[postgis-users] Getting TopologyExections when trying to node linestrings to create an overlay
Rémi Cura
remi.cura at gmail.com
Wed Feb 18 04:07:53 PST 2015
Hey,
last chance for you ^^
I successfully was able to remove errors in 2 ways :
- first your geometry array contains a lot of duplicated geometry.
( rest of this is based on geom_set_id= 1)
I suspect you forgot a where in an inner join, or something like this.
Removing the duplicate in your data seems to solve the problem
(removing duplicate : DISTINCT ON (geom))
I generated each pair of geometry in this set, none gave the error
individually (which lessen the chance of a legit GEOS problem)
- second, without removing the duplicates, a combination of tanslate and
snaptogrid(0.5) was sufficient to also remove the error.
I would recommend that you analyse a little bit your data.
For instance, simply deuplicating on geom make your data set going from
--15317 geometry in arrays to --9934
Snapping to a 0.5 grid before deduplicating further reduce the data set to
9617 (which might indicates that somewhere in your workflow you have a
precision related issue).
I never used ARCGIS, but I can bet you that ARCGIS is not precision-safe.
The only product that is truly safe is CGAL, which comes with other type of
constraints.
You could also probably use GRASS safely if cleaning the data with v.clean
on import.
Anyway, there is no safe tool, only safe way to use it. You could very
easily create another SRS that is a translation of your original srs to
increase precision.
So you simply change your workflow to ST_Transform before computing, then
ST_Transform after computing.
Lastly, I don't see the interest of using an array of geom here because you
don't need to be able to do my_array[N] (no need to access).
So you could simply use a geometry collection, so your input is a geom, and
not a geom[].
Cheers,
Rémi-C
2015-02-18 10:39 GMT+01:00 BladeOfLight16 <bladeoflight16 at gmail.com>:
> On Mon, Feb 9, 2015 at 6:25 AM, Sandro Santilli <strk at keybit.net> wrote:
>
>> First of all I confirm it still happens with GEOS="3.5.0dev-CAPI-1.9.0
>> r4038".
>> Second, I took a look at a random set (geom_set_id=1) and I found it
>> pretty
>> big. That's to say you could probably further reduce the dataset for the
>> ticket. That set contains 109 polygons, I can get the error by attempting
>> to union the boundaries of the first 40 in that set, and I'm sure you can
>> further reduce the input.
>>
>
> Thanks for taking a look. I'll work on doing that when I can find the
> time, but I don't expect that to be a fast process at all. Even just
> checking for the pairwise case took a decent amount of time to develop the
> query and took overnight to finish running (even with optimizations like a
> bounding box intersection test). I don't really have any good heuristic
> that could narrow down the possibilies for reproducing, so I don't see much
> option other than to brute force it possibly with some kind of filter.
> That's why I didn't put more effort into shrinking the input set to begin
> with.
>
> Is a PostGIS database dump an okay format to provide the shapes, or would
> you prefer something else? I suppose I could dump groups into shapefiles or
> something like that if it's more convenient.
>
> On Mon, Feb 9, 2015 at 7:00 AM, Rémi Cura <remi.cura at gmail.com> wrote:
>
>> this is a precision related issue, coordinates are way too big and should
>> be translated.
>>
>
> I understand what you mean by that (as in floating point problems due to
> size), but these coordinates are pretty typical. This is a standard UTM
> projection, zone 15N in middle America. It's even predefined in PostGIS'
> list of spatial references. I'm told ESRI had this kind of problem years
> ago, but they dealt with it as far as I know. While I would choose PostGIS
> over ESRI any day, this could be viewed by my coworkers as a good argument
> against using PostGIS; it represents a serious reliability concern since it
> applies to a broad range of functions and 2D projections. Basically, if you
> use any projection with coordinates of this size (of which there are a good
> number), there seems to be no telling when any function will just blow up
> in your face at random.
>
> On Mon, Feb 9, 2015 at 7:04 AM, Rémi Cura <remi.cura at gmail.com> wrote:
>
>> Hey,
>> I executed your data,
>> the following command solve the problem (with very recent GEOS for me)
>> (POSTGIS="2.2.0dev r12846" GEOS="3.5.0dev-CAPI-1.9.0 r0" PROJ="Rel.
>> 4.8.0, 6 March 2012" GDAL="GDAL 2.0.0dev, released 2014/04/16"
>> LIBXML="2.8.0" RASTER)
>>
>
>>
> [Snip]
>>
>
>>
> The change compared to your approach : convert input to table of simple
>> polygons, (no array, no multi).
>> Then translate to improve precision in geos computing
>> Then the union.
>> I don't really understand what you are trying to do,
>> but ist_union seems dangerous and quit ineffective for that .
>>
>
> I'm looking at this function call in your code: ST_Union(
> ST_MakePolygon(ST_ExteriorRing(geom)) ). That call seems to remove all
> holes and then create the union of all the covered areas (a single
> multipolygon that covers all areas covered by the originals). That is not
> what I'm trying to do; I already have an outer boundary that I could use
> for that purpose.
>
> Here's what I'm trying to do. I'm trying to create an overlay (as I said
> in the first sentence of my original e-mail but didn't elaborate on), in
> the sense that the term is used by these two articles:
> http://boundlessgeo.com/2014/10/postgis-training-creating-overlays/ or
> http://trac.osgeo.org/postgis/wiki/UsersWikiExamplesOverlayTables. What
> they do is they create a sort of Venn diagram, if you will; they take all
> the polygons and create a new polygon for each area with a different set of
> overlappying polygons. Both of them use ST_Union in the same way I am: they
> take a set of linestrings and union them to create a fully noded
> linestring, and then they use this linework to create a bunch of new
> polygons. I'm leveraging ST_Boundary instead of ST_ExteriorRing in my code
> because I need to preserve holes, but that shouldn't change the results as
> far as I know. In what way is ST_Union "dangerous" and "quite ineffective"
> for the purpose of noding lingstrings? If that's true, I'm apparently not
> the only one who has some misconceptions, since both these articles use it
> the same way. I should also note that I don't show the whole process here;
> I'm only showing the part where I'm noding the linestrings because that's
> where this error occurs. Once I have the polygons, I also go back and
> relate them to the attributed rows in the original tables (of which there
> are several).
>
> That said, I'm trying out the translate and cutting out multi, but I am
> still using an array in my actual code for a reason. Namely, I need to be
> able to do this with *different* sets of geometries. These geometries
> come as the result of selecting polygons from 3 or 4 different tables, each
> having completely disparate sets of attributes. So basically, I need to be
> able to use an arbitrary query to get the group of polygons to be passed
> into a reusable overlay process. As a result, a function is a natural fit.
> I opted to use a normal function that takes an array. The only other option
> I can think of is some kind of aggregate function, which I didn't
> investigate doing as I'm not sure whether that might be more or less
> reliable. If you think an aggregate would be better or know of a better way
> to accomplish that, I'm all ears.
>
>
>> Of course reducing the number of useless points before union make it 10
>> times faster .
>>
>> DRoP TABLE IF EXISTS unioned_poly ;
>> CREATE TABLE unioned_poly AS
>> SELECT ST_Union(
>> ST_Buffer(
>> ST_MakePolygon(
>> ST_ExteriorRing(
>> ST_SImplifyPreserveTopology(
>> geom
>> ,10
>> )
>> )
>> )
>> ,1 )
>> )
>> FROM unique_polygon
>> GROUP BY geom_set_id
>> (17 sec)
>>
>
> 10 meters is a lot of precision to lose. That's over a tenth of a football
> field. I might be willing to simplify by 1 meter at most, but even that's a
> little big for my taste. My client and users expect to see the same shapes
> that got input come back out.
>
> Thanks again to everyone who took a look.
>
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