Help on extract function
On 02/12/2011 02:05 PM, Sean O'Riordain wrote:
Hi Rahul, First look at str(polygon, max.level=2) to understand how a shapefile is represented in R, then have a look at str(polygon at data) which is a dataframe - you can add a new column to this dataframe like... polygon at data$newcolumn <- 1:nrow(polygon at data) i.e you can treat this dataframe like any other as long as you're in read-only mode or just adding columns... if you add rows or delete rows or modify ID fields then beware!!! these rows are linked to the actual polygons held elsewhere as arrays... don't touch for the moment I'd suggest! :-)
You can, if you avoid modifying object slots directly, but use the appropriate methods. So to select the first ten polygons with their attributes you should NEVER do something like: polygon at data = polygon at data[1:10,] # NO! but rather polygonSel = polygon[1:10,] For addressing or adding columns you can also use polygon$existingcolumn polygon[["existingcolumn"]] and assigning: polygon$newcolum <- 1:nrow(polygon at data) which will have the same effect as polygon at data$newcolumn <- 1:nrow(polygon at data) ... whatever you prefer! -- Edzer
then you can save out the shape file again... cheers Sean Dublin Ireland On 12 February 2011 05:16, Rahul Raj <rahulosho at gmail.com> wrote:
Dear all, I am using the command "extract(raster, polygon)".
I want to run this command in loop to extract the raster cells values by
polygon number in each iteration.
I also want to attach one attribute column to shape file and write the
shape file with added attribute.
Please help me how can I do this in R
Quick reply is highly appreciated and Thanks in advance
Rahul Raj
Indian Institute of Remote Sensing, India.
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Edzer Pebesma Institute for Geoinformatics (ifgi), University of M?nster Weseler Stra?e 253, 48151 M?nster, Germany. Phone: +49 251 8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de http://www.52north.org/geostatistics e.pebesma at wwu.de