[GRASS5] v.support and large datasets (gshhs from GMT)
David D Gray
ddgray at armadce.demon.co.uk
Fri Dec 8 14:49:15 EST 2000
Markus Neteler wrote:
> Hi all,
> to provide a high resolution vector coastline dataset I have
> converted the
> G S H H S
> Global Self-consistant Hierarchical High-resolution Shorelines
> from Paul Wessel and Walter. H. F. Smith
> A problem arises from v.support which doesn't run on large files (at all or
> or in reasonable time, process is still running here). If you are interested
> to test GRASS with a *large* vector file, I have prepared the GMT dataset
> for you:
> To describe it's resolution:
> (From the GMT page:) The file gshhs_h.c (High resolution) is at 0.2 km, I
> have converted gshhs_f.c (Full resolution) at 0.?km (no value given). So I
> have converted the *full* resolution dataset gshhs_f to GRASS ASCII vector
> format for you to download at:
> ftp://www.geog.uni-hannover.de/outgoing/gshhs_f_grassascii.tar.gz (58MB)
> -rw-r--r-- 1 neteler users 226115131 Dez 8 17:42 dig_ascii.gshhs_f
> -rw-r--r-- 1 neteler users 10561544 Dez 8 17:42 dig_att.gshhs_f
> -rw-r--r-- 1 neteler users 2166637 Dez 8 17:42 dig_cats.gshhs_f
> You can use this lat/long world coastline dataset within "global dataset"
> location which you get from here (or you define an empty world lat/long
> Add the missing directories:
> and move the files into their directories (rename to gshhs_f or similar of
> course). Then
> v.in.ascii in=gshhs_f out=gshhs_f
> v.support gshhs_f op=build
> However, this takes muuucchhh time... Does anyone see a chance to speed
> up the v.support topology build function? Maybe it would be useful to
> add progress output (percentage) to v.support.
I don't know the details but execution time against file size grows at a
rate that is `more than linear' for large files. So large files, like
vector grids, do build very slowly. I think the answer is segmentation.
Perhaps the technique of `walls' where a triangulation topology is built
from a set of points could be modified to build areas. Then only small
regions are built at a time working out from the segment boundaries.
This needs to load the respective segments into a network in memory to
be efficient. A rework of the build function is desperately needed. This
could be in place for 5.1. Anyone else has had thoughts about this?
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