[postgis-users] getting a square/rectangle arround a point

[Daniel Grum]

Dear Mailing-List,

I want to create a square/rectangle around a point-->via box2d(geometry)
or buffer(geometry, double precision) for example.

If I have this rectangle I want to intersect this with another
polygon--> saved in the (geodata) table: wald_by column: the_geom, to
get information about the area the the "lumberjack" can dig.
The point in the middle of the rectangle = the point that is saved in a
table in my database in the table: holzfaeller in the column the_geom!

Is this possible to solve?
Do I have to save this calculated square/rectangle in an extra/own table?

Thanks for every help.

Daniel
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[Pavel Iacovlev]

st_intersection(st_envelope(st_buffer(t1.the_geom, $radius)), t2. the_geom)

2009/8/18 Daniel Grum <danie...@unibw.de>:

--
http://iap.md, The future is open

[Daniel Grum]

Daniel Grum schrieb:
> Pavel Iacovlev schrieb:

>> st_intersection(st_envelope(st_buffer(t1.the_geom, $radius)), t2.
>> the_geom)
>>
>> 2009/8/18 Daniel Grum <danie...@unibw.de>:
>>
>>> Dear Mailing-List,
>>>
>>> I want to create a square/rectangle around a point-->via
>>> box2d(geometry) or
>>> buffer(geometry, double precision) for example.
>>>
>>> If I have this rectangle I want to intersect this with another
>>> polygon-->
>>> saved in the (geodata) table: wald_by column: the_geom, to get
>>> information
>>> about the area the the "lumberjack" can dig.
>>> The point in the middle of the rectangle = the point that is saved in a
>>> table in my database in the table: holzfaeller in the column the_geom!
>>>
>>> Is this possible to solve?
>>> Do I have to save this calculated square/rectangle in an extra/own
>>> table?
>>>
>>> Thanks for every help.
>>>
>>> Daniel
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgi...@postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
>>>
>>
>>
>>
>>

> I try solve the problem with your message:
>
> SELECT st_intersection(st_envelope(st_buffer('holzfaeller.the_geom',
> 10, 'quad_segs=8')),wald_by.the_geom);
>
>
> I get this error message:
>
> FEHLER: parse error - invalid geometry
>
> ********** Fehler **********
>
> FEHLER: parse error - invalid geometry
> SQL Status:XX000
>
> What do you think is the problem in this case.
> Is there a understaning mistake in my SQL order.
>
> Thanks,
> Daniel
>
I thing I solved the problem:

SELECT st_intersection(st_envelope(st_buffer(a.the_geom,1)),b.the_geom)
FROM public.holzfaeller a, public.wald_by b
(WHERE a.gid=3); //not necessary

Thank You Pavel for the fast answer!

[Daniel Grum]

Daniel Grum schrieb:

The problem now:

this order calculates a lot of points and the intersection with the
layer "wald_by" can't be solved because the geo data typ is:
GEOMETRYCOLLECTION.

I tried to generate a rectangle with ST_GeomGromText instaead of the
st_buffer that generates a circle:

ST_GeomFromText('POLYGON((st_x(a.the_geom)-2.0
st_y(a.the_geom)+2.0,st_x(a.the_geom)+2.0
st_y(a.the_geom)+2.0,st_x(a.the_geom)+2.0
st_y(a.the_geom)-2.0,st_x(a.the_geom)-2.0
st_y(a.the_geom)-2.0,st_x(a.the_geom)-2.0 st_y(a.the_geom)+2.0))')

PostgreSQL don't alerted me with a mistake, but if I would look for the
generated data, PostgreSQL isn't able to do this and said "Bricht ab".

What can be the reason for this?

Thanks

[Kevin Neufeld]

Unless I misunderstand what you are after, why don't you simply expand the bounding box of the point by your required
amount?

ie.
SELECT
ST_AsText(geom),
ST_AsText(ST_Expand(geom, 2))
FROM (SELECT 'POINT(5 5)'::geometry AS geom) AS a;

st_astext | st_astext
------------+--------------------------------
POINT(5 5) | POLYGON((3 3,3 7,7 7,7 3,3 3))
(1 row)

-- Or create a rectangle from a point
SELECT
ST_AsText(geom),
ST_AsText(
ST_Envelope(ST_MakeLine(
ST_MakePoint(ST_X(geom) - 3, ST_Y(geom) - 2),
ST_MakePoint(ST_X(geom) + 3, ST_Y(geom) + 2)
))
)
FROM (SELECT 'POINT(5 5)'::geometry AS geom) AS a;

st_astext | st_astext
------------+--------------------------------
POINT(5 5) | POLYGON((2 3,2 7,8 7,8 3,2 3))
(1 row)

-- Kevin

[Daniel Grum]

Kevin Neufeld schrieb:

Your request nr.2 is perfect, thank you.
But can use this created polygon to selcet a part of another polygon, in
my case public.wald_by.the_geom.
Or do I have to save the selcted information in a table and use this for
the new request???
Because I need to selcet the other polygon to calculate the area of the
selected part, of public.wald_by.the_geom !

--Daniel

[Kevin Neufeld]

You mean like this?
SELECT ST_Intersection(
poly.geom,
ST_Expand(pt.geom, 2))
FROM mypolygontable poly, mypointtable pt
WHERE poly.geom && ST_Expand(pt.geom, 2);

Yes, you can expand the bounding boxes on fly and use it to intersect another geometry.

Cheers,

[Daniel Grum]

Kevin Neufeld schrieb:

> You mean like this?
> SELECT ST_Intersection(
> poly.geom,
> ST_Expand(pt.geom, 2))
> FROM mypolygontable poly, mypointtable pt
> WHERE poly.geom && ST_Expand(pt.geom, 2);
>
> Yes, you can expand the bounding boxes on fly and use it to intersect
> another geometry.
>
> Cheers,
> -- Kevin
>
> Daniel Grum wrote:
>> Your request nr.2 is perfect, thank you.
>> But can use this created polygon to selcet a part of another polygon,
>> in my case public.wald_by.the_geom.
>> Or do I have to save the selcted information in a table and use this
>> for the new request???
>> Because I need to selcet the other polygon to calculate the area of
>> the selected part, of public.wald_by.the_geom !
>>
>> --Daniel

The SQL you have given me, seems to be not correct, because the area is
NULL.
Here the geometry that came as an result:
"0107000020EB7A000000000000"

I changed the second paramter in the ST_Expand from 2 to 20 and after
this from 20 to 200 but the result was the same.
I tried to load the result of the SQL --> I saved it as a VIEW.
But in mapbender there wasn't shown any area???!!!

-- Daniel

[Daniel Grum]

Kevin Neufeld schrieb:

> You mean like this?
> SELECT ST_Intersection(
> poly.geom,
> ST_Expand(pt.geom, 2))
> FROM mypolygontable poly, mypointtable pt
> WHERE poly.geom && ST_Expand(pt.geom, 2);
>
> Yes, you can expand the bounding boxes on fly and use it to intersect
> another geometry.
>
> Cheers,
> -- Kevin
>
> Daniel Grum wrote:
>> Your request nr.2 is perfect, thank you.
>> But can use this created polygon to selcet a part of another polygon,
>> in my case public.wald_by.the_geom.
>> Or do I have to save the selcted information in a table and use this
>> for the new request???
>> Because I need to selcet the other polygon to calculate the area of
>> the selected part, of public.wald_by.the_geom !
>>
>> --Daniel
>

It works, thx