Dear geo-list,
I'm just starting using R and the rgdal package to manage and create some
shapes and I am hitting some speed issues while using the %over% function. R is
significantly slower than ArcGIS.
Here is what I'm doing (I've shorten the code to make it easier to follow, I
don't expect it to be reproducible).
# I first load a shape giving the perimeter of the territory of interest
terr <- readOGR("D:\\Couches GIS\\06152\\contour", "contour_carto06152_Project")
# I then want to make a grid of points. The grid is very dense and rectangular
# to do so, I first make a gridtopology
grille.plac.temp1 <- GridTopology(...)
# and get all the coordinates from the grid
grille.plac.temp2 <- coordinates(grille.plac.temp1)
# and finally transform it to a SpatialPointsDataFrame object
grille.plac <-
SpatialPointsDataFrame(grille.plac.temp2,data.frame(1:nrow(grille.plac.temp2)),proj4string
=CRS(proj4string(terr)))
# The grid is very big as it is a rectangle in which you can fit the territory
shape
# (which is of irregular shape). I then want to reduce it's size by keeping only
# to points from the grid that are in the territory. I use the %over% function
to do
# that. This is the slow part.
system.time(plac.dans.contour<-grille.plac%over%contour)
user system elapsed
1420.23 9.57 1435.17
# I then make a new shape with only the points from the grid that are in the
territory
grille.plac.reduce <- grille.plac[as.vector(!is.na(plac.dans.contour)),]
This code works fine and produce what I want it to produce. However, the %over%
functions takes 24 minutes to run, while ArcGIS can run it in less than 1
minute with the intersect tool. Is there a more efficient function than %over%
to do what I want? Is it normal that ArcGIS is that much faster than R in that
case?
Thanks for your help,
Bastien Ferland-Raymond, M.Sc. Stat., M.Sc. Biol.
Division des orientations et projets spéciaux
Direction des inventaires forestiers
Ministère des Ressources naturelles et de la Faune du Québec
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