[GRASS-user] Working with a large number of maps

[Glynn Clements]

Craig Leat wrote:

> I have 139 high resolution images, which I need to use as a background map
> for some digitizing work. Loading 139x3 layers into gis.m is a real pain and
> I can think of three workarounds.

> 3.  Use r.patch to build one mosaic for each band.
> I currently have r.patch running. It's been going for about 10.5hrs and has
> reached 15%. Does this sound reasonable? I am starting to wonder if I'm
> expecting too much. Here are my region settings:
> 
> GRASS 6.3.0RC3 (msunduzi_lo31):~ > g.region -p
> projection: 99 (Transverse Mercator)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      -3264858.18753122
> south:      -3295545.14772988
> west:       -92005.31394386
> east:       -43582.78768323
> nsres:      0.35000012
> ewres:      0.35000019
> rows:       87677
> cols:       138350
> cells:      12130112950

How do the individual maps relate to the overall region?

If they are (approximately) tiles, I would suggest using a
hierarchical approach, patching groups of adjacent tiles into larger
tiles, then patching the larger tiles together, so that each r.patch
operation has relatively few maps, and the earlier stages have
relatively small regions.

The total time taken by r.patch is roughly proportional to the total
number of cells in the region multiplied by the number of input maps,
so the combination of a large region and a large number of maps will
be slow.

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