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

[Glynn Clements]

Markus Metz wrote:

> > 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.

Note that the round-trip conversion isn't *inherently* lossy.

It's possible to convert any binary floating-point value to decimal
exactly.

Arbitrary decimal values cannot be converted to (binary)
floating-point values exactly, but decimal values which were generated
from a floating-point value will convert back to the original value
provided that sufficient decimal digits are used. The decimal
representation doesn't have to be exact, only sufficiently accurate to
be distinguishable from the adjacent floating-point values.

Conversion to and from text is only lossy because the number of digits
required to avoid this is excessive for any other purpose (double
precision can represent the circumference of the earth to within 9
nanometres; no actual measurement will be that precise).

-- 
Glynn Clements <glynn at gclements.plus.com>