". These two give me an pretty accurate GR for the 1:50 000 topos produced by The Swedish Geodetic organisation. They also publish on their web-site the necessary parameters for a custom user grid that, together with the WGS84 datum, give an even better result (a more accurate transform than the built-in Garmin one).



The eTrex has in its list of datums all three Australian datums: AGD66, AGD84 and GDA. Together with a GDA-map I would then choose Datum=GDA and Position format=UTM/UPS (I guess!) to get eastings and northings that correspond to the map.



If I had an older map with the AGD66 or AGD84 datum, both of which I can choose in the Setup menu, what Position format (grid) would I choose to get GR:s that correspond with it (if I want eastings and northings, not Lat/Lon)?



Ante   
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