[postgis-users] split line at polygon edge

François Hugues hugues.francois at irstea.fr
Thu Mar 17 14:17:08 PDT 2016


Empty geometries are returned when there is no intersection and I think we forgot something obvious. When you want to intersect geometries you need to add WHERE ST_Intersects (a.geom,b.geom). 

Things should work better and faster. 

HugThanks Remi-C and Hugues for your suggestions, they got me what I needed!

I first tried Remi-C's example, since I was curious about how it would turn
out. It gave me an error mentioning that it could not convert
GeometryCollection to LineString. This error brought me back to what Hugues
mentioned. So I used ST_Summary() to verify the GeometryCollections, which
appeared to be empty (0 elements), and mixed in I noticed the LineStrings,
MultiLineStrings. Since the Collections seemed to be empty I opted to
separate out the linestrings I using a function Hugues mentioned

Specifically I used:
ST_GeometryType(geom) like '%Line%'

to get both linestring and multilinestrings.

In the end it took 2 statements, even though I knew someone much more
proficient then myself could do it in one.

My final statements where:
#create table public.temp_trail_div1 as select st_intersection(t.geom,
d.geom) as geom,t.trails_id,
d.id as trail_system from public.temp_trails as t, public.divisions as d;

#create table public.temp_trail_div_sep as select * from
public.temp_trail_div1 where ST_GeometryType(geom) like '%Line%';

This seems to have done the trick, for now. Could someone enlighten me on
how that might be done in one statement?

Thanks again,

On Thu, Mar 17, 2016 at 5:45 AM, Rémi Cura <remi.cura at gmail.com> wrote:

> Hey,
> two things :
> recent version of QGIS are boringly strict about geometry type,
> so if you want to be able to add the corresponding postgis layer to qgis,
> you may have to explicitely cast the result.
> QGIS also require a unique identifier per row,
> which you can fabricate with row_number() for instance
> ​​
> ----------------------------------------------------------------
> CREATE TABLE my_table AS
> SELECT row_number() over() AS qgis_unique_id,
>     st_intersection(t.geom, d.geom)::geometry(linestring,4326) AS geom
>     ,t.trails_id, d.id
> FROM public.temp_trails as t, public.polys as d;
> ----------------------------------------------------------------
> Cheers,
> Rémi-C
> 2016-03-17 8:15 GMT+01:00 François Hugues <hugues.francois at irstea.fr>:
>> Hello,
>> Dis you take a look at the query result ? I think you should first try
>> to see what is the type of geometry returned using ST_GeometryType(). You
>> may have some geometrycollections and I’m not sure QGis can handle  it. In
>> this case you could extract lines using ST_CollectionExtract().
>> To achieve what you want to do, you’ll be able to compare your original
>> lines table with the result of your query using ST_Difference().
>> Regards,
>> Hugues.
>> *De :* postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] *De
>> la part de* Garret W
>> *Envoyé :* jeudi 17 mars 2016 04:11
>> *À :* postgis-users at lists.osgeo.org
>> *Objet :* [postgis-users] split line at polygon edge
>> Hi Ive been looking for a way to take several hundred lines and split
>> them where they intersect a polygon while also giving them the ID of the
>> polygon they fall in. Ive seen many posts on splitting polygons. But its
>> been difficult for me to adapt those examples.
>> Ive been able to get an output from this:
>> select st_intersection(t.geom, d.geom),t.trails_id, d.id
>>  from public.temp_trails as t, public.polys as d;
>> Its giving me the line and IDs that I wanted but the geom is unreadable
>> for some reason by QGIS.
>> 99.9% of the lines fall within a polygon. Id like to still hang on to
>> those few lines that arent contained in a polygon. They should just be
>> split with no ID added
>> Im using; postgis 2.2, postgresql 9.5
>> Thank you
>> Garret
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