[GRASSLIST:289] RE: Reduction of raster filesize by subsectioning into smaller maps?

[Patton, Eric]

Thanks, Glynn. That clears things up.  

I suppose the null threshold filter would still be useful if one was trying
to automate the export of regions of a map less than some null-to-data
ratio; skipping windows that contain more than some cutoff value  would save
on one r.mapcalc iteration per window.

--
Eric Patton 
epatton at nrcan.gc.ca

-----Original Message-----
From: Glynn Clements [mailto:glynn at gclements.plus.com] 
Sent: Tuesday, March 21, 2006 1:15 PM
To: Patton, Eric
Cc: 'GRASSLIST at baylor.edu'
Subject: Re: [GRASSLIST:276] Reduction of raster filesize by subsectioning
into smaller maps?


> 1) Does an increase in the number of null cells within a region 
> increase the raster's file size on disk? If I increase the region 2x 
> in both N-S and E-W dimensions, but add no valid data only null 
> values, I'm getting a bigger raster regardless, correct?

Correct. Although increases in horizontal resolution won't necessarily
produce proportional increases in file size, due to compression. 
Increases in vertical resolution will.

 
> So, to sum in one sentence, I want to make a big diagonally-shaped 
> raster smaller by cheating and making a bunch or smaller maps that 
> cover data areas only (and subsequently have low null content) then 
> patching the smaller ones together. But I'm worried that this whole 
> process just puts Humpty back together again and I'm left with just as 
> many nulls to data as I started with.

Your understanding is correct.

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