[postgis-users] polygon minimum width

Tue Oct 13 21:03:34 PDT 2009

Your query is not distinguishing between the left and right river linestring.

If there were TWO of them sharing the same name and ID then one way to do this using PostgreSQL 8.4 is:

drop table river;
create table river (
  id integer,
  name varchar(32),
  the_geom geometry
);

insert into river (id,name,the_geom)
values (1,'Barito',ST_GeomFromText('LINESTRING(-237.5 -138.5, -79.5 -31.5, -119.5 101.5, 41.5 166.5, -51.5 251.5)',0)),
        (1,'Barito',ST_GeomFromText('LINESTRING(42.5 271.5, 216.5 163.5, 70.5 84.5, 4.5 -4.5, -39.5 -58.5, -79.5 -151.5, -40.5 -243.5)',0));

select *
   from river;

id;name;the_geom
integer;character varying;geometry
1;"Barito";"010200002000000000050000000000000000B06DC000000000005061C00000000000E053C00000000000803FC00000000000E05DC000000000006059400000000000C044400000000000D064400000000000C049C00000000000706F40"
1;"Barito";"0102000020000000000700000000000000004045400000000000F870400000000000106B4000000000007064400000000000A051400000000000205540000000000000124000000000000012C00000000000C043C00000000000404DC00000000000E053C00000000000F062C000000000004044C00000000000706EC0"

SELECT min(ST_Distance(r.the_geom, r.the_geom)) as Min_Width
   FROM River as r
  WHERE r.Name = 'Barito';

min_width
double precision
0

The above SQL is, for each row it selects,  pushing the same geometry into the ST_Distance function.
What we need to do is feed the left and the right linestring into the same function as follows. Firstly we
need to give each row its own unique id....

select row_number() over (order by the_geom) as rin, the_geom
   from River r
  where r.Name = 'Barito';

rin;the_geom
integer;geometry
1;"010200002000000000050000000000000000B06DC000000000005061C00000000000E053C00000000000803FC00000000000E05DC000000000006059400000000000C044400000000000D064400000000000C049C00000000000706F40"
2;"0102000020000000000700000000000000004045400000000000F870400000000000106B4000000000007064400000000000A051400000000000205540000000000000124000000000000012C00000000000C043C00000000000404DC00000000000E053C00000000000F062C000000000004044C00000000000706EC0"

Now we can use this to separate our river into its two halves and then, via a cross join, feed them into the ST_Distance function.

SELECT min(ST_Distance(l.the_geom,r.the_geom)) as Min_Width
   FROM (select row_number() over (order by the_geom) as rin, the_geom
           from River r
          where r.Name = 'Barito' ) as l,
        (select row_number() over (order by the_geom) as rin, the_geom
           from River r
          where r.Name = 'Barito' ) as r
  WHERE l.rin = 1
    AND r.rin = 2;

min_width
double precision
48.2597140480546

Hope this helps.

regards
Simon
On Wed, 14 Oct 2009 13:37:08 +1100, Surya Tarigan <surya.tarigan at yahoo.com> wrote:

> Dear List,
>
> I have polyline depicting river banks. I have queried the minimum distance between left and right river bank as follows:
>
> SELECT min(ST_Distance(r.the_geom, r.the_geom)) as Min_Width
> FROM River as r
> WHERE r.Name = 'Barito';
>
> It returns zero ('0') result. The left and the right river bank have the same attributes (ID, Name, etc), that is why I use ST_Distance(r.the_geom, r.the_geom). Should I give different ID or Name for left and river bank. How the argument of ST_distance should look like.. The argument that I use above must be not correct (i.e. ST_Distance(r.the_geom, r.the_geom)).
>
> Any clue would be appreciated
>
> surya
>
>
>
>
> ________________________________
> From: Kevin Neufeld <kneufeld at refractions.net>
> To: PostGIS Users Discussion <postgis-users at postgis.refractions.net>
> Sent: Fri, October 2, 2009 12:35:17 PM
> Subject: Re: [postgis-users] polygon minimum width
>
> The linestrings wouldn't need to be continuous as long as you attribute them left and right bank.  Then your query would be to find the shortest distance between two linear datasets (left and right banks).  A cross product query would help you here.  Or, ST_Collect your banks into two multilinestrings, one for left and one for right bank.  Then compute the distance between them.
>
> But you are still going to have problems at the head and mouth of the river.  There, the left and right banks touch, so the minimum distance between the left side of the river and right side of the river is zero.
> -- Kevin
>
> Surya Tarigan wrote:
>> Hallo Nicklas
>> fortunately I have linestring version of the polygon, but it seems that the linestirings are not continuous. Does st_distance still apply? Or are there any postgis function to make the polyline continuous?
>>  kind regards,
>>------------------------------------------------------------------------
>> *From:* "nicklas.aven at jordogskog.no" <nicklas.aven at jordogskog.no>
>> *To:* PostGIS Users Discussion <postgis-users at postgis.refractions.net>
>> *Sent:* Thursday, October 1, 2009 3:50:37 PM
>> *Subject:* Re: [postgis-users] polygon minimum width
>>
>> Hallo
>>  I think the easiest way is to make a linestring of the polygon.boundary and then cut the line in the start and end so you get two more or less paralell lines.. Then you can use st_distance to find the shortest distance between them.
>>  Hope that helps
>> Nicklas
>>
>> 2009-10-01 Surya Tarigan wrote:
>>
>> >
>> >
>> Dear list,
>> >
>>  >
>> I  have a river polygon with polygon length about 20 km.  How can I query the minimum width of  the river polygon. I tried to search previous threads, but I could not find any clue.
>> >
>>  >
>> kind regards,
>> >
>>  >
>> surya
>>
>> >
>>
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>>
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