[GRASSLIST:288] Re: Reduction of raster filesize by subsectioning into smaller maps?

[Glynn Clements]

Patton, Eric wrote:

> Ok, I'll try to explain what I mean. I have certain bathyemtry rasters that
> are pretty unusual in shape - see attached - this map pretty much maximizes
> the null/data ratio for the given region which completely encloses it's
> shape. So I have a couple questions:
> 
> The first question is to clarify my understanding of how nulls are
> represented in Grass rasters.
> 
> 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.

> 2) If (1) is true, then can a raster such as the one in the attached image
> be made smaller by tiling together a number of smaller regions that only
> cover the areas with data? Or will the patched product of these smaller maps
> only approximate (very closely) the total size of the original raster?
> 
> 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>