[GRASS-dev] GRASS to Numpy [was: Re: List of new features in GRASS 7 (in trac)]

[Glynn Clements]

Tim Michelsen wrote:

> O question on this here:
> > Improved Python API:     * read/write GRASS rasters to/from NumPy
> How do I trigger this?
> 
> Will a command r.out.numpy save the raster in a pickled array?
> How will the geographic information be retrained?

The grass.script.array module defines a class "array" which is a
subclass of numpy.memmap with .read() and .write() methods to
read/write the underlying file via r.out.bin/r.in.bin.

Example:

	import grass.script.array as garray
	a = garray.array()
	a.read("elevation.dem")
	b = garray.array()
	b[...] = (a / 50).astype(int) * 50	# or whatever
	b.write("elev.50m")

The size of the array is taken from the current region.

The main drawback of using numpy is that you're limited by available
memory. Using a subclass of numpy.memmap lets you use files which may
be much larger, but processing the entire array in one go is likely to
produce in-memory results of a similar size.

Unfortunately, I don't think that it's possible to define an array
subclass which provides the data in chunks.

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