[geos-devel] Non Noded intersection bugs when performing Geometry overlay

James Sewell James.Sewell at lisasoft.com
Mon Feb 27 01:25:36 EST 2012

Hello All,

I'm never sure which list to fire these GEOS problems from PostGIS issues to, so I'll try them both this time.

I'm using the following:

PostgreSQL 9.1.2 on i686-pc-linux-gnu, compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44), 32-bit

POSTGIS="2.0.0alpha7SVN" GEOS="3.3.2-CAPI-1.7.2" PROJ="Rel. 4.7.1, 23 September 2009" LIBXML="2.6.26" USE_STATS

I have been closely following the updates to GEOS / PostGIS at the end of last year with regards to overlay routines. Since the advent of ST_UnaryUnion and the updates in the BinaryOp code as of r3552 I have been trying to get our overlay code working again. We currently do the following:

*         Break left layer and right layer into smaller regions (both are multipolygons)

*         For each region

o   Use ST_Snap to snap the left layer (which in this case is less accurate) to a collection of the right layer at 10E-8

o   Dump the left and the right layers to linework and collect them to two geoms

o   Perform a ST_UnaryUnion on each layer, then collect them together

o   Perform a ST_UnaryUnion on the collection (I found using this two tiered Union approach sometimes resolved nodding errors, obviously at the cost of speed though). If I get a non-noded intersection problem it is here 100% of the time.

o   Polygonize the output

o   Perform overlay map to pull through gids from original left and right layers

At one point I did translate to the origin before any Union operations, but this broke down as the linework got more complex (which unfortunately happens every month as new polygons are added).

At the moment the problem I face is the same as my old problem (when using ST_Union, see http://trac.osgeo.org/geos/ticket/392). When performing the second ST_UnaryUnion of the collected left and right lineworks (which are valid) I get the following:

GEOSUnion: TopologyException: found non-noded intersection between LINESTRING (149.169 -37.739, 149.169 -37.7391) and LINESTRING (149.169 -37.7391, 149.169 -37.739) at 149.16916209449002 -37.739087184984776

This year I've been familiarising myself with the code and reading through all the old posts I can find which apply to this.

I just wanted to throw this list post out there to see what others had to say about this issue. It seems to have been round for a while now, and from what I can tell it's not an easy fix.
Yum install
Is it likely to be a precision issue? I am currently upgrading my GCC version so I can try this http://trac.osgeo.org/geos/ticket/504, does anyone have any thoughts on if that might help?

Or is the issue more likely to be as described here? http://trac.osgeo.org/geos/ticket/459

Sorry for the long post, just hoping to get some discussion going. I will try to post a fresh testcase which causes the error tomorrow.

James Sewell
Solutions Architect
Ph: +61 3 8680 3250 Fax: +61 3 8680 3299
Level 9, 601 Bourke St, Melbourne Vic 3000


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