<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<div class="BodyFragment"><font size="2"><span style="font-size:10pt;">
<div class="PlainText">So maybe I found the answer to my question, but it has left me confused.<br>
<br>
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.<br>
<br>
buffer1.png shows the original points (blue triangles) and the circular buffers (in grey with "X" centroids).<br>
buffer2.png shows the same points and buffers in digitizing mode<br>
<br>
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.<br>
<br>
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.
<br>
<br>
When I calculate the square area of each area feature linked to each attribute record, it returns a correct value for a circular area.
<br>
<br>
So maybe I can just ignore the extra stuff? But I worry a bit that this will show up incorrectly in some analyses.<br>
<br>
</div>
</span></font></div>
<div class="BodyFragment"><font size="2"><span style="font-size:10pt;">
<div class="PlainText"><br>
<br>
Michael<br>
____________________<br>
C. Michael Barton<br>
Director, Center for Social Dynamics & Complexity <br>
Professor of Anthropology, School of Human Evolution & Social Change<br>
Arizona State University<br>
<br>
voice: 480-965-6262 (SHESC), 480-727-9746 (CSDC)<br>
fax: 480-965-7671 (SHESC), 480-727-0709 (CSDC)<br>
www: <a href="http://www.public.asu.edu/~cmbarton">http://www.public.asu.edu/~cmbarton</a>,
<a href="http://csdc.asu.edu">http://csdc.asu.edu</a><br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
On Jun 4, 2013, at 10:54 AM, Michael Barton <michael.barton@asu.edu> wrote:<br>
<br>
> So this is odd.<br>
> <br>
> I have 166 points, with a few cases where 2 points are in the same location<br>
> I created a set of circular buffers around the points with <br>
> <br>
> v.buffer -t input=neolithic_sites type=point output=sites5km distance=5000<br>
> <br>
> This indeed created a set of overlapping (non-merged), circular buffers.<br>
> <br>
> BUT, it creates 212 areas and 100 islands according to the command output and metadata report (v.report).
<br>
> <br>
> There are only 166 attribute table entries.<br>
> <br>
> So what are the extra areas and the islands?<br>
> <br>
> Michael<br>
> ____________________<br>
> C. Michael Barton<br>
> Director, Center for Social Dynamics & Complexity <br>
> Professor of Anthropology, School of Human Evolution & Social Change<br>
> Arizona State University<br>
> <br>
> voice: 480-965-6262 (SHESC), 480-727-9746 (CSDC)<br>
> fax: 480-965-7671 (SHESC), 480-727-0709 (CSDC)<br>
> www: <a href="http://www.public.asu.edu/~cmbarton">http://www.public.asu.edu/~cmbarton</a>,
<a href="http://csdc.asu.edu">http://csdc.asu.edu</a><br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> <br>
> On Jun 3, 2013, at 12:20 AM, Markus Metz <markus.metz.giswork@gmail.com> wrote:<br>
> <br>
>> On Mon, Jun 3, 2013 at 1:08 AM, Hamish <hamish_b@yahoo.com> wrote:<br>
>>> Michael wrote:<br>
>>>>> Is there a way to create buffers around features that<br>
>>>>> overlap when the features are near to each other instead of<br>
>>>>> merging?<br>
>>> <br>
>>> Markus M wrote:<br>
>>>> In GRASS 7, with v.buffer -t<br>
>>> <br>
>>> for the sake of completeness, in 6.3.0 you could do this with<br>
>>> the debug= option, but that did not continue into the replacement<br>
>>> 6.4 module. There you'd have to run it in a loop then combine with<br>
>>> v.patch.<br>
>> <br>
>> To be precise, the 6.3 debug= option as well as the v.patch solution<br>
>> produce really overlapping areas with broken topology. The 7.0 -t flag<br>
>> preserves the categories of the input features and cleans topology.<br>
>> Areas in the output can have more categories, one for each input<br>
>> feature in case of overlapping buffers. Thus you have clean topology<br>
>> and you can extract single buffers. See the example in the 7.0 manual.<br>
>> <br>
>> Markus M<br>
> <br>
<br>
</div>
</span></font></div>
</body>
</html>