Sandro,
I replaced ST_Simplify by ST_SimplifyPreserveTopology, and as a
result, like you expected, the coastline of Norway was simplified at
the 0.25 iteration.
So, why not use ST_SimplifyPreserveTopology by default? Or is that
function generally a lot slower than ST_Simplify?
Michiel
Quoting "Michiel J. van Heek" <michielvanh...@nine-e.org>:
Hello Sandro,
Thank you for your reply.
These are the warnings for edge 11652, the coastline of Norway (on
the screenshot in my previous post):
WARNING: Simplification of edge 11652 with tolerance 0.5 failed:
SQL/MM Spatial exception - geometry crosses an edge
WARNING: Simplification of edge 11652 with tolerance 0.25 failed:
SQL/MM Spatial exception - curve not simple
WARNING: Simplification of edge 11652 with tolerance 0.125 failed:
SQL/MM Spatial exception - curve not simple
WARNING: Simplification of edge 11652 with tolerance 0.0625 failed:
SQL/MM Spatial exception - curve not simple
WARNING: Simplification of edge 11652 with tolerance 0.03125
failed: SQL/MM Spatial exception - curve not simple
WARNING: Simplification of edge 11652 with tolerance 0.015625
failed: SQL/MM Spatial exception - curve not simple
WARNING: Simplification of edge 11652 with tolerance 0.0078125
failed: SQL/MM Spatial exception - curve not simple
The "curve not simple" makes we want to try your suggestion to use
ST_SimplifyPreserveTopology instead of ST_Simplify. I will try that
tomorrow (it is late in my timezone) and let you know the outcome.
The ticket you mentioned seems to be about creating topology from
geometry. That part - SELECT CreateTopology('countries_topology',
4326) - ran flawless in my case. (Long, but without errors.) The
ticket is not about simplifying topology, or is it?
Best,
Michiel.
Quoting Sandro Santilli <s...@keybit.net>:
On Fri, May 25, 2012 at 06:36:39PM +0200, Michiel J. van Heek wrote:
Dear list,
I am trying to simplify a dataset of country boundaries by using
topology functions, as described in Strk's Blog:
http://strk.keybit.net/blog/2012/04/13/simplifying-a-map-layer-using-postgis-topology/
This has worked quite well, except for coastlines with islands
nearby. The simplified coastline would probably intersect the
island, or pass the island at the wrong side, leading to invalid
topology. Does that mean that countries with coastal islands such as
Canada and Norway (see attached image) cannot be simplified this
way?
I think you should figure out more about why the simplification
fails. The function in the post raises a WARNING on each failure,
what does that warning say ? I'm asking because there's also the
case of self-topology breakage, which could be prevented by using
ST_SimplifyPreserveTopology rather than ST_Simplify.
If your assumption about why simplification fails is correct you
could make things better by splitting the complex edge near the
islands (see ST_ModEdgeSplit).
Ideally, islands in the way of a to-be-simplified coastline should
be merged with the mainland. (I have already removed the smaller
islands, but I think I should not remove the bigger islands too.)
The function in the post is still only considering each edge in
turn and has no knowledge of which faces are defined by those
edges, let alone which faces define your feature (mainland, island).
You'll need something more sofisticated for that, and I'm not sure
there's an easy general handling for such cases. I mean, there could
still be the case in which simplifying an edge would intersect an
island which doesn't belong to the same feature...
Let me know what you found when looking closer at the failure messages.
There's a ticket to make those messages more useful to understand
the problem [1], if you hit that please add to that ticket, and maybe
provide also a patch :)
[1] http://trac.osgeo.org/postgis/ticket/1817
Finally: make sure to run the latest and greatest postgis & GEOS.
There have been a few fixes and improvements lately.
--strk;
,------o-.
| __/ | Delivering high quality PostGIS 2.0 !
| / 2.0 | http://strk.keybit.net - http://vizzuality.com
`-o------'
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