[postgis-users] Getting TopologyExections when trying to node linestrings to create an overlay

[BladeOfLight16]

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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