[gdal-dev] Extra Decimal places added to ASCII gdal_translate mosaic

Mon Mar 10 10:55:36 PDT 2014

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.

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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