[GRASS-user] Re: v.kernel -- how to get useful results

Aren Cambre aren at arencambre.com
Tue Feb 15 22:04:06 EST 2011


I did this:
*v.random output=random_vector n=10000*

**Then I ran this:
v.kernel --verbose input=random_vector output=random_vector_heatmap

That just made a large yellow square. So I changed *stddeviation* to 100 and
got a yellow square speckled with teal and dark blue pixels after 7 seconds.
Then I changed it to 1000, and after 232(!) seconds, I got something that
actually resembles a heat map!

It appears the random_vector map is 100000x100000 units, so I can see why
1000 is necessary to get a reasonable map.

I then ran *r.info map=random_vector at de_maps* and got this (cut down to key
*Type of Map:  vector (level: 2)*
*Number of points:       10000*
Projection: Lambert Conformal Conic
N:            100000
S:   -99975.58519242
E:    99987.79259621
W:   -99975.58519242
That looks like it's around 200000x200000 units?

So I ran *v.info map=master_grass7 at de_maps* on my original vector data and
got this key data:
*Type of Map:  vector (level: 2)*
*Number of points:       2057980*
*Projection: Lambert Conformal Conic*
*N:  2781558.27080444*
*S:    422254.4557289*
*E:  12145354.2426632*
*W:   767479.81703687*

I went back to the Map Display window for this vector and noticed that about
all I am interested in is a box bounded by 1269430.31,1210876.00 on the NW
side and 1325045.16,1164615.03 on the SE side. Does this mean I should
define a region with this as the bounding box, then re-run v.kernel and hope
it works on just that region?


On Tue, Feb 15, 2011 at 12:15 PM, Marcello Gorini <gorini at gmail.com> wrote:

> > Aren said:
> >Where is resolution adjusted?
> >http://grass.fbk.eu/gdp/html_grass64/v.kernel.html doesn't make that
> clear.
> GRASS works with the region concept. You can adjust any region settings
> with
> the command g.region, like:
> g.region res=desired_resolution -p
> The -p is for you to see the resulting changes.
> >Am I supposed to have a pre-made raster layer already that corresponds to
> my
> >vector layer?
> No, you probably don't. I would suggest you play around with v.random to
> create some random points and then use v.kernel to make the heat map. Do
> that with different number points, stdev and resolutions to get the feel of
> it.
> Maybe in this way you will understand what is going on with your specific
> case.
> >I really appreciate your advice!
> Any time! Sorry not to help you more.
> >I'm not upset with the list, was just afraid this was too esoteric or
> poorly stated.
> I think I myself have been a little upset a few times when I did not get
> any
> responses to my querries :) that's why I mentioned, I guess. But I from
> Brazil, so either I get upset or I get really happy :)
> Cheers,
> Marcello.
> --
> View this message in context:
> http://osgeo-org.1803224.n2.nabble.com/v-kernel-how-to-get-useful-results-tp6018010p6028684.html
> Sent from the Grass - Users mailing list archive at Nabble.com.
> _______________________________________________
> grass-user mailing list
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