[Gdal-dev] GeoTiff and nodata values

[Frank Warmerdam]

Kor de Jong wrote:
> Hi,
> 
> I am writing some tests for the PCRaster raster driver and have a question 
> about the nodata handling of the GeoTiff driver.
> 
> For one test I read several kinds of PCRaster files using the GDAL PCRaster 
> driver. Each dataset is written using CreateCopy and read again. This way I 
> want to check that a written copy of a PCRaster file equals the original.
> 
> When I use the PCRaster driver to create copies of the originals all goes 
> well.
> 
> Using the GeoTiff driver I noticed that for rasters in UINT1 (UInt8) and INT4 
> (Int32) all goes well too, but for data in REAL4 (Float32) the missing value 
> of the GeoTiff file is set to 0 instead of 1e-30 which is used by the 
> PCRaster driver. The cell values of the original PCRaster file and the 
> GeoTiff file do equal eachother, it is just that the nodata value of the 
> GeoTiff file is not set to the value used in the PCRaster driver. In the 
> other cases (UINT1, INT4), the nodata value is correctly copied into the 
> GeoTiff.
> 
> Does anyone know why the nodata value is handled differently by the GeoTiff 
> driver, depending on the type of the cell values?

Kor,

I have committed a change to the GTiff driver that should fix this.  The
nodata values for GTiff are ascii encoded and stored in a string tag.  The
"%f" format was being used which does not permit exponents.  So 1e-30 would
be internally rounded to zero.  I have changed the format to %.16g which
I believe generally will preserve the precision of any double value.

Best regards,
-- 
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