[GRASS-user] Set default cell type to FCELL?

[Glynn Clements]

Rainer M Krug wrote:

> What I would like to do is tho change the default type from CELL to
> FCELL - i.e. whenever a new raster is created, it will *always* and
> *automatically* be saved in a FCELL raster, even if it only contains
> integer values, and not in a CELL raster.

This isn't possible. If a module generates a CELL map, it's invariably
because it's performing the calculations using integers. Converting
the final result to floating-point wouldn't help; if the integer
calculations overflow, converting the result to floating-point won't
"undo" the overflow.

Some modules will generate either integer or floating-point output
depending upon the type of the inputs.

> so
> 
> oldmap * 1
> 
> results in a raster of the same type as oldmap,

Arithmetic operations on mixed types promote to the lesser type to the
greater type, where CELL < FCELL < DCELL.

> where
> 
> oldmap * 1.0
> 
> would result in a FCELL?

Actually, it will result in DCELL. r.mapcalc follows C syntax:
floating-point literals are double precision unless suffixed with an
"f", so "oldmap * 1.0f" will result in FCELL (unless oldmap is DCELL).

But float(oldmap) or double(oldmap) do the same thing but avoid the
gratuitous multiply operation.

Also:

1. If you are having issues with overflow, you need to promote values
before calculations, e.g. "float(x) * y". Using "float(x * y)" will
result in x*y overflowing, then the overflowed value will be
converted.

2. Single precision (FCELL) only has a 24-bit mantissa, so integers
greater than 2^24 (16777216) cannot always be represented exactly. 
Double precision (DCELL) has a 53-bit mantissa.

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