[GRASS-user] How to categorize my areas

[Paulo van Breugel]

On Tue, Dec 15, 2015 at 9:15 AM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:

> On 15/12/15 00:22, Martin Landa wrote:
>
>> Hi,
>>
>> 2015-12-15 0:18 GMT+01:00 Alec Ventura <alecventura at hotmail.com>:
>>
>>> I run the v.buffer with the -t flag and everything is ok now
>>>
>>
>> I wonder if attributes should be copied by default (without need to
>> set up any flag).
>>
>
> v.buffer fusions the geometries of buffers by default. Keeping the
> attribute table does not make sens in that case as one buffer geometry can
> be the result of many different input geometries. This also means that you
> cannot retrieve the buffer of a specific input geometry.
>

> When you use the -t flag, buffers are cut up into separate parts and each
> one that is the result of several overlapping buffers has multiple
> categories. As the man page says: "The resulting buffer areas can have
> multiple categories, and multiple buffer areas can have the same category.
> The buffer for the input feature with category X can thus be retrieved by
> selecting all buffer areas with category X".
>



The above is actually a nice and more explicit explanation of what v.buffer
does, especially for (new) users that get confused by the consequences of
GRASS' topological vector format and handling. Why not add this to the
manual page, perhaps something along the line of:

"v.buffer fusions the geometries of buffers by default. Categories and
attribute table will not be transferred (this would not make sense as one
buffer geometry can be the result of many different input geometries). To
transfer the categories and attributes can be done with the *t* flag. This
will result in buffers being cut up where buffers of individual input
geometries overlap. Each part that is the result of overlapping buffers of
multiple geometries will have multiple categories corresponding to those
geometries. Multiple buffer areas can also have the same category. The
buffer for the input feature with category X can thus be retrieved by
selecting all buffer areas with category X (see example below). "

(this would replace: "Categories and attributes can be transferred with the
*t* flag. The resulting buffer areas can have multiple categories, and
multiple buffer areas can have the same category. The buffer for the input
feature with category X can thus be retrieved by selecting all buffer areas
with category X (see example below).")




>
> Keeping in mind GRASS' topological vector format the current behavior
> seems the most adequate to me.
>
> Moritz
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