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

[Jonathan Moules]

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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>

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