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

[Marcello Gorini]

Aren Cambre wrote:
> 
> I am not getting any responses. :-) Am I doing something really dumb? Is
> this the right forum for this kind of question?
> 
> Thanks,
> 
> Aren
> 
> On Sun, Feb 13, 2011 at 8:02 AM, Aren Cambre <aren at arencambre.com> wrote:
> 
>> Maybe I could clarify. Here's the command:
>> *v.kernel --verbose input=master_grass7 output=master_grass7a_heatmap
>> stddeviation=.0001*
>>
>> Here's the command output:
>> *STDDEV: 0.000100*
>> *RES: 200.000000 ROWS: 1000 COLS: 1000*
>> *
>> *
>> *Writing output raster map using smooth parameter=0.000100.*
>> *
>> *
>> *Normalising factor=0.000000.*
>> *Maximum value in output: 0.000000e+000.*
>>
>> If I change stddeviation to *100*, here's what I get:
>> *STDDEV: 100.000000*
>> *RES: 200.000000 ROWS: 1000 COLS: 1000*
>> *
>> *
>> *Writing output raster map using smooth parameter=100.000000.*
>> *
>> *
>> *Normalising factor=64826.350188.*
>> *Maximum value in output: 0.000000e+000.*
>>
>> Either way, the resulting raster is a large salmon/pink square. I have no
>> idea what I'm doing wrong. I've tried varying the *stddeviation
>> *parameter
>> from 100 to .0001 and some values in between, and I don't get anything
>> different. Heck, I just now used 1000000 and .000001 and got nothing
>> different.
>>
>> I was really hoping to make a high res heat map of these 2.5 million
>> points
>> and overlay on to a street map layer.
>>
>> Aren
>>
>>
>> On Fri, Feb 11, 2011 at 9:45 PM, Aren Cambre <aren at arencambre.com> wrote:
>>
>>> I am not clear how to get useful results out of v.kernel. I have a
>>> vector
>>> layer imported from PostGIS that has about 2.5 million points. I want to
>>> make a high density raster to show where these points are clustered. I
>>> can
>>> get the vector layer to display in GRASS just fine, albeit without
>>> attributes because of http://trac.osgeo.org/grass/ticket/1274.
>>>
>>> It's not clear to me, based on
>>> http://grass.fbk.eu/gdp/html_grass64/v.kernel.html, how you can get it
>>> to
>>> do something besides make a big, pinkish (salmon-red?) square?
>>>
>>> Aren
>>>
>>
>>
> 
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> 

Hey,

I had tried playing around with v.random and v.kernel to see if I could
understand your problem, but I couldn't find any solutions so I didn't post
it. But since you got upset with GRASS community, I will share what I found
out anyway :)

I tried with different numbers of points and with different resolutions and
I was amazed that you could make v.kernel work with 2.5 million points in a
200 meter resolution with a 1000000 stdeviation!! My quadcore computer took
so much that I gave up.

I noticed that it goes slower with increasing resolution, increasing number
of points and increasing stdev. I also noticed that you have to set a much
higher stdev for smaller resolutions and that this value is by no means
intuitive. 

For instance: for my 2.5 meter resolution, I got meaningful results for
1,000 points using a stdev of 30. However, this same 30 stdev in my 1,000
resolution region gives me the "salmon/pink" square. So, in this case, a
10,000 stdev begins to give me good results.

So, my humble opinion is that you should try stdevs greater than 100, but
still much smaller than 1,000,000. I believe that it is just a matter of
tuning these parameters with respect to your resolution and number of
points.

Hope this helps (or at least, keep you in the list :) )

Marcello.


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