<div dir="ltr"><div><div><div>A couple quick thoughts on approach 2:<br><br>1) The union need only apply to intersecting geometries, rather than the whole dataset. This helps considerably with memory footprint, but in some edge cases could still be a real problem.<br>
<br></div>2) We need Mr. Davis to hurry up and demonstrate streaming geometry processing so we can port it to c and put it in postgis... . :)<br><br></div>And so, with some kibitzing , this morning I contribute no code to the conversation... . Apologies-- I've got a deadline elsewhere... .<br>
<br>Best,<br></div>Steve<br><div><div><div><br><br><br><br><br></div></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Wed, May 8, 2013 at 10:27 AM, Stephen Woodbridge <span dir="ltr"><<a href="mailto:woodbri@swoodbridge.com" target="_blank">woodbri@swoodbridge.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi all,<br>
<br>
This question comes up reasonably often on the pgRouting list and has been posted he on occasion under titles like "How to break streets at intersections?"<br>
<br>
It seems to me that this would be a good function to create in either postgis or pgrouting.<br>
<br>
THE PROBLEM:<br>
<br>
I have a table of 10's of thousands of street segments to 10's of millions of street segments. These street segments are LINSTRING or MULTILINESTRING geometries with some arbitrary number of attribute columns. The geometries may cross one another and are not noded correctly for use with pgRouting.<br>
<br>
THE RESULTS:<br>
<br>
We want to process the table and create a new table with the same structure (see comment about primary key below), and in the new table all the geometries are broken at intersections and all the new pieces of the original segment that have been broken have the original attributes propagated to them. So if the original segment has column foo='abc' and was split into 3 new segments, each of the three new segments would also have foo='abc'. The exception to this might be that the new table needs a new primary column as the old primary key will now be duplicated for the multiple parts.<br>
<br>
POTENTIAL SOLUTIONS:<br>
<br>
1. I think one way to do this would be to create a topology and load the table into it, then extra a new table from the topology. Although I'm not sure of the specifics for doing this or the efficency of doing it this way.<br>
<br>
2. Another way seems to be using a query like:<br>
<br>
select (st_dump(bar.the_geom)).* from (<br>
select st_union(foo.the_geom) as the_geom from mytable foo<br>
) as bar;<br>
<br>
And then taking each of the dump.geom objects and using st_contains to find which original segment it belonged to so we can move the attributes to the new segment. This method also loose any association to the original record and forces the use of st_contains to re-associate the new segments to the original segments.<br>
<br>
My concern with this is that the st_union has to load the whole table which may be 10's of millions of street segments and this will likely be a memory problem. Also running the st_contains() does not seems to me to be optimal.<br>
<br>
3. Is there a good recipe for doing this somewhere that I have not found? or other better approaches to this problem?<br>
<br>
What would be the best way to add tolerance to the problem? using snap to grid?<br>
<br>
Thoughts on how to do this efficiently?<br>
<br>
Since I'm working on the pgRouting 2.0 release I thought this might be a nice function to add to that if we can come up with a generic way to do this.<br>
<br>
Thanks,<br>
-Steve<br>
<br>
<br>
-- Example to demonstrate st_union above<br>
select st_astext((st_dump(bar.the_<u></u>geom)).geom) from (<br>
select st_union(foo.the_geom) as the_geom from (<br>
select 'MULTILINESTRING((0 1,2 1))'::geometry as the_geom<br>
union all<br>
select 'MULTILINESTRING((1 0,1 2))'::geometry as the_geom<br>
union all<br>
select 'LINESTRING(1 1.5,2 2)'::geometry as the_geom<br>
) as foo<br>
) as bar;<br>
<br>
"LINESTRING(1 1.5,2 2)"<br>
"LINESTRING(1 0,1 1)"<br>
"LINESTRING(1 1,1 1.5)"<br>
"LINESTRING(1 1.5,1 2)"<br>
"LINESTRING(0 1,1 1)"<br>
"LINESTRING(1 1,2 1)"<br>
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</blockquote></div><br></div>