[GRASS-user] [GRASS-dev] how to make non-merged buffers?

[Michael Barton]

So maybe I found the answer to my question, but it has left me confused.

I'm attaching a zip file of 2 images of buffers created around a set of points. This is a closeup of a part of my complete dataset.

buffer1.png shows the original points (blue triangles) and the circular buffers (in grey with "X" centroids).
buffer2.png shows the same points and buffers in digitizing mode

It seems like a centroid is created for every overlap as well as the non-overlapped part of each buffer. I don't know if an area is defined for each overlap too. However, my guess is that this is why I have 212 areas created for 166 points I am buffering. I'm not sure what is being defined as "islands", but there are 100 of these in all.

Centroids are placed in the center of each overlapping and non-overlapping area. That means that area centroids may not correspond spatially to the original points. This is clear in each of the 2 images.

When I calculate the square area of each area feature linked to each attribute record, it returns a correct value for a circular area.

So maybe I can just ignore the extra stuff? But I worry a bit that this will show up incorrectly in some analyses.



Michael
____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

voice:   480-965-6262 (SHESC), 480-727-9746 (CSDC)
fax:          480-965-7671 (SHESC),  480-727-0709 (CSDC)
www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu











On Jun 4, 2013, at 10:54 AM, Michael Barton <michael.barton at asu.edu> wrote:

> So this is odd.
>
> I have 166 points, with a few cases where 2 points are in the same location
> I created a set of circular buffers around the points with
>
> v.buffer -t input=neolithic_sites type=point output=sites5km distance=5000
>
> This indeed created a set of overlapping (non-merged), circular buffers.
>
> BUT, it creates 212 areas and 100 islands according to the command output and metadata report (v.report).
>
> There are only 166 attribute table entries.
>
> So what are the extra areas and the islands?
>
> Michael
> ____________________
> C. Michael Barton
> Director, Center for Social Dynamics & Complexity
> Professor of Anthropology, School of Human Evolution & Social Change
> Arizona State University
>
> voice:         480-965-6262 (SHESC), 480-727-9746 (CSDC)
> fax:          480-965-7671 (SHESC),  480-727-0709 (CSDC)
> www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu
>
>
>
>
>
>
>
>
>
>
>
> On Jun 3, 2013, at 12:20 AM, Markus Metz <markus.metz.giswork at gmail.com> wrote:
>
>> On Mon, Jun 3, 2013 at 1:08 AM, Hamish <hamish_b at yahoo.com> wrote:
>>> Michael wrote:
>>>>> Is there a way to create buffers around features that
>>>>> overlap when the features are near to each other instead of
>>>>> merging?
>>>
>>> Markus M wrote:
>>>> In GRASS 7, with v.buffer -t
>>>
>>> for the sake of completeness, in 6.3.0 you could do this with
>>> the debug= option, but that did not continue into the replacement
>>> 6.4 module. There you'd have to run it in a loop then combine with
>>> v.patch.
>>
>> To be precise, the 6.3 debug= option as well as the v.patch solution
>> produce really overlapping areas with broken topology. The 7.0 -t flag
>> preserves the categories of the input features and cleans topology.
>> Areas in the output can have more categories, one for each input
>> feature in case of overlapping buffers. Thus you have clean topology
>> and you can extract single buffers. See the example in the 7.0 manual.
>>
>> Markus M
>

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