Q. Does the GDA Converter software keep indices on MapInfo
tables?
A. Yes, along with the style and any custom
projection bound statements.
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Q. How accurate is the conversion?
A. The GDA Converter software uses the most
accurate method possible - the distortion grid method. Accuracies
between 0.1 m to 0.3 m can be achieved using this method (compared
to accuracies between 1 m and 10 m by other methods). Where
it is not possible to use this method because of grid coverage,
the second most accurate is used - the 3-Dimensional Similarity
method. This method usually has an accuracy of 1 metre.
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Q. Can't I just do a "Save copy as..."
command in MapInfo to a GDA projection to convert my tables?
A. You can use this method but it isn't accurate
or consistent. The reason why this method doesn't produce
accurate results is because AGD and GDA use different ellipsoids
with different centres. MapInfo's method of reprojecting coordinates
need to be on the same ellipsoid for an accurate result. Depending
on where the coordinates are positioned, you can be quite
a few metres out. If your data requires an accurate conversion
it is recommended that the distortion grid method is used.
The GDA Converter software uses the distortion method.
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Q. What are the distortion grid extents
within Australia?
A. Below is a diagram showing the extent of
the AGD66 and AGD84 grids over Australia.
Map 1 - ADG66 to GDA94 Transformation Grid Coverage *
Map 2 - ADG84 to GDA94 Transformation Grid
Coverage *
The ADG84 coordinate system is not over TAS, NT, NSW or VIC
as these state/terrority governments had adopted the ADG66
datum. In areas where the grid is not present and you are
converting from ADG84, the 3-Dimensional Similarity method
will be used.
* Maps 1 & 2 were sourced from publication
'The Geocentric Datum of Australia Technical Manual' version
2.2 produced by the Intergovernmental Committee on Surveying
Mapping (ICSM). Copyright Commonwealth of Australia 1998.
This publication can be found at http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.2.pdf.
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Q. How fast is the GDA Converter software?
A. There are many factors that influence the
speed of the converter such as computer hardware, disk access
speed, the type of objects being converted and the number
of indices. As an indication, the following conversion took
24 seconds.
Hardware: Pentuim III 900Mhz, 256Mb RAM, local
disk access
File: 10,000 property parcels with a total of 108,000 points
Time taken: 24 seconds
No other GDA conversion software tested was
able to perform the same conversion under this time.
One customer averaged one hour of conversion
time for every gigabyte of MapInfo data.
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Q. Why does MapInfo say my converted table
is in Longitude/Latitude (NAD83) rather than Longitude/Latitude
(Australia GDA94)?
A. Unfortunately MapInfo sometimes displays
the incorrect projection name but the underlying coordinates
will be correct as NAD83 uses the same ellipsoid as GDA94.
This stemmed from the fact that earilier versions of MapInfo
didn't have a dedicated ellipsiod number in it's projection
list for GDA longitude and latitude coordinates.
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Q. Where can I find out more information
on GDA?
A. For more information on GDA, visit the sites
mentioned on the links page.
