[gdal-dev] Extra Decimal places added to ASCII gdal_translate mosaic
Even Rouault
even.rouault at mines-paris.org
Mon Mar 10 12:21:22 PDT 2014
Le lundi 10 mars 2014 18:55:36, Jonathan Moules a écrit :
> Hi Etienne,
> I was looking for something like that but the
> http://www.gdal.org/frmt_gtiff.html page didn't list it explicitly.
>
> That said, looking at the gdalinfo now (a little late) I see that it's
> already defaulting to Float32. I also tested with the "-ot Float32"
> arguement, but there was no change. I'm counting 16 significant figures; I
> only want 5 or 6.
TIFF stores floating point values as IEEE754 floats. Talking about significant
figures doesn't make much sense. You could test using Float64 with the hope
that 50.597 can be exactly represented as a Float64. Otherwise you'll have to
do the rounding when reading back from TIFF.
>
> Cheers,
> Jonathan
>
> On 10 March 2014 17:49, Etienne Tourigny <etourigny.dev at gmail.com> wrote:
> > Have you tried using a 36bit (float) instead of 64bit (double) output?
> >
> > double is usually the default output type and has more significant
> > digits
> >
> > (~16) than your source, so it's representation is "ok" in double format
> >
> > see [1] for an explanation on floating-point precision
> >
> > the following argument would do this "-ot Float32"
> >
> > [1] http://en.wikipedia.org/wiki/Floating_point
> >
> > Etienne
> >
> >
> > On Mon, Mar 10, 2014 at 2:25 PM, Jonathan Moules <
> >
> > jonathanmoules at warwickshire.gov.uk> wrote:
> >> Hi List,
> >> Bringing back an old thread but with a slight change.
> >>
> >> I'm now trying to get GDAL to mosaic and export from the ASCII files to
> >> a GeoTIFF.
> >> But the GeoTIFF writer in GDAL is doing the exact same thing as the
> >> ASCII writer was - it's taking *50.597* (source value) and converting
> >> it into something like *50.596895342423*.
> >>
> >> I've tried using both gdal_merge and going via a VRT (gdalbuildvrt -
> >> gdal_translate). The result is the same.
> >>
> >> I also tried the *DECIMAL_PRECISION* trick, but gdal informs me:
> >>> Warning 6: Driver GTiff does not support DECIMAL_PRECISION creation
> >>> option
> >>
> >> How do I get GDAL to merge these into a GeoTIFF without altering the
> >> data?
> >>
> >> Thanks,
> >> Jonathan
> >>
> >>
> >> On 24 January 2014 16:12, Jonathan Moules <
> >>
> >> jonathanmoules at warwickshire.gov.uk> wrote:
> >>> Thanks Norman. I'd already seen that page but failed to read it!
> >>> However, having tested it, it's not functioning as the manual says.
> >>>
> >>> As best I can tell, DECIMAL_PRECISION=3 should give me 3 decimal
> >>> places. But what it instead does it give me 3 significant figures. So
> >>> 98.354 has become 98.3.
> >>> This becomes a problem because many of my heights are hovering around
> >>> 100. So some will either have to be 1 character too long to ensure no
> >>> data loss for the rest.
> >>>
> >>> Is this a bug? The documentation on the page linked to is quite clear
> >>> it should be decimal places.
> >>>
> >>> Less of an issue, but it also didn't make any difference to:
> >>>> xllcorner 432000.000000000000
> >>>> yllcorner 242000.000000000000
> >>>> cellsize 2.000000000000
> >>>
> >>> GDAL 1.10.1, released 2013/08/26
> >>>
> >>>
> >>> Cheers,
> >>> Jonathan
> >>>
> >>> On 24 January 2014 15:11, Norman Vine <nhv at cape.com> wrote:
> >>>> try
> >>>>
> >>>> gdal_translate -of AAIGrid -co DECIMAL_PRECISION=3 abc.vrt abc.asc
> >>>>
> >>>> see http://www.gdal.org/frmt_various.html
> >>>> HTH
> >>>> Norman
> >>>>
> >>>> On Jan 24, 2014, at 10:01 AM, Jonathan Moules <
> >>>> jonathanmoules at warwickshire.gov.uk> wrote:
> >>>>
> >>>> Hi List,
> >>>> I'm trying to mosaic some ASCII grid tiles into a single large ASCII
> >>>> file (which can then easily be handled).
> >>>>
> >>>> I'm using this process:
> >>>>> REM Create list of files
> >>>>> dir /b /s *.asc > asc_list.txt
> >>>>>
> >>>>>
> >>>>>
> >>>>> REM Turn into VRT
> >>>>> gdalbuildvrt -srcnodata "-9999" -vrtnodata "0" -a_srs "EPSG:27700"
> >>>>> -input_file_list "asc_list.txt" abc.vrt
> >>>>>
> >>>>>
> >>>>>
> >>>>> REM actual mosaicing
> >>>>> gdal_translate -of AAIGrid abc.vrt abc.asc
> >>>>
> >>>> The input files are like:
> >>>>> ncols 500
> >>>>> nrows 500
> >>>>> xllcorner 432000.000
> >>>>> yllcorner 243000.000
> >>>>> cellsize 2
> >>>>> nodata_value -9999.0
> >>>>> 98.354 98.449 98.658 98.874 99.038 99.096
> >>>>
> >>>> But the outputs are like:
> >>>>
> >>>> fncols 500
> >>>>
> >>>>> nrows 1000
> >>>>> xllcorner 432000.000000000000
> >>>>> yllcorner 242000.000000000000
> >>>>> cellsize 2.000000000000
> >>>>> NODATA_value 0
> >>>>>
> >>>>> 98.353996276855469 98.448997497558594 98.657997131347656
> >>>>>
> >>>>> 98.874000549316406
> >>>>
> >>>> Suddenly everything has a dozen extra decimal places. Not only does
> >>>> this induce the False Precision problem (we don't have heights to the
> >>>> femtometer!), it also massively increases the filesize.
> >>>>
> >>>> How can I get this to not happen?
> >>>>
> >>>> Thanks,
> >>>> Jonathan
> >>>>
> >>>>
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