qgis – Cutting Multiple Lines with Multiple Polygons: Performance Issues and Solutions

performanceqgisvector

I have a line-shp (80mb) and a polygon-shp (5mb) and want to intersect them. In result, the lines (contours) should be cut by the polygons.

After using one of the tools "Vector -> Geoprocessing -> Dissolve/Clip/Intersect" to do so, the loading bar keeps at 0%. Same by marking just 10 of the 6000 contours to intersect them. The CPU usage is at 50% (one core 100%) all the time, even for more then 30minutes…

What is the problem? How to solve it? But don't tell me, I need a new computer! :)

My config: AMD II X2 240 ; 4GB RAM; xubuntu 12.04; Qgis 1.8

Best Answer

(i recently had to piece this together for Linux Mint 12 KDE, i expect ubuntu should be identical. You can also refer to http://postgis.refractions.net/documentation/manual-1.4/ch02.html and http://postgis.17.n6.nabble.com/Installing-a-fully-functional-PostGIS-2-0-on-Ubuntu-Linux-GEOS-GDAL-issues-td3566227.html)

Install and configure PostgreSQL (v9.1)

install via Synaptic or

sudo apt-get update
sudo apt-get install build-essential
sudo apt-get install python-dev libreadline-dev
sudo apt-get install postgresql-9.1 postgresql-server-dev-9.1 postgresql-contrib-9.1 postgresql-plpython-9.1

change password method 1

sudo passwd postgres
sudo -u postgres psql postgres

enter password twice on prompts

at psql prompt

postgres # \password postgres

enter password twice on prompts

OR change password method 2

sudo -u postgres psql
postgres # ALTER ROLE postgres WITH ENCRYPTED PASSWORD 'newpassword';
postgres # \q

Install and configure pgAdmin3 (v1.14)

default distro repo only has v1.12 which lacks support for the Postgres 9.1 maintenance features, so instead, do the following:

add apt keyring using standard port (firewall workaround)

gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv 1F9ADD375CA44993
gpg --export --armor 1F9ADD375CA44993 | sudo apt-key add -

add ppa

sudo add-apt-repository ppa:rhonda/pgadmin3

using nano, open ppa list and replace oneiric with natty (this is cheating, but it works), then apt-get update:

sudo nano /etc/apt/sources.list.d/rhonda-pgadmin3-oneiric.list 
sudo apt-get update

pgAdmin3 v1.14 will now be available to Synaptic, or

sudo apt-get install pgadmin3=1.14.0~beta1-1~ppa1~natty1

Setup pgAdmin3 bin location. Launch pgAdmin3, File > Options > PG bin: /usr/lib/postgresql/9.1/bin/

Install and configure PostGIS (v1.5)

install via Synaptic or

sudo apt-get install postgresql-9.1-postgis

then

createdb tmplt_postgis
psql -d tmplt_postgis -f /usr/share/postgresql/9.1/contrib/postgis-1.5/postgis.sql
psql -d tmplt_postgis -f /usr/share/postgresql/9.1/contrib/postgis-1.5/spatial_ref_sys.sql
psql -d tmplt_postgis -f /usr/share/postgresql/9.1/contrib/postgis_comments.sql

(if these paths are incorrect, check Synaptic > postgresql-9.1-postgis > rclick > properties > installed files)

If postgis was compiled with shp2pgsql-gui then add the plugin to pgadmin3. (i think this method is hamfisted, oh well. All you need is a text ini file as shown below)

sudo mkdir /usr/share/pgadmin3/plugins.d
su
cat > /usr/share/pgadmin3/plugins.d/postgis.shp2pgsql-gui.ini
;
; pgShapeLoader (Linux):
;
Title=PostGIS Shapefile and DBF loader
Command=$$PGBINDIR/shp2pgsql-gui -U $$USERNAME -d $$DATABASE -p $$PORT -h $$HOSTNAME
Description=Open a PostGIS ESRI Shapefile or Plain dbf loader console to the current database.
KeyFile=$$PGBINDIR/shp2pgsql-gui
Platform=unix
ServerType=postgresql
Database=Yes
SetPassword=No

[Ctrl-D] to exit input mode

# exit

to exit su

Done.

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