Giovanni,
That's a good point. In both JTS and GEOS CascadedUnion is limited to
operating in main memory. (However, it may be that GEOS running under
PostGIS is more memory-efficient, or is able to utilize a larger memory
space).
There's not really an easy solution for working with JTS CascadedUnion
and very large datasets. (This also applies to using CascadedUnion in
JEQL - although JEQL itself is not memory-bound, it can process data in
a streaming fashion. But the CascadedUnion algorithm is not stream-based).
Some alternatives that come to mind are:
- use 64-bit Java and lots of memory
- implement your own "GriddedUnion" in PostGIS (and maybe involving
JTS). I can provided details if you like.
Martin
G. Allegri wrote:
Thanks Martin.
I see that jts 1.11 works fine on a subset of my dataset. I have
hundreds of thousands od polygons to merge... I have to find the way
to read them to pass the collection to JTS without receiving
java.lang.OutOfMemoryError (just happened!).
What do you suggest to build the collection needed by CascadedUnion
without havind to create a huge Collection of geometries?
I will take a look at JEQL. Thanks for the hint, I didn't know it...
Giovanni
2010/6/22 G. Allegri <gioha...@gmail.com>:
I'm using Postgis 1.5.1 to compute a Union on a quite large
multipolygons dataset (some hundreds of thousands of features), but
I'm facing the following error:
NOTICE: TopologyException: found non-noded intersection between
LINESTRING (1.7318e+006 4.77959e+006, 1.7318e+006 4.77958e+006) and
LINESTRING (1.7318e+006 4.77958e+006, 1.73174e+006 4.77954e+006) at
1.7318e+006 4.77958e+006
ERROR: GEOS union() threw an error!
I know it's a known issue, but I haven't been able to find a good,
replicable, solution. The input geometries result valid (in simple
feature meaning).
What is causing this? I have overlapping polygons, but I thought the
cascaded union algorithm computed the necessary noding on the
linestrings.
I've supposed it depends on tolerances/precision model, but I don't if
it makes sense and how, eventually, tune it.
Thanks a lot,
Giovanni
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
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