Thanks for your answer and link, Moritz.
Of course, slow or fast are relative terms. May be I wasn't clear
enough. Let's me explain better:
The core of my processing is simple:
1. extract point from point map (v.extract)
2. create buffer (v.buffer)
3. clip polygon map with buffered point (v.clip)
4. calculate areas (v.to.db)
This takes only 8 minutes. So we can agree to call it "fast".
The problem is: I have to do this for 60 points, in 5 different buffer
sizes, for 14 sets of polygon maps, that is 4200 times.
At this rate, this process will take 4200 x 8 minutes = 33600 minutes,
or about 23 days and 8 hours.
That's why my question was if there was setting I might activate to
reduce computation time like, for example, limiting the computational
region to the area under the buffer. In the meantime since I posted my
question I read more carefully the v.clip manual and see that there is a
switch (-r) to use in a defined region. This is what I am using now.
I will have a look at your script, but for what I see it does not seem
critically different from my working routine. I believe I may explore
another path, namely converting the polygons to a raster of suitably
small pixel size and calculating the areas using r.stats instead. may be
that's significantly faster.
Regards and thanks for the help!
/H.
On 2018-05-13 17:05, Moritz Lennert wrote:
Le Sun, 13 May 2018 13:21:51 +0200,
Hernán De Angelis <dhdeange...@comhem.se> a écrit :
Hello GRASS users!
I am working to make a certain vector process faster and more
effective and would like to ask you for some advice on how to proceed.
I have a vector map containing points and a collection of vector maps
containing polygons. My task is to calculate the area covered by
these polygons en each polygon map but only under buffers of certain
sizes around each of the points. I understand how to do this. My
problem is how to do it faster and more effectively.
Each polygon map contains around 2 million of them. The point map has
only ~60 points. Both cover the entire country (Sweden). The buffer
zones extend only a relatively small area around each point (up to
some tens of km). Selecting points and generating buffers around them
is easy and fast. The problem appears in the overlay operation. The
vast majority of polygons are not involved in the computations but
both v.clip and v.overlay seem to need to process them all. This
takes a long time.
I wish there could be a way of limiting the number of features
involved in the computation to the size of each buffer zone in order
to cut down the processing time. I understand (and confirmed
empirically!) that setting the computational region does not work for
vector operations in GRASS (except for v.in.region). I therefore
wonder what can be done in this respect.
I am not new to GRASS but have not used it for heavy duty tasks in a
long time, my skills have become rusty! Any hint will be greatly
appreciated!
I'm a bit surprised that v.overlay is so slow for you. In the the
little alternative (faster) script in [1] I got quite fast results
using v.overlay with other modules. Maybe it can help you identify a
faster way.
Moritz
[1] https://trac.osgeo.org/grass/ticket/3361.
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