[Gdal-dev] Defining Australian Map Grid in GDAL

[Andrew Williams]

OK I've narrowed it down. Jacob is correct, there is a problem handling AGD datums. With the TFW file in the same directory as the source image I've attenpted to translate the image using the following command line
 
gdalwarp -s_srs "+proj=utm +zone=55 +south +a=6378160 +rf=298.250000000055" -t_srs "+proj=latlong +datum=WGS84" -co "TFW=YES" image_in.tif image_out.tif
 
What is happening is that the system is being converted from grid to latlong but the source datum is still adopting WGS84. Therefore 200 odd metres out when computed to latlong.
 
I have checked the varios csv's such as gcs, pcs, datum and ellipsoid and the correct parameter appear to be in place.
 
The TFW only defines the top left corner of the image and the pixel dimension. Why would GDAL assume WGS84 when I have defined override parameters?
 
Do I need to take Jacob's file and insert it into one of these other csv's? Any other ideas?
 
regards
Andrew

	-----Original Message----- 
	From: Delfos, Jacob [mailto:jacob.delfos at maunsell.com] 
	Sent: Wed 3/1/2006 11:02 AM 
	To: Andrew Williams 
	Cc: gdal-dev at lists.maptools.org 
	Subject: RE: [Gdal-dev] Defining Australian Map Grid in GDAL
	
	

	Andrew, 

	You have to be careful with AGD-based projections in Proj4, because the definitions that exist in Proj4 are not correct (problem with datums). I'm attaching a file with correct definitions for AGD-based projections. I have myself spent several days trying to figure out why my coordinates were up to 200m off, only to realise that Proj4 does not appear to use the AGD84 and AGD66 datum correctly.

	Just for the record: 
	AMG66 = UTM + AGD66-datum 
	AMG84 = UTM + AGD84-datum 
	MGA94 = UTM + GDA94-datum (where GDA94 can be substituted with WGS84) 

	Be careful with the acronyms. You wouldn’t be the first to start construction in the wrong spot :) 
	For your image, I guess just override the projection, and project from Latlong AGD84 (epsg:4203, bottom of file I sent) to Latlong WGS84.

	Regards, 

	Jacob 
	  

	JACOB DELFOS
	GIS ANALYST
	Maunsell Australia Pty Ltd
	629 Newcastle Street, Leederville, WA 6007 
	PO Box 81, Leederville, WA 6902 
	Western Australia 
	ABN 20 093 846 925
	
	Tel     + 61 8 9281 6185
	Fax    + 61 8 9281 6297
	jacob.delfos at maunsell.com 

	<<agd84_agd66_proj4.txt>> 

	» -----Original Message----- 
	» From: gdal-dev-bounces at lists.maptools.org 
	» [mailto:gdal-dev-bounces at lists.maptools.org <mailto:gdal-dev-bounces at lists.maptools.org> ] On Behalf Of 
	» Andrew Williams 
	» Sent: 28 February 2006 22:27 
	» To: gdal-dev at lists.maptools.org 
	» Subject: [Gdal-dev] Defining Australian Map Grid in GDAL 
	» 
	» Hello folks. 
	» I'm having trouble with a Geotiff. 
	» The scan is a topographic map sheet. The cartography and grid 
	» on the sheet is based on AMG (Australian Map Grid). It is a 
	» UTM projection based on the Australian National Spheroid, 
	» ellps=aust_SA I believe. 
	»  
	» My problem is that the GeoTiff has been georefernced as UTM 
	» ellps=GRS80 datum=WGS84 incorrectly. 
	»  
	» My intent is to gdalwarp the image to latlong WGS84. 
	»  
	» I have tried for the better part of a day to attempt to 
	» override or otherwise correct the georeferncing. Without success. 
	»  
	» I have been very successful with GDAL in the past, but this 
	» has stopped me. 
	»  
	» The problem is that I can't seem to fully define AMG "enough" 
	» to allow GDALwarp to reproject the image. It seems to default 
	» to the source as being UTM WGS84. 
	»  
	» I attempted to gdal_translate the image using the corrected 
	» definition and then gdalwarp image, but that also failed. 
	»  
	» the definition I have been using is "+proj=tmerc 
	» +ellps=aust_SA +lat_0=0 +lon_0=147 +x_0=500000 +y_0=10000000 
	» +k=0.9996 +south" 
	»  
	» regards 
	» Andrew 
	» 
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