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

[Markus Metz]

On Tue, Jun 4, 2013 at 9:28 PM, Michael Barton <Michael.Barton at asu.edu> wrote:
> 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.

Categories of the centroids correspond to categories of the input
features. A single output centroid can correspond to multiple input
features.

The equivalent to overlapping areas are in GRASS areas with centroids
multiple categories.

HTH,

Markus M

>
> 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
>
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> 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
>>
>>
>>
>>
>>
>>
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>> 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
>>
>