[GRASS-user] Precision of raster/vector float values

[Markus Metz]

On Thu, Aug 23, 2012 at 2:40 PM, Johannes Radinger
<johannesradinger at gmail.com> wrote:
>>> I have a GRASS raster map which contains small values (e.g 2.5e-15) among
>>> others.
>>
>> This is (dangerously) close to the IEEE limit of double precision
>> floating point values. If GRASS tests for fp precision, it regards
>> (absolute) values smaller than 1.0e-15 (GRASS_EPSILON) as zero.
>>
>> v.what.rast prints fp values with 10 decimal places. The equivalent
>> issue has aready been fixed in d.what.rast, now also for v.what.rast
>> in trunk r52857. For FCELL maps, it prints 7 decimal places, for DCELL
>> maps it prints 15 decimal places. Printing means here that fp values
>> are first converted to text, then uploaded to the attribute table.
>>
>
> So if I understand you correctly: 1.0e-15 is the smallest number for
> values in attribute columns although the values in a DCELL raster
> can be  smaller? A bottleneck is v.what.rast which converts
> the queried raster values into text with 15 decimal points?
> So is there any (future) way to have similar precision in a DCELL
> raster and its corresponding point vector (r.to.vect - v.what.rast)?

The short answer is no because binary values have to be converted to
text which for fp values is lossy.

The precision of double values in attribute tables is probably
dependent on the module loading values to the table and maybe also the
db driver.

>
> Or is there any other recommendable way to get a "list" resp. loopable
> object that contains all raster cells (x,y and value) without loosing
> the precision of the original raster?

Hmm. Something with r.mapcalc? Any *.[in|out].ascii will suffer from
the lossy conversion of binary -> text -> binary values.

Also be aware of the difference between precision and accuracy. For
example, floating point DEM can have a precision in the nanometer
range, but its accuracy may be in the meter range.

In your case, rescaling could help.

Markus

>
> /Johannes
>
>> Markus M
>>
>>>
>>> Some additional info: I am working on Ubuntu 12.04 whit GRASS 6.5SVN. My
>>> point vector  is
>>> connected to an sqlite db - table (standard sqlite connection).
>>>
>>> Of course I thought about a work around (e.g. multiplication by a large
>>> scalar). Any other suggestions?
>>>
>>> Best regards,
>>>
>>> Johannes