# [postgis-users] Need a "polygon box" of a table instead of the bounding box that returns extent function

Oscar Zamudio cmntlk at gmail.com
Mon May 3 10:47:14 PDT 2010

```I must to recognize that today I did not research further about that, I just
remember something about convex hull from my scientific research background.
Anyway in my case if the convex hull of the streets table doesn't work in
the straight manner, I still can make a new table containing  first and last
point of every line. A convex hull over them should do the work.

On Mon, May 3, 2010 at 1:14 PM, Martin Davis <mbdavis at refractions.net>wrote:

> I believe that the convex hull of a set of lines is the same as the convex
> hull of the vertices of the lines.  So it should do what you want.
>
> But if you have a reference showing otherwise I'd be interested to see it.
> Oscar Zamudio wrote:
>
>> ha, ha!!,
>> My knowledge come from Maths ,and certainly not from Postgis!. The concept
>> of Convex Hull (in algorithms and math fields) is always applied to set of
>> points and I remember there was a big challenge to find a good method for
>> lines (some years ago). But I don't mind Math anymore, if this function
>> works to solve my problem, it will be ok for me.
>> I will let you know later about my results.
>>
>> On Mon, May 3, 2010 at 11:29 AM, strk <strk at keybit.net> wrote:
>>
>>
>>
>>> On Mon, May 03, 2010 at 11:18:03AM -0300, Oscar Zamudio wrote:
>>>
>>>
>>>> Thanks strk,
>>>> As far as I know convex hull basically applies to a collection of
>>>>
>>>>
>>> points...,
>>>
>>>
>>>> nevertheless, I will try with my set of lines.
>>>>
>>>>
>>> How far does your knowledge come from ? :)
>>> http://postgis.refractions.net/docs/ST_ConvexHull.html
>>>
>>> --strk;
>>>
>>>  ()   Free GIS & Flash consultant/developer
>>>  /\   http://strk.keybit.net/services.html
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>>
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>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
>
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