Creating mask

[Adalberto da Silva]

Hello!

	I have a very large dataset I will use to generate a DEM by
interpolation with s.surf.tps. I have 16 digitised maps, some 150
thousands points each, and it takes about 3 Gbytes temp files for
elevation, slope and aspect output to make the interpolation of every
4-maps set. My HD is not such an enormous one as I can trust it to do
the entire job in a single pass.

	Output files are not so big, almost 600 Kbytes each for this 4-maps
set. So I need to cut my entire dataset down in smaller pieces and patch
them together for final output. I wish this works.

	I tried to do this redefining regions and interpolating inside every
smaller chunk, but the final patchwork was awful! Every rectangle were
quite obvious to see because the interpolating function disregards any
point outside working region and therefore I got a badly generated
perimeter strip in every block.

	From the manuals, I read this could be better done by masking instead
of defining 'sub-regions' because masks can constrain my output for a
required rectangle but needed data outside can still be computed.

	Questions:

	First: is this right or a misunderstand due to my bad reading?

	Second: case (1st) is right, how can a generate raster maps for using
as masks? I will need a series of 'walking windows' in order to scan the
entire area before final patching.

	Thanks,


	Adalberto da Silva
	Instituto Astronomico e Geofisico
	Universidade de Sao Paulo - Brasil
	adalbert at iag.usp.br