EXTERNAL: Re: [gdal-dev] retrieve specific tiff tags

[Garrett, Benjamin D]

Thanks Frank.  I understand now but that is too bad because I have GeoTiff files that have data like this for tag 34737 GeoAsciiParams:

QT5 DEM|Null: -9999.000000, Non-Null Min: -21.640174, Non-Null Avg: 14.014185, Non-Null Max: 49.668544|

And another geotiff file with data like this for tag 34737 GeoAsciiParams:

Digital Elevation Model, Null: -10000|WGS 84 Ellipsoid|

The reason I need this data is specifically that I need a way to pull the Null/noData value from this file. I have searched through all of the other tiff tags in these files and this is the only place where I could find this information.

When I use gdal to read these two geotiffs, I call:

double noDataValue = poBand->GetNoDataValue(&bSrcHasNoData);

but bSrcHasNoData is always false.

These geotiffs are publicly available so if you are interested I could send you the links.

Ben

-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Tuesday, August 17, 2010 9:11 PM
To: Garrett, Benjamin D
Cc: 'gdal-dev at lists.osgeo.org'
Subject: EXTERNAL: Re: [gdal-dev] retrieve specific tiff tags

Garrett, Benjamin D wrote:
> Hi,
> 
>  
> 
> I would like to be able to retrieve the value of this tiff tag:
> 
> 34737                    GeoAsciiParams
> 
>  
> 
> from a geotiff file.   How do I do this using GDAL?  I've searched the 
> FAQ, the mailing list via google, this page: 
> http://www.gdal.org/frmt_gtiff.html , etc.
> 
>  
> 
> When I dump the metadata for my file using "gdainfo", the tag above 
> doesn't show up (even though I know it is present in the file by dumping 
> it via other 3^rd party tools). I assume this is because I don't know 
> what the right domain name is to pass in via -mdd.  But I haven't found 
> any list of domain names that might be appropriate for tiff.

Ben,

This tag is considered internal structural information of the file,
and is not directly made available.  However, it will generally contain
things like the PSC citation name, datum name and such that are used to
build the projected coordinate system name.  So much of the contents will
typically come through in that fashion.

If you want to access such underlying TIFF tags you should really be
using libtiff, and TIFFGetField().  I would note that libgeotiff also
does not provide direct access to this tag.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent