Hi all
I have to develop an application that will build full topology from
shapefiles as input data.
I'm used to coding in C but I've begun using Python for a couple of weeks.
I wanted to know if Python could work fast enough when the input data are
rather big.
I made a simple test to find intersections between polygons, with a big
shapefile as input data (75,000 polygons, buildings and houses, located on a
25 x 25km2 area)
Both programs have been run on the same iMac (Mac OS X, Leopard).
On one hand, an executable, written in C, that calls directly the libgeos_c
library. It ended after 4 minutes : 2100 intersections (points, linestrings
and polygons).
On the other hand, few Python lines (see below) that use ogr api to read
shapefile and Shapely to find intersections. I stopped it after 70 minutes,
at iteration 8100 (20 % done) : about 6 hours estimated for the whole
shapefile ! I expected smaller differences.
What do you think about these huge differences ? Is it mad to use such big
files with Python/Shapely ?
Or did I make a beginner's mistake in my code ? Is it a problem with the
MacOS version of Shapely ? of Python (2.5.1) ?
Thanks
Pascal
#=======================================
import os,sys,osgeo.ogr
from shapely.wkb import loads
from shapely.geometry import Polygon
source = osgeo.ogr.Open(sys.argv[1])
layer = source.GetLayer()
objet = layer.GetNextFeature()
liste = []
while objet:
liste.append(loads(objet.GetGeometryRef().ExportToWkb()))
objet = layer.GetNextFeature()
print len(liste)
for i in range(len(liste)-1):
ref = liste[i]
if i % 100 == 0:
print str(i)
os.system("date")
for j in range(i+1,len(liste)):
sec = liste[j]
if ref.disjoint(sec) == False:
print "intersection entre " + str(i) + ' et ' + str(j)
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