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

[Oscar Zamudio]

Well,
I found many proposed solutions to my problem and just to let you all
know,the concept I was asking for is : "Concave hull".
Google search for it !


On Wed, May 5, 2010 at 5:28 PM, Oscar Zamudio <cmntlk at gmail.com> wrote:

> Now that I get this polygon I found that it encloses all the streets in my
> table and follows some more or less smooth path around them, much better
> than a bounding box. But....to be honest, it is not the best suitable for my
> purposes. I want a footprint of the whole bunch of streets. I mean: suppose
> for a minute I have a table with only two equal length streets that crosses
> each other at 90 degrees at the middle of them. With the proposed mechanism
> I will get a square.
> What I want to return from this cross is a closed cross shaped polygon that
> sorrounds more or less smoothly the actual cross. Maybe I need to use some
> buffer extent over the lines (to be defined) that helps to get the final
> shape. This polygon is what I want to use as an area shape to query about
> its overlaps with a point.
> The need of such process is because I have many streets table for cities
> that are closer to each other. Using bounding boxes concept they overlaps in
> some regions for neighbour cities. This makes more complex the process of
> finding to which city belongs a point that falls in the overlapping region.
> Maybe a recursive process with dividing squares can be a good method. An
> initial box that includes all the strets (extent) is divided in to four
> squares. From these ones, the squares that contains streets (true) are left,
>  the other (false) are discarded. Every true square again is divided in 4
> subsquares and so on ..up to a minimum size of square (maybe 100 m x 100
> m).
> I don't know if anyone has implemented or tried something like this or even
> if there's already a solution to this problem.
> Can anybody help me?
> Thanks in advance,
>
>
> On Wed, May 5, 2010 at 11:23 AM, Nicolas Ribot <nicolas.ribot at gmail.com>wrote:
>
>> > Nicolas,
>> > What I want to do is an insert of the type INSERT INTO...VALUES .. as
>> > follows:
>> >
>> > INSERT INTO boundaries ( the_geom, the_name ) VALUES
>> >  (some_geometry_data,'some_arbitrary_name')
>> >
>> > My problem is that I want to replace the some_geometry_data value by the
>> > result of the SELECT query. And of course I am not an expert user of SQL
>> > statements so I don't know how to do that..
>> > from my_street_table;
>> > Maybe your example can work, I will try it later and let you know.
>> >
>> > INSERT INTO boundaries ( the_geom, the_name ) SELECT
>> > ST_ConvexHull(ST_Collect(the_geom)) as the_geom, 'mytablename' from
>> > my_street_table;
>> >
>>
>> So yes, the example, (or, better, the link to the INSERT command
>> reference) will help you with the query.
>> (When inserting with a SELECT, the VALUES keyword is not used).
>>
>> Nicolas
>> _______________________________________________
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>>
>
>
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