[pgrouting-users] Isochrones as polygons

[Simon Georget]

Hi Ross,

Thanks for your reply,

I've tried, indeed, to increase the distance with no success. I get the
same error. That sounds strange.
Of course, I read Anita Graser blog posts but on her examples she calculate
isochrones points by points. I want to automatize the task, that's why I
want to use a function.

I had no time last days to work on that. I'll try next week.

Do you you if 'pgr_pointsAsPolygon' relies on 'pgr_alphashape' function. I
got a strange error message when trying to implement it directly ?

bye, simo

On Thu, May 14, 2015 at 12:11 PM, McDonaldR <McDonaldR at angus.gov.uk> wrote:

>  Hello Simon
>
>
>
> Have you tried increasing your distance to 10000?  The error message says
> your distance is too short and not enough points are available to create
> the alphashape.
>
>
>
> As an aside:
>
>
>
> Look at this post:
> http://anitagraser.com/2011/09/25/a-closer-look-at-alpha-shapes-in-pgrouting/
>
>
>
> And maybe at this post:
> http://anitagraser.com/2013/07/07/public-transport-isochrones-with-pgrouting/
>
>
>
> That should help you create the polygons from driving distance.
>
>
>
> Ross
>
>
>
>
>
> *From:* pgrouting-users-bounces at lists.osgeo.org [
> mailto:pgrouting-users-bounces at lists.osgeo.org
> <pgrouting-users-bounces at lists.osgeo.org>] *On Behalf Of *Simon Georget
> *Sent:* 13 May 2015 18:48
> *To:* pgrouting-users at lists.osgeo.org
> *Subject:* [pgrouting-users] Isochrones as polygons
>
>
>
> Hi list,
>
> I'm trying to generate isochrones as polygons in the way it is explained
> in that post  :
>
>
> https://smathermather.wordpress.com/2014/08/06/drivetime-analyses-pgrouting/
>
>
>
> I'm not familiar with postgres/postgis and I get a bit confused when
> trying to understand the difference between pgr_alphashape() /
> pgr_pointsAsPolygon(). I've read a lot through the web but did not find lot
> of examples scirpts.
>
>
>
> I'm able to generate the drivingDistance() but I get lost when trying to
> get points to generate my polygons.
>
> Is there any sample script somewhere combining the use of
> pgr_drivingDistance() with pgr_pointsAsPolygon() ?
>
> Here is an attempt to adapt the script from smathermather with my data.
> Note that I want to run it on multiple start points :
>
>
> ---------------------
> CREATE OR REPLACE FUNCTION nn_search (geom geometry) RETURNS
> int8 AS $$
>
> SELECT id FROM roads_idf_l93_vertices_pgr AS r
> ORDER BY geom <#> r.the_geom
> LIMIT 1;
>
> $$ LANGUAGE SQL VOLATILE;
>
>
> -- cette fonction renvoie un polygone
> -- elle prend en paramètre l'id du noeud et le cost
> CREATE OR REPLACE FUNCTION alpha_shape (id integer, cost integer) RETURNS
> geometry AS $$
>
> WITH alphashape AS(SELECT pgr_alphaShape('WITH
>
> DD AS (
>     SELECT seq, id1 AS node, cost
>     FROM pgr_drivingDistance(''SELECT id, source::int4 AS source,
> target::int4 AS target, length::float8 AS cost FROM roads_idf_l93'',' || id
> || ', ' || cost || ', false, false)
>     ),
>
> points AS (
>     SELECT v.id::int4 AS id, x::float8, y::float8
>     FROM roads_idf_l93_vertices_pgr v, DD d
>     WHERE v.id = d.node)
>
> SELECT * FROM points')),
>
> alphapoints AS (
> SELECT ST_Makepoint((pgr_alphashape).x, (pgr_alphashape).y) FROM
> alphashape),
>
> alphaline AS (
> SELECT ST_MakeLine(ST_MakePoint) FROM alphapoints)
>
> SELECT ST_MakePolygon(ST_AddPoint(ST_MakeLine,
> ST_StartPoint(ST_MakeLine))) AS the_geom FROM alphaline
>
> $$ LANGUAGE SQL VOLATILE;
>
> CREATE TABLE alpha_test AS
> WITH dest_ids AS (
> SELECT nn_search(geom) AS id FROM gare_idf_l93
> )
> -- cost 1000 m
> SELECT alpha_shape(id::int, 1000)::geometry, id FROM dest_ids;
>
> -----------------------
>
> Executing this script return the following error :
>
> ERROR:  Distance is too short. only 2 vertices for alpha shape
> calculation. alpha shape calculation needs at least 3 vertices.
> CONTEXT:  SQL function "alpha_shape" statement 1
> ********** Erreur **********
>
> ERROR: Distance is too short. only 2 vertices for alpha shape calculation.
> alpha shape calculation needs at least 3 vertices.
> État SQL :XX000
> Contexte : SQL function "alpha_shape" statement 1
>
>
>
> Any help would be appreciated. Thanks
>
> simo
>
>
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