[Gdal-dev] Defining Australian Map Grid in GDAL

[Frank Warmerdam]

Andrew Williams wrote:
> 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"

Andrew,

The +south switch in PROJ.4 does not have any meaning with proj=tmerc.  For
proj=utm it basically determines that +y_0=10000000 instead of +y_0=0 as it
does in the northern hemisphere.

I tried:

gdal_translate -a_srs '+proj=tmerc +ellps=aust_SA +lat_0=0 +lon_0=147 
+x_0=500000 +y_0=10000000 +k=0.9996' openev/utm.tif wrk.tif

Then gdalinfo wrk.tif and get:

PROJCS["unnamed",
     GEOGCS["Australian Natl & S. Amer. 1969",
         DATUM["unknown",
             SPHEROID["unnamed",6378160,298.2500000000049]],
         PRIMEM["Greenwich",0],
         UNIT["degree",0.0174532925199433]],
     PROJECTION["Transverse_Mercator"],
     PARAMETER["latitude_of_origin",0],
     PARAMETER["central_meridian",147],
     PARAMETER["scale_factor",0.9996],
     PARAMETER["false_easting",500000],
     PARAMETER["false_northing",10000000],
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]]]

So, the adjusted ellipsoid seems to be properly stored in
the file.

What exactly are you trying and why do you think the results
are wrong?

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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