Hmm. Another use case for having topology in PostGIS ... if only :)
Well, I think my approach would be to iterate through your polygons and
do as you suggest, use ST_Difference, but use a collection of all the
lines that overlap the polygon and the polygon as parameters.
1. First identity all the lines that potentially overlap a polygon
CREATE TABLE overlapping_contours AS
SELECT a.id AS poly_id, b.id AS cont_id, b.geom
FROM poly a, contours b
WHERE a.geom && b.geom;
2. Remove all original contour lines so you don't get duplicates later
DELETE FROM contours WHERE id IN (SELECT cont_id FROM overlapping_contours);
3. Create multilinestring of the contours around every polygon (to be
used as input into ST_Difference)
CREATE TABLE grouped_contours AS
SELECT poly_id, ST_Collect(geom) AS geom
FROM overlapping_contours
GROUP BY poly_id
4. Subtract the polys from the multilinestrings
UPDATE grouped_contours a
SET geom = ST_Difference(a.geom, b.geom)
FROM polys b
WHERE a.poly_id = b.id;
5. Expand and insert the mlines back into the contours table
INSERT INTO contours (geom)
SELECT (ST_Dump(geom)).geom
FROM grouped_contours;
None of this code is tested, and obviously you'll need to add indexes to
your intermediate tables, but it might be a start. Note that PostgreSQL
does not use more than one CPU at a time per query. I could see this
taking quite a long time. To speed things up, you might try
partitioning your polygons into 4 logical groups and run things
concurrently. IE, in step one, add the filter AND a.id < 50000. Then,
at the same time, run another query using AND a.id >=50000 AND a.id <
100000. Just a thought.
Cheers,
Kevin
Christy Nieman wrote:
Hello,
I have been having difficulty figuring out how I might go about doing
a difference of many lines and polygons. For context, I have many (>
3,000,000) contour lines that I want to difference based on many (>
180,000) lakes. My goal is to remove the sections of the lines that
are within the polygons. I know that I can use ST_Difference(aLine,
aPolygon) to do this for one feature, but, what would the syntax look
like to go though all my line and all my polygons and return just the
lines/parts of lines that are outside of the polygons (and do this in
as quickly and resource-friendly way as possible)? The machine I am
working on is a relatively well-powered desktop (quad core, 6GB RAM).
Thanks for any insight/suggestions,
Christy
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